What is Air Layering?

by Andy Walsh

Introduction by Brent Walston

Air layering is the process of removing a large branch or section of the trunk of a tree to create another tree. Before the branch is removed it is girdled, protected with peat moss or other media and the girdled section is allowed to root. After rooting the branch is removed from the tree. This is a very common practice in bonsai to obtain another tree from an unwanted branch or to save a thick trunk section that was going to be removed anyway. Andy Walsh posted a short but very informative article on the physiology of this process on the Internet Bonsai Club mail list. Knowing how a tree forms roots at an air layer site provides powerful information for not only understanding the process, but also a vehicle for answering your own questions and solving your own problems in air layering.
BW

Transport of Food, Water, and Nutrients

Under the bark of trees (dicotyledonous ones) there is a layer of cells called the phloem. This tissue transports carbohydrates and other photosynthates (including auxin) down from the leaves to the lower parts of the plant. Beneath the phloem layer is another layer called the xylem that transports water and mineral nutrients from the roots and soil up to the leafy parts of the tree. Beneath the xylem is another xylem layer called the secondary xylem. These xylem layers are thicker and deeper into the wood of the tree than the phloem layer. Lying on top of these layers just under the bark is a layer of actively dividing cells called the cambium.

The Air Layering Process

In the process of airlayering, the bark, the cambium, and the phloem layer are removed by cutting away about a 1 inch wide ring of these tissues from around the circumference of the shoot. The xylem however is left intact. This is known as girdling. Generally, synthetic auxins (in a vehicle of talc powder or by liquid) are applied to the site where the tissues have been removed. (Although applying auxin is the general practice today it is not necessary for many trees). Wet sphagnum moss (or another moisture retentive soil) is then bunched around and over this girdled site and covered with plastic and sealed.

What Happens at the Air Layer Site

The removal of the bark, cambium, and phloem, but not the xylem, prevents carbohydrates and photosynthates from flowing down the trunk past the girdling site but still allows water and mineral nutrients to flow upward to the leaves. This keeps the leafy portions of the shoot from drying out and maintains them with an adequate supply of nutrients. The removal of the actively growing cambium layer prevents the regeneration of phloem and healing over of the wound. Because of this the carbohydrates and photosynthates flowing down the trunk collect at the girdling site. The presence of these excesses of carbohydrates and photosynthates (esp. auxin) at the girdling site, plus the presence of the water in the sphagnum moss, causes dormant adventitious buds in the area to grow into roots. When there are enough roots to sustain the shoot independently the shoot is cut off of the tree and then planted or potted.

The Difference Between Air Layers and Cuttings

The propagation of plants by cuttings occurs by the same principles and has very similar circumstances. The difference is that the shoot is removed from plant at the start and water and nutrients flow up the shoot from the cut site by capillary action instead. This kind of propagation can only be done with small and thin shoots since the flow of water is insufficient for larger branches. Airlayering solves this problem and allows the creation of new plants from very large parts of trees.

Crabapples for Bonsai

by Brent Walston

Introduction

Flowering Crabapples, Malus species, and cultivars are one of the best flowering trees for bonsai. They are covered with beautiful flowers in early Spring before the foliage appears and little green apples appear in Summer, ripening to all different colors in the Fall when the foliage begins to turn to shades of yellow, orange and red. Winter brings a nice twiggy skeletal outline punctuated by the persistent fruit. New cultivars are constantly being developed giving us an ever increasing selection of flower color, fruit color and size. They are easy to grow and quite pest and disease resistant.

Cuttings versus Grafts

The problem of using Crabs for bonsai up to now has been the difficulty in obtaining good low grafts. This has been solved by growing Crabs for bonsai from rooted cuttings. There are only a few people doing it, but cutting grown crabs can be obtained from several mail order nurseries now. Grafted crabs suffer from unattractive and often mismatched unions making an imperfect trunk line. You can try burying the understock an inch at a time by making vertical slices in the bark and applying hormone, then covering the area with good bonsai soil to try to move the root zone upward. You may also try air layering. Budded crabs have about eight inches of straight understock trunk with the scion making a right turn at the union.
Additionally there is the problem of understock suckering. Whenever the top of the plant is pruned, the understock is very often stimulated more than the scion resulting in an explosion of sucker growth. This makes large trunk crabs very difficult to train.

Cutting Propagation of Cultivars

Most of the cultivars are quite easy to propagate, but the conditions are very specific. Unlike other members of the Malus genus, Crabs do not usually root from hardwood cuttings. They must be propagated from semi-hardwood cuttings taken from June through about early August. For some reason talc based hormones work better than solution formulations. The usual hormone strength (IBA ) is 0.8%. If you have a place at home that stays shady, cool, and damp you might try some. Take 6 inch cuttings, remove most of the leaves by cutting off about 3/4 of each one, dip in hormone and place in a clean flat of 4 parts perlite to one part peat and loosely cover with plastic. Do not let them dry out. They will root in about 4 weeks.
Unfortunately the cultivars of Malus coronaria do not easily root from cuttings. They callous nicely, but I have never rooted a single cutting. I particularly like this species because the flowers are very fragrant. One cultivar, 'Klehm's Bechtel Improved' is extremely fragrant, the sweet but not cloying perfume can travel many feet from the tree itself. This is my personal test for fragrance, if I have to stick my nose in to smell it, I don't consider it to be very fragrant. 'Bechtel' is also one of the few truly double flowered Crabs. The pink buds look like little roses. It also blooms much later than other Crabs, waiting until May when the foliage has already covered the tree. I am currently low grafting this cultivar.
Another interesting aspect of growing Crabs from cuttings is that the plants are much more vigorous. This isn't really noticed until you put them into the ground to fatten them up. I find that the cutting grown specimen grow 2 or 3 times as fast as the grafted ones.

Training Young Material

Once you have the trunk development you desire, then it is time to put them in bonsai pots.
If you put a pencil thick cutting in a bonsai pot you, you will have to wait many years to get a one inch trunk tree. I begin training cuttings immediately. They grow very fast in training pots, and require pruning only weeks after being potted up. As soon as the roots fill the little plastic pots, I cut back the leader to about three inches, leaving a section of close internodes if possible. This facilitates growing and choosing low branches later on. Multiple trunk cuts will also make nice soft, or sometimes radical bends in the trunk. This process is repeated, adding a few inches of trunk each time. Some taper will result from this, but not a lot. You do get nice crooked trunks without sacrificing much growth. The wind blows that pots over anyhow if I don't prune them down.
By the time I get them in four inch pots I have some pretty nice looking plants suitable for shohin, but the leaves are really a little large for Shohin with the exception of perhaps M. sargentii or M. x zumi cvs. This takes only two to three years. Next they go into one gallon cans for more development. One inch trunks can be obtained in about three to four years from the original cutting. The trunks will have nice little crooks and bends in them, and the plants should be flowering regularly, with little fruits following.

Developing Trunk and Taper

It is difficult to get good taper on Crabs despite the fact that they are relatively fast growing. They always want to be cylinders, albeit fat cylinders as they get older. The way around this problem is to reduce the whips to within about 6 inches after letting them grow completely wild for 2 or 3 years. This can be in large containers or in the ground. The plant will respond by throwing many branches below the cut in all directions that eventually bend back upward. Let all these grow wild until winter. Then head them all back to within several inches of the trunk, but preserve a new leader from the top of the plant.
Repeat this process as long as you like. What happens is that the multitude of low branches will form an enormous lower trunk. These lower branches can be removed (they will be too large for bonsai ) when the tree is dug up and new scaffolding branches trained once it is placed in a smaller training pot. Trees in the ground will develop 4 to 6 inch trunks in 5 years with this method.
For an in depth discussion on growing larger trunks go to the article Developing Large Trunks for Bonsai.
Once potted for bonsai it is time for branch development. Branches should already be in place before potting except for possibly the small branches in the apex. I let them grow wild after potting up. After they begin to fill the pot, which only takes a few months, I cut back the long extensions that have grown on my selected branches and begin pinching them regularly to get nice close internodes the rest of the way out the branch. If a branch needs more caliper you can let it grow longer for a while. Branches in the top of the tree have to be watched very closely because most of the growth is directed there and they will soon be larger than the lower branches if you don't pinch them back regularly.
Most crabs will need repotting every year due to their fast growth rate. Larger bonsai may need it every other year or so. If they begin to push out of the pot or decline, it is time to repot. You can repot all season long if you are careful and an experienced bonsai enthusiast. Beginners should stick to spring and fall.
It is never possible to get very much ramification on crabs so don't worry about it, if you can get four or five sets of forked joints on a branch you will have a very nice tree. They can be defoliated in spring for smaller leaves and tighter internodes.

Pests and Diseases

Crown gall is sometimes a 'problem'. This is a parasite that makes warty nodules on the surface roots and crown of the plant. It seems to cause no adverse symptoms, and some of my trees have lived with it for years. The wonderful thing about it is that it creates spectacular nebari. I have one plant in training that has about a two inch trunk, but the gall covers about ten inches of root surface, which will nearly fill the pot when it is finished.
All sorts of bugs and critters love to eat apple leaves so you will have to deal with all of them, including deer. Snails and slugs put big holes in the leaves. Caterpillars of various sorts will roll up in the leaves. They are the first plant in the spring to get aphids. Mites will move from their favorite (Quince) to crabs when the pressure gets heavy. Woolly aphids are serious pests and hard to eradicate since they will live underground on the roots as well as the upper portions of the plant. They can be controlled with a good systemic insecticide.
Diseases are less of a problem, the most common being powdery mildew which is easily controlled with Funginex or simply giving them more sun and keeping the foliage dry at night. Cedar Apple Rust is a serious problem in many areas of the country. It can be mitigated by the use of fungicides and by keeping junipers out of the immediately vicinity of your apples.

Culture

Crabs are greedy little plants and require regular feeding and high calcium levels for lots of flowers and fruit. Stop the nitrogen in about August or September for better flowers. Regular, thorough watering is a must. They suffer little from root rot and can be placed in a somewhat denser soil mix to ease the watering requirement in summer. Full sun in most areas is best. In the very hottest, driest areas, full morning sun and afternoon shade may be necessary to keep them from drying out. They require a period of winter chill to set flowers, but this is easily obtained in most areas, other than perhaps the deep south.

Cultivars for Bonsai

The best cultivars for bonsai are the smaller leafed ones that develop some degree of taper on their own and are inclined to produce low twiggy branching no matter what the top of the tree is doing. These include 'Hopa', 'Hime Ringo', 'Mary Potter', sargentii and zumi cvs. Good plants for fruit are 'Snowdrift' which has a profusion of small yellow persistent fruit, and 'Sugar Tyme'. These are some of my favorites, but virtually any crab can be used.
A cultivar new to us is 'Louisa', a true weeper, the branches hang practically straight down. The buds are deep pink, opening to a beautiful true pink. The fruits are gold yellow with a blush of red and a little less than half an inch. It is a newer cultivar and very disease resistant. Father Fiala (Flowering Crabapples) proclaims it to be "An outstanding crabapple". I don't know if it will be possible to propagate it from cuttings, this will have to wait until next year. This one is quite exciting. The arc of the weeping branches is naturally about six to eight inches so that in a larger bonsai wiring the branches down would not even be necessary.
Another new cultivar for us this year is 'Weeping Candied Apple', it has reddish leaves and good red new growth with dark pink-purple flowers and small red fruit. It is an aggressive grower and very easy to propagate. It's weeping character will not be evident in bonsai, although it's branches could be wired down.
'Hime Ringo' has double light pink flowers, but there is some trouble with the name. I have not been able to trace the name or lineage to a species, and I am afraid that this is another example of Japanese naming by 'group' or 'type' rather than species. I have my doubts as to whether all the plants being sold in this country as 'Hime Ringo' are in fact the same cultivar. The clone we are producing is an excellent subject for bonsai. It produces many low angular branches with ease.
Really good red forms are 'Lizet' and 'Royalty'. Unfortunately both of these have been really difficult for me to propagate from cuttings. I may have to get this one from tissue culture.
The smallest leaf form that I have been able to find is M. sargentii. These have really small lobed leaves, almost like oak leaves. They reduce nicely to about an inch. The species grows to about ten feet and is very wide spreading rather than upright, it has tremendous bonsai potential. The buds are pink, opening to almost pure white. The cultivar M. s. 'Tina' is a superior selection that has very dark pink buds at the same time as other buds have opened to pure white flowers for a thrilling display of color in the spring. This should be one of the finest cultivars of all crabs for bonsai, although it will be rather slow growing.
'Sugar Tyme' has the most fantastic small fruit. They are red and persistant. Here they usually last until after the first of the new year. As the fruit ages, it takes on a reddish fluorescence that is an incredible sight on a cloudy day with the backdrop of dark wet branches. Not only that, but it sets fruit in huge clusters of fifty in a spur. I have taken several hundred fruit from a single small tree. It has pinkish white flowers in the spring, medium sized leaves and is not overly aggressive.

And finally

I recommend that any of the standard apples be considered for bonsai, although most will have to be used for three to four foot bonsai to make them work. I love the 'Moon through the branches' style where one apple is left on the small bonsai. In fact the sight of such a pear specimen many years ago was instrumental in my voyage toward bonsai from landscape work.

Beginner's Page: What Are Bonsai?

by Brent Walston

Click on the highlighted words for more information in that area

The simplest definition for bonsai is a plant in a pot. But bonsai is so much more than a tree treated like a house plant. It is plant art, or sculpture to be more accurate. Bonsai require skilled care to survive and to blossom into magnificent specimen plants.
Many nurseries and vendors sell inexpensive plants in pots and call them bonsai. With care they could become bonsai, but they are not bonsai. On the internet we have the adopted the term 'Mall Bonsai' or 'mallsai' for these plants. Some of these people are true bonsai folk and do their best to see that your plant survives by giving you at least basic care instructions. Sadly, other vendors could care less. We recommend that you do not purchase bonsai from persons unwilling or unable to give you care information.
Bonsai are not houseplants. For the most part they are outdoor plants and must remain outdoors all year long. This is because most bonsai are temperate climate plants that require a period of dormancy. This is true for evergreens such as juniper as well. There are also indoor bonsai, but these are usually limited to tropical and subtropical species, and even these species are happier outside in the summer. Since the roots of outdoor bonsai are exposed, they must be carefully over wintered in cold climates to prevent the roots from reaching killing temperatures.
Little bonsai do not become big bonsai. They are grown under training conditions until they reach the desired size and trunk thickness and are only then transferred to bonsai pots. Plants grow very slowly once they are in bonsai pots, that is the object of the pot, to slow down growth to reduce leaf and twig size. Most of the inexpensive (and some not so inexpensive) mall bonsai have had little or no training whatsoever. This is why we do not consider them to be bonsai. Trees must becarefully trained to achieve the beautiful shapes most people associate with bonsai.
The training of bonsai is done mostly by manipulating the trunks and branches through pruning and wiring. Anyone can learn how to do this with a few hours of basic instruction from a teacher or by reading books. Of course, advanced skills are honed over a period of a lifetime, and the more your practice, the more you will learn.
Bonsai must be root pruned and repotted occasionally. Since the object is to keep the tree the same size, they usually go back into the same pot after root pruning. Root pruning is also a training technique and a plant may be invigorated or slowed down by timely pruning. The trees are repotted with fresh special soil designed specifically for bonsai. The most important quality of bonsai soil is that it drains very much faster than usual potting soils.
Bonsai must be fertilized regularly while actively growing. This is most easily achieved by fertilizing with a good soluble fertilizer every two weeks. Use a fertilizer with trace elements such as Miracid or Miracle Grow at the full strength recommendation. Bonsai soils are not very good at holding nutrients, so regular full strength feeding is a must.
Watering improperly is probably the most common problem for beginners. Plants must be watered thoroughly when they become slightly dry. Watering should be done from the top, not by submerging the plant, and they should be watered until they are saturated. You will see water pour from the drain holes when they receive enough. This method of watering helps to prevent salt buildup in the soil.
Determining when a plant is slightly dry can be tricky. We recommend that you lift the pot (if it is a small bonsai). There is a substantial difference in weight between a well watered bonsai and one that needs water now. You will learn this 'feel' very quickly. You can also dig your finger down about an inch into the soil. If it is dry to this level, it should be watered. Another method, recommended by Michael Persiano, is to insert a chopstick into the soil as sort of a dipstick. Leave this stick in the soil and pull it out periodically to test the water 'level'. If the soil has adequate water the stick will be damp.
Most people in hot summer areas find that watering is necessary every day when the temperature is above 80F. Sometimes, watering is necessary twice a day. In general, indoor plants require less frequent watering than outdoor plants.

Understanding Pruning

by Brent Walston

Introduction

An IBC'er asks:
"I want to have a better idea as to how a particular pruning action will affect the tree's development as well as how to use these predictors in styling."
And Brent answers:
The first thing you have to do is learn how trees grow. Read the article Principles of Plant Growth. It is drastically oversimplified, but it will get you going in the right direction. Then get a college text on Plant Physiology. I don't think it matters a whole lot which one, but wade through it. You might only get through one page at a time on the crapper in the morning, that's what I do, but read it at a rate that allows you to understand and apply what you are learning to actual growth in plants. There is another excellent article on plant physiology that all hort folk should read, although it is in a very tiresome style: Plants as a System The key to understanding pruning is to understand plants as a system. That is the beauty of the crown pruning article. Removing a bud or a stem with buds on a plant does a whole lot more than just improve the looks of the plant. It makes changes all down the stem and right to the roots. Timing is also very important. You can shorten or lengthen resultant internode growth just by pruning at different times of the year. Much of this is explained in my Principles article.

Removing Food Storage

Removing root storage in winter will have the opposite effect. LESS food will find its way into a full complement of buds, causing shorter internode length and smaller leaves. This is what happens in dormant root pruning and repotting. Removing roots during the growing season doesn't significantly upset the food balance, but it does upset the water uptake/ transpiration balance and thus must be accompanied by top pruning, foliage reduction, or environmental change to keep from stressing the plant.

Hormones and Pathways

Hormone changes are no less important but not quite as obvious as food and water balance. The two most important for us are cytokinens, produced by the roots, and auxins, produced by the leaves and buds. These two hormones are in constant communication via the plants vascular pathways. Woody plants typically show strong growth at the branch tip (terminal bud) and the root tip. Strong terminal buds or terminal shoot growth (early in the season) produce a strong auxin signal that does two things. It suppresses bud break at all the buds behind it on the branch and stem. It also travels down the pathways to the root tip where it serves as a powerful growth regulator for the root tip. There it is destroyed. The strongly growing root tip produces cytokinen which follows the same pathway back to the terminal bud or shoot where it serves as a strong growth regulator. As you can see, this is a self reinforcing cycle. Unaltered, this cycle produces a plant that grows strongly at the branch tips and at the ends of the roots.

Ramifications of this Cycle

Now for the ramifications of this cycle, especially if it is altered. First, unaltered, this cycle favors the terminal shoots and suppresses inner and lower growth. This is why trees lose inner and lower branches as they grow. As leaves photosynthesize, they feed themselves, their shoot and the local stem along the pathway. Excess production goes to the root. As long as this food and the auxin go to the root, this pathway will remain open and enhanced. Lower and inner leaves are shaded and produce less food and auxin. As long as the food production is in balance, the leaves will remain, new buds will be set, but shoot growth will be limited or absent. When production falls below balance, the auxin signal falls off and the roots will wall off this pathway. The leaves and buds along this pathway will eventually die from lack of water and nutrients from the roots.

Disrupting the Pathway

>Now, alter the pathway. If you prune out the terminal bud and growth during the growing season, you do two things. You remove the food and auxin along this pathway to the roots. The response by the roots will be to wall off those pathways. Simultaneously, you are removing the strong auxin signal that has been keeping the buds behind the terminal bud suppressed. These buds are now released. They begin to grow, produce food and auxin, and the roots enhance the new pathway. Now we have secondary bud break. Ordinarily, only the bud following the terminal bud will break, but sometimes one, two or three other lower buds in the line will also break. But the first bud left at the end of the stem will feel the effect first and break first. It gets a jump on the other buds and soon begins to produce auxin just like the removed terminal bud and the suppression will begin again. Thus we have a limited number of buds breaking.
>The strongest response to terminal bud removal will occur in winter because there will be no bud break (and subsequent suppression) until soil temperatures begin to warm up and the roots begin to grow, signaling the buds to break into leaves. Thus winter pruning will produce the most bud break by interrupting apical dominance (terminal bud suppression) for the longest period. Summer pinching of the terminal bud will ordinarily give you only a two bud break, resulting in good ramification. So winter is the time for most heaving pruning, spring is the time for ramification pruning.

Plants Shift Gears in Summer

>Late summer is the time for no pruning. Mid to late summer is the time of the year that the plant changes gears. It has spent all spring growing leaves and stems. The terminal buds have continued to grow into new shoots each time they formed with little delay. This will often give you three or four internodes in a single season. The health of the plant, and the length of the season affect how much growth you get. By about mid season or a little later, a normally growing plant will stop opening new terminal buds. It retains its leaves and continues with food production, but in fact it is not growing (breaking buds or extending shoots). Now the cycle shifts back to the roots, which have spent all season busily pumping up water and nutrients to the top and only growing in response to the expanding shoots. As the terminal buds form or set as in bud set) there is an increased auxin signal to the roots, and the food which had been fueling shoot growth is pumped instead to the roots. This is a time of explosive root growth, and it will continue into the fall until soil temperatures begin to fall below about 50 to 60F. Even as the leaves begin to stop photosynthesizing in the fall, carbohydrate withdraw from the leaves and stems continues and the food is shifted to storage in the roots. The terminal buds retainenough food so that they only need water in the spring to break into leaf (a very handy little tidbit and another story about Bareroot Pruning).

Fall Root Growth and Pruning

This fall root growth is valuable knowledge for late summer and fall repotting (with some very important provisos, see the article on Fall Repotting). The phenomenon of bud set is also important to understand, because it is an important step in the process of developing a dormant state. When the terminal bud sets, it sends a very strong auxin signal; there is almost no chance for any new shoot growth anywhere on the plant. That's good because winter is coming and new growth would be burned off by early frost. You can interrupt bud set by removing the terminal bud by pruning. Thus if you prune in late summer or early fall, you can get new shoot growth from the newly released remaining buds. This is why nitrogen feeding in late summer or fall gets such a bad rap. It isn't the nitrogen (in most cases) that is fueling the new growth, it is the pruning. Nitrogen plays little role in releasing buds, but it plays a strong role in the expansion of new shoot growth. The common knowledge of the importance of feeding with 0-10-10 is a myth, although it does little harm.

And finally

These are the basic tools you need to analyze for yourself how, when, and where to prune. Take your time, it will take you several years to digest this experientially. Istill have to think about it when I prune, that is, all the ramifications of what I am really doing. But that is changing, and I am beginning to incorporate these principles into my psyche so that I don't really have to analyze, but rather intuitively know what to do. This is the horticultural counterpart to the artistic principles. We can teach you about balance, proportion, the golden section, (and physiology), but you won't really be able to do decent bonsai until you own these principles and no longer have to think about them while working.

Root Pruning Bare Root Seedlings

by Brent Walston

Introduction

This article was an Internet Bonsai discussion between Brent Walston and Rick Choate. Rick is asking follow up questions about a previous article Brent had written on root pruning field grown bare root seedlings.
Brent had described his practice of removing the long tap root that these seedlings have and placing them in small pots about three inches deep. The tops were left unpruned even though some were almost three feet tall. The object was to get a good shallow root system for bonsai very quickly. The seedlings were then placed in light shade for the rest of the season.
Rick
I remember this good information you previously posted but don't remember why I didn't ask some questions at the time. Before going on with my questions, I think it appropriate to emphasize this is dealing, at least initially, with trees during their dormant period, isn't that right?
Brent
Yes, that is correct, fully dormant deciduous material only. Attempts to do this with conifers has had mixed results.
Rick
[You previously wrote] I cut off all the tap root leaving only 3 inches below the crown (that part where the trunk of the tree met the earth). I do this even if it leaves only a stub with no side roots. I plant these whacked trees in 3 inch pots with high quality well drained soilless mix. I leave all of the top.
I understand the small pot, you want to fill the pot with roots as soon as possible. However, I'm not clear on the advantages of leaving all of the top.
Brent:
The advantage of leaving all the top is that you let the roots determine how many buds can be sustained rather than just guessing and whacking some top material off at random. If you guess wrong and remove too much top growth you are robbing the plant of potential photosynthesizing leaves. Replacing these leaves would come at the cost of growing new shoots and buds. This would be a net loss to the plant at the start. By leaving all of the top, the roots will only open as many buds (which contain their own food) as can be sustained, probably through how much food and water can be pumped up the xylem by the diminished root system. The portion of the stem at the top that cannot receive food and water dies. Leaves open at the other bud sites, no new shoot growth is necessary to start food production. The cost to the diminished roots is minimal. Usually no new shoot growth will begin until the roots are regenerated.
Rick
[You also previously wrote] The reduced root system will limit the amount of leaves that will bud out and the size of the leaves, but you will get the maximum leaf surface possible and thus the fastest recovery and new root growth possible. Let them grow undisturbed until the roots completely fill the pot, it may take the entire season. If it does, then they may be pruned down to the first desired curve in the trunk the following winter when you normally do your winter pruning. I cut mine down to 4 to 6 inches. The following spring you will have a beautiful fat trunked little tree that can be grown out or treated as small bonsai. If the roots fill the pot by July, you may cut it then and you will get the same result a season earlier, although this a a little riskier, and depends on your zone and over wintering care.
I know you generally style your trunks by clip and grow, but have you ever wired one at the time of potting. I try to get my bare root stock in during the fall so (at least according to Andy [Walsh]) that would be a good time to wire, if I chose to do so. Also, let me make sure I understand something. I would suspect that in my climate the roots would fill this pot within a few months. I am assuming that when you say pruning the trunk in the summer is a little riskier it is because in more northern climates the new growth may not harden before winter, is that correct?
Brent
With smaller material, wiring the trunk should be no problem. There is no physiological problem of which I am aware. Much of the material I get is quite large, three quarter to one inch trunk, two to three feet tall and quite stiff, so mechanical manipulation would usually be difficult, especially with no soil as an anchor. Of course as you have pointed out my trees are cut down to four to six inches anyhow so there is little that wire would do. Wiring would be more useful if you were not going to cut your trees as far back as I do.
Pruning the tops down in summer is riskier for two reasons. One is that if you live in a cold winter area there may be insufficient time for the new growth to harden off. That is not a problem here in Northern California. The second reason is that by pruning them back to sticks again (the foliage is almost always above the point at which I prune) you ask the roots to use their stored food that you just worked so hard to achieve to move back up again and fuel a new burst of foliage. This depletes the root system again. There must be sufficient time before the end of the season for new growth to once again to pump up the roots.
I find that this manipulation of root (tissue) stored food and food making leaves to be extremely valuable in planning the growth of bonsai. It is actually a very simple concept with considerable ramifications, but few people seem to think in these terms.
I just finished 'topping' the field grown seedlings that went through this process this spring. The roots were a little lighter that I would have liked. I think a little heavier fertilizer and more light once they started growing would have been in order. The losses were surprisingly low, less than five percent. Most of the buds had opened almost to the tops of all the stems with small leaves and no shoot growth, so this strategy really seems to have worked. If I had pruned almost any amount of material off the top when potting them, I would have taxed the roots with making new shoot and leaf growth or robbed them of potential leaves from existing buds.
Rick:
[You also wrote] I have compared this method to the normal practice of pruning the top and the roots and have found it to be superior, producing a better root system in a shorter period of time.
Since, from previous posts, I know you care abut developing a good nebari, as do I, when do you start working on developing it, the next repotting season?
Brent
That is correct. After root pruning the long tap root, there is usually nothing left but a stub, no side roots whatsoever. The following spring there will be many large fleshy roots radiating from the cut end of the tap root. And they do radiate beautifully. Since I jam the end of the cut tap root into the bottom of the pot to get as much soil covered root tissue as possible, it is easy to pop them out of the pot and look at the new root growth. There is almost always a perfect ring of radiating thick roots. They are still a little fragile the following spring so you must proceed with caution, and it probably best to straighten them out but leave them all until the following season.
Rick
And lastly, do you know why this method seems to work better than the technique of pruning the top as the same time as the roots? Do you think it allows the tree to focus its energy toward 'healing' one area at a time (as Andy [Walsh] suggested in his post on heated trees)?
Brent
Pruning the top will result in a net energy (food) loss to the roots in the short run by either forcing roots to fuel expansion of new shoots and buds to increase production or by indiscriminately limiting the leaf surface area without regard to how many leaves the remaining roots can sustain.

Intact Rootball Vs. Rootbound

by Brent Walston

edited by Robert Potts

Introduction

A continued discussion of healthy roots by Brent Walston, taken from postings on Bonsaisite Forums.
RP

What's an intact rootball?

An intact rootball is when you can knock the nursery can or pot off the root ball and it won't fall apart. This is sort like the advice of bending the branch to the point where it is just about to break. How in the world do you know without doing it? There are several tricks. You can wiggle the stem. If it stem moves in the pot, don't try unpotting it. If the stem seems pretty solid, try the next test. Try to pick the plant up by the stem. If the surface starts to give before you can pick it up, it is too soon. If you can pick the plant and pot up by the stem, do the next test. With a surface just under to pot to catch it, knock the pot off the roots. Don't pull it off. The proper procedure is to hold the stem in one hand and give the rim of the pot a sharp rap with the palm of your other hand. If the pot drops off cleanly and the rootball doesn't fall apart, you can pull it and inspect it. If the pot falls to the surface and the root ball collapses back into the pot, it's too soon. That's why you want something just under the bottom of the pot.

Why inspect the rootball?

People don't inspect rootballs nearly enough. Hardly ever do I read a post asking for help where the pot has been knocked of and the roots inspected, even though this is the most revealing test you can perform. I do it all the time for healthy and sick plants just to see what is going on. A healthy growing plant will have a nice intact rootball with lots of lovely white growing root tips.

Rootbound?

At what point do plants stop growing in a pot? It's a bit involved, but the simple answer is that new growth stops or slows when they become rootbound. So what's rootbound? Rootbound is when there is no effective space for new roots to occupy. Roots effectively occupy the entire volume of space between the soil particles. One of the first symptoms of being rootbound is, in fact, that plant growth slows despite favorable environmental conditions (light, water, fertilizer, etc). The second symptom is that rootbound plants begin having difficulty taking up fertilizer. This is undoubtedly related to the inability to form new root tissue. You see this as a chlorosis despite the fact that they have been properly fertilized.
People often confuse leaves with new growth. New growth is the process of continually opening the terminal bud of a stem (shoot formation). Plants can be, and frequently are, alive and relatively healthy with absolutely no new growth. This happens when plants are severely rootbound, there is a lack of fertilizer, or after a trauma such as barerooting. The existing buds will open, leaves will form, but no shoots will develop. This condition will persist until conditions change. I have seen many plants survive year after year without shoot growth. New growth each year consists of a succession of opening terminal and axillary buds in the spring without any shoots to form an internode. If you look closely at the stems there are just a pile of leaf bundle scars piled up on one another. Talk about close internodes!
Rootbound plants need to be rootpruned and shifted at the nearest appropriate opportunity. This usually means winter, because unlike shifting an intact rootball, rootbound plants must be rootpruned to initiate proper root growth.
It is difficult to tell when a plant is rootbound just by observing the roots. I think it is better to determine 'rootbound' by both the symptoms of growth (or lack thereof) and the physical density of the roots. For our purposes (bonsai), trees should be rootpruned and repotted long before they reach rootbound conditions. This doesn't happen overnight. There is a long gradual procession of slowing growth over time, usually several years before all new growth stops. It is clearly evident what is happening if you stop to look.

Root growth patterns are species dependent

Some species quickly occupy the soil mass uniformly (Buxus). And yes, there are species that love to occupy the bottom of the pot with roots, but not the top,Cedrus and Quercus come to mind. But given enough time, both of these genera manage to occupy the entire soil mass, albeit over many years. While doing this, shoot growth is present, but obviously slowed. Growth of this kind presents a problem when root pruning and repotting because you often don't really know where the root crown is. It is very easy to buy a rootbound nursery plant, slice off the bottom portion of the roots with a saw or axe, and then find out you just cut off the bottom of the trunk. You have to proceed slowly and carefully when root pruning rootbound trees. It is difficult and arduous.

And finally

In this, as well as most of the other articles at this website, I have tried to point out that container growing is a system. Rarely can you change one condition without changing others or changing the growth dynamics. At first it may seem like a daunting task to understand the interrelationships, but it is a necessary to learn these processes to be able to successfully manipulate plant growth.
Related Articles: Overpotting and Root Pruning

Overpotting

by Brent Walston

edited by Robert Potts

Introduction

Overpotting a bonsai tree into a container that is too large is a mistake that many bonsaists make unknowingly. The following is an explanation of why this is a problem. It is taken from several posts on the Bonsaisite Forums, by Brent Walston.
RP

Let's start with the physics

Water will drain from a pot until the lowest level of saturated soil (that can be supported) is reached. At this point drainage stops and this saturated layer remains saturated, no more water will drain out, ever. The height of this column of soil depends on the nature of the mix. A coarser soil will have a lower (shallower) column or layer of saturated soil than a finer mix. The total retained amount of water is less for a coarser soil.
Water can be removed from this saturated layer in two ways: evaporation (the water will be wicked upward as water evaporates from the surface), or by the absorption of water by the roots (powered by foliage transpiration). Of these two, removal by transpiration is by far the most effective. To prove this to yourself, just place two pots of identical soil next to each other, one with an established plant in it, the other with no plant. Water them thoroughly and then compare the weight of the pots over the period of one hot summer day.
If the plant is not root established, it cannot remove very much water by transpiration. This leaves too much water in the parts of soil without roots. In the short run, this is not much of a problem. In a proper environment, the plant will grow and will root establish quickly so that the saturated level is wicked dry in a day or two after a few weeks or months of growth.
However, if the pot is so large that the saturated level cannot be removed by normal root colonization, problems begin. This is not dependent on the soil type. With coarse soils a larger pot could be tolerated, but there are still limits to the space that can be quickly root colonized.

What happens if the limits are exceeded?

If you are using an organic amendment such as bark, you will experience accelerated soil composting. This means that you will lose your effective soil particle size more quickly than if you used a smaller pot which is wicked dry daily. This is the most common effect. You use a pot that is too large and it stays too wet. The organic amendment quickly decays in this wet environment, particle size decreases, soil collapses, the saturated level increases, even more water is retained, roots eventually remain in standing water, root failure occurs with, or without, the presence of a pathogen. Using only stable inorganic amendments would avoid this scenario, but there are other problems.
Even if the above doesn't occur, what kind of root growth occurs in a volume that is not wicked dry daily? When you water properly, a new charge of air is pulled into the pot by the volume of water draining from the drain holes. Carbon dioxide and other gases are purged from the soil. The longer you leave these gases in the soil, and the longer you wait to introduce a fresh charge of oxygen, the poorer the roots will be. If you create a situation such as overpotting that doesn't require daily watering, then you don't obtain an optimal soil growing environment.

And finally

The best environment is a soil that dries out daily. The best potting practice is to shift to the next larger size pot after each time the plant becomes root established as evidenced by forming an intact rootball. UC Davis studies have shown this, and I have conducted my own studies with Acer palmatum, which have verified it to my own satisfaction. It is not a marginal effect; the resulting growth improvement is significant.