Nursery Shopping for Bonsai Material

by Brent Walston

Introduction

I used to go to nurseries all the time in search of material for bonsai. Now I just go out to the growing grounds and pick up whatever strikes my fancy. I could never completely work it all by myself in a hundred years. For most of you, nursery shopping for potential bonsai material will be the way that you acquire your collection. The following article offers some guidelines for effective shopping, or what to look for.

First, don't be afraid to get dirt on your hands and knees. I can always tell the real aficionados by how much dirt they get on their knees grubbing around the surface of the pot looking for a good nebari or trunk characteristic. The foliage and the branches are of our little importance, in most cases they will be discarded anyhow.
Look for a good nebari, the crown and surface roots of the plant. This is the single hardest element to obtain, if a tree has a good nebari and nothing else to offer, buy it, grow a trunk, then grow branches. Nobody said this was going to happen overnight. Look for a radical swelling at the base that soars into a tapered trunk. Look for surface roots that smoothly merge into this crown. The roots should come out radially. Circular roots can be problem and usually will have to be discarded.
If a tree lacks a good nebari, it still may offer other qualities too good to pass up. The second element to look for is a good trunk. Traditional 'masculine' trees will have thick trunks and mature bark. The more taper you can get the better. A thick trunk with no taper can be dealt with, but one with taper is better. The usual rule for such trees is that the height of the tree will be six times the diameter of the trunk at the base. So already you must form some sort of picture in your head of what the final tree will look like. At least picture how tall it will be with this size trunk. Will this work? Is there a branch that can be bent upward for an apex? Can the top be broken and a jin apex carved? Will the tree need to be grown out some more before it has sufficient trunk or height? Trees with smaller trunk proportions are perfectly acceptable, they usually have a more feminine appearance, softer and more sinewy. It just depends what you want and what the tree has to offer.
If you are looking for immediate gratification, or pretty quick bonsai, you must analyze the branching. If this thing is going in a bonsai pot soon, under one year, then some existing branches must be used. The first branch is usually one third the height of the tree. Does the tree have such a branch? It should be one third or less than the diameter of the trunk at the point at which it is attached, or it will be too fat. Skinny branches can be grown out, fat branches are a serious problem and can only be solved over a number of years by growing out the trunk. Choosing or finding a second branch must also include a decision on the front and back of the tree. Usually the first two branches are at the left and right of the tree and slightly toward the front, they make an angle of somewhere around 120 degrees. The third or back branch is usually 120 degrees from the first or second branch but not directly behind the tree. The back branch may also be the second branch but only rarely the first. The other branches can usually be grown out later.
PATIENCE! Ok, you have analyzed the tree and it has good possibilities, buy it and take it home. If it is not spring with the sap running, you can go ahead and do some styling, remove unwanted branches, wire and bend others, remove some surface soil to expose the nebari, reduce the top. In general, have a good time. If it is an evergreen do not remove more than one half the foliage while playing with it. Slap your fingers if they itch to remove just one more little branch.
If it is deciduous and it is dormant, have at it. If it a species that buds back nicely, and most do, you can work it down to the trunk alone. Check with someone to make sure the species will support this. Beech, for instance, usually will not. If it is the growing season but not early spring you can easily remove half or more of the foliage with your manipulations. If it is less than six weeks until the end of the season wait until it goes dormant or you risk throwing new growth that will not have time to harden off.
What about the roots? My advise is to leave them alone. This is where beginners get into trouble. They work a plant too much, too soon, and it cannot support all the changes. Do as much top work as you can and leave it in the existing can or put it in a larger can without disturbing the roots if it needs to grow out some more. Wait for the next opportunity to do root work. You can continue to clip and trim and wire branches and have a good time watching your little tree progress. It will be very easy to care for because it has excess root capacity compared to the canopy. It will not dry out easily and it will be easy to water and fertilize.
If you worked on the top during the winter wait at least until the following fall to do root work. If you worked the top after the spring flush you can work the roots in the fall, but it would be safer to wait until the following late winter or early spring. If you work on the top during the summer wait until the following fall. In other words give the plant an entire growth cycle before root pruning. I have used this formula many times and it has rarely failed me. Every time I lose a tree it is usually because I don't follow my own rules and do too much too fast. Learn to enjoy the tree in the nursery pot. I know the fascination for beginners is to get the tree in a little pot, but just having a tree in a pot is not bonsai.I have thousands of little trees in nursery pots and I enjoy them every bit as much as the ones in the bonsai pots.
Now that I have given you all the things to look for in nursery plants, I will try to make specific suggestions for what to do when you get to the nursery.
Take a plant species description book with you, do not rely on what the sales help tell you, unless you have dealt with this nursery before, and you know for a fact that they have honest, knowledgeable help. In the west any nursery worth its salt will have a copy of Sunset's Western Garden Book right on the counter for you to use, but it is better to take you own so you have it with you when you are actually looking at the plant.
If you live in the west you can use the maps in the front of 'Sunset' to find out what zone you live in to make proper plant choices according to cold hardiness and other factors. The 'Sunset' system is far to superior to USDA zones, and they do not correspond. Sunset's maps are much more detailed and consider far more information than the blanket USDA zones. If you don't live in the west take some time to read through this section to find the zone that most closely matches your area and use this number to aid in plant selection. Since the book includes zones from San Diego to the mountains of Washington almost the entire country should be able to find a comparable zone except the deep south and the upper Midwest.
Armed with this kind of information about plant material, you can make intelligent choices about the cold hardiness, watering requirements, and growth habits of any particular plant that strikes your fancy.
Since this is mostly for beginners, I suggest that you look for deciduous material, or hardier evergreens such as juniper. Stay away from pines unless you have several years of experience under your belt. Some may disagree with me, but I don't consider pines other than Pinus mugo to be beginner plants. Most pines in nurseries are already wrecked anyhow for bonsai.
For the most part stay with one gallon material since this will be inexpensive and no big deal if it succumbs to over ambitiousness. Also stay away from grafted material unless you know what to look for in grafts and the pitfalls of high grafts, ugly grafts, mismatches, etc (See the article on grafting for bonsai). Go for seedling material and cuttings. This will also be the least expensive material.
Bargain basement areas are often a great resource for bonsai shoppers. Here you will find the large trunked root bound material at good prices already dwarfed for you. However, pass up stuff that looks like it is on its last legs unless you have some experience. Take rootbound material home and immediately pot it up to the next larger size regardless of what size the final bonsai will be. This will insure the survival of the plant and invigorate the top, so you can do some work on it the following season. Do not overwork root bound material because it has no reserves, you must reinvigorate the plant first.
Look for plants with small leaves and twiggy branching, they will make the best bonsai candidates. In general stay away from plants with compound leaves. These are plants that have leaves that are usually large with many small leaflets. These plants will usually not ramify (create small twiggy branches).
If you want evergreens, stay with tough plants like juniper and cedar, if you are in an appropriate area. Broad leaf evergreens may or may not be easy, you will have to consult your text.
Deciduous material such as maples, hornbeam, hackberry, elm, hawthorn, linden, Malus (apple), Prunus (plum, peach, apricot), and Liquidambar are fairly easy to work with.

And finally

Remember, the strong impulse to have that little tree in a bonsai pot will become a very boring experience if that is your only interest in bonsai. Most people have much more fun with their collections of pre-bonsai and bringing their trees along, watching them grow, planning their future, and finally potting them up as bonsai. These trees, born of love, are much more valuable than mall bonsai.

Integrated Pest Management for Bonsai

by Brent Walston

Introduction

Being a nurseryman means that, at times, I have to deal with pests and diseases. I hate dealing with them, it is the least satisfying part of my job, but it is also a fact of life in the nursery business.

What is IPM?

All the rage these days is IPM, integrated pest management. Instead of going out and periodically blasting everything in sight, IPM teaches that it is better to treat bugs and diseases with levels of increasingly invasive (toxic) techniques. The first level is cultural. Change the environment if possible to deal with the pest or disease. If you continually get powdery mildew, move your plants into the sun and keep the foliage dry at night. Aphids, mites and other sucking insects can be controlled to some degree by regular blasts of water to keep them from getting established. This level is completely non toxic and should be the first thing to do.
If cultural controls fail or are impossible to apply, the second level is to use biological controls. Use predator mites, lady bugs to control aphids, etc. BT is now available for a wide range of pests.
If biological controls fail or are nonexistent for the pest, the last level is to use chemical controls in increasing order of toxicity. The least toxic used first.

Monitoring Pests and Diseases is the Key

I practice IPM in my nursery whenever I can, and the best thing I can say about it is that it increases my awareness of the environment in the nursery. Secondarily, it has saved me lots of money by reducing the amount of pesticides that I must use. Commercial pesticides are very expensive. A single quart of Avid cost $200. A fact that was painfully brought home to me one day as I accidentally tipped over a bucket with about $20 worth.
The premise of IPM is that we never 'get rid of' anything. Pests and diseases are at best controlled, kept at minimum levels where the damage is acceptable. Pests and diseases are a fact of life, and we will always have to deal with them. The question is how will we deal with them.
The key to successful use of IPM is monitoring. Without monitoring the pest levels, one is doomed to blasting full blown infestations with the strongest stuff possible. Monitoring requires one to weekly check representative samples of the crop for pests and diseases. In my nursery this takes between one and two hours, but it is a fun job and a good excuse to get out of real work.
The essential tool for monitoring is a good 5X hand lens. Several insects are too small to see with the naked eye if you are over 40 like me. Two-spotted mites are one of my biggest problems and they are nearly invisible without the lens. I look for speckled discolored leaves, traces of yellow and red, then look at the undersides for traces of debris. A healthy leaf will be completely smooth with the exception of hairs for some species. An infected leaf will be covered with 'dirt', little bits of debris, webbing and eggs. The eggs are translucent spheres just visible even with the lens. Red spider mites are similar but are larger and a little easier to see with the naked eye and leave visible webbing. Red spider mites have a reddish color and are fairly easy to control. Two-spotted mites are translucent with two dark spots on their back and are the devil to control. Both have life cycles of about 5 to 7 days so follow up spraying is always necessary.

Knowing the Enemy and Making Changes

Fungal and other disease problems can also be monitored, with or without magnification. Early detection can save heavy spraying and foliage damage. A really good way to learn about these diseases is to monitor your plants and watch the progress of the pest or disease with the hand lens, you will see all the symptoms. Then, when you need professional help, you can provide the necessary information to get the problem identified and solved. Half the problem is knowing the enemy.
There are fungal diseases for which there is no cure, only cultural control, level one. In the case of some root rot problems cultural changes may be all that is necessary. In the process of instituting such cultural changes, I was recently forced to look at how I water and how wet the plants were staying. As a result I discovered other problems related to over wet conditions and have now acted to correct the problem by hand watering only the plants that are dry. This is the kind of thing that IPM is really good at, it forces one to observe and to act at the lowest level of toxicity, and often this is the very best solution, far better than the chemical warfare to which we have been raised to expect.

Managing a Micro-Eco System

Pesticides are not necessarily a fact of life. I think a really good example is my operation. I have a one acre growing facility where plants are lined up according to species and grown like corn. Since it is a commercial venture and not for public display, efficiency is the key word. Things are much more crowded than they should be, but I have no where else to go. As a result I have to deal with insects and diseases which thrive on this arrangement. It is a never ending battle of fungal diseases and bugs, especially mites and aphids.
On the other hand I have a retail store where things are spaced evenly and attractive, a good diversity of plants and much hand watering which has a good washing action. I stock plants at my store with plants from the growing grounds. Now I never try to intentionally infest my store with pests so I am always looking for clean stock to bring in, but in over seven years of operation I have never sprayed for mites in my store. It amazes me, but I attribute it to good cultural practice, probably some natural predators in the area, and the hand watering which tends to limit mite infestations.
This is not to say mites have never gotten in there. I find them occasionally, but they never reach levels that endanger or disfigure my plants. I find them on their favorite targets but they never seem to spread. I monitor them all the time to make sure the damage is nonexistent or minimal. There is a natural balance going on in the store that is obviously not going on in my growing grounds. The natural balance has never been upset and I don't intend to upset it by spraying unless I really have to. I think that once this symmetry of natural control and balance is upset it is extremely hard to reacquire.
This is an incredibly complex topic and I think there is room for systemic insecticides for aphids without upsetting the balance, but I also believe that natural controls for even this pest if carefully managed would obviate a need for the chemicals.
The problem is that spraying for pests will usually upset any natural balance of predator organisms which may be present. Predators are not always other insects or mites. There are beneficial fungi and bacteria as well, possibly even beneficial viruses as yet undiscovered. There are even minerals in water that can favor or diminish the pests or the predator (beneficial) organisms. Very little is known about any of this but we see it all the time. My advice is: If it ain't broke, don't fix it. If insects, mites and fungal diseases are not a serious problem leave them alone or use the least toxic method possible first. If your trees are in danger then the balance probably has already been upset and using more toxic methods will probably do little more damage. This is the essence of IPM.

And finally

I think the key word here is balance. We, as humans, seem destined to unbalance the entire earth. Can we save our selves and the environment and our little bonsai plants by becoming aware?

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.

Using Principles of Growth to Manipulate Plants for Bonsai

by Brent Walston

Introduction

This is an article that will show you how to apply basic concepts of plant growth to bonsai training techniques.
Growing plants for bonsai means developing a nursery plant to the point of final styling and placing it in a pot. These plants have a variety of names, i.e. pre- trained or pre-bonsai, etc. The idea is to grow a plant specifically for bonsai rather than landscape. Since our ends are very much different than landscapers the techniques are also very much different. Some plants, such as pines need special attention from the very beginning to become good bonsai, others such as most deciduous trees are much more forgiving and may still be used after achieving some size, but without any previous bonsai training.
The approach that I would like to take is that of explaining and applying plant growth principles so that you may apply these principles and concepts to your particular situation. Once understood, these principles are a very powerful tool for manipulating plant growth. And bonsai is probably the epitome of plant growth manipulation.

Principle 1:

Leaves (needles) increase the size of the woody parts of the plant and the size of the roots.

Well, that seems obvious. But most of us fail to use this concept to its maximum advantage. If you want a really large trunk, don't prune your plant. The removal of leaves will only slow it down. This is of course over- simplified but nonetheless true. If you root prune a plant, leave as many leaves as the roots can support to generate new roots as quickly as possible. If you do this while a deciduous plant has its leaves, you must reduce the foliage comparably to prevent overtaxing the root's ability to supply water. It is imperative to keep the plant cool while new roots are being regenerated.
Leaving the entire stem of deciduous plants works particularly well for bare-root plants or severely root-pruned dormant deciduous plants. The remaining roots will only stimulate as many buds as they can support, so do not top prune the plant. (See the article "Root Pruning Bare Root Seedlings)

Principle 2:

Leaves manufacture plant food, roots store plant food.

OK, obvious again, but what are the ramifications? Top pruning a plant at the end of the season (fall or winter) leaves all of the food intact to stimulate new growth in the spring. A full complement of food with no where to go will stimulate new buds and the new growth will be explosive and coarse, some deciduous plants may send out an eight foot sprout one inch thick in a single season (or more!). Severely top pruning a plant just after it leafs out in the spring uses up most of the stored food because the roots must send out a second burst of food to stimulate even more buds. This depletion will cause very weak new growth and will slow the plant down.
Not pruning a plant in winter or spring leaves the maximum number of buds to be stimulated into flowers and leaves and twigs, this taxes the roots to the fullest and will produce the smallest leaves, and the closest internodes (spaces between the leaves) on the new stems. Confining roots, as in a bonsai pot, limits their ability to store food, which in turn will diminish the leaves and internodes even further. This is the basic mechanism for dwarfing a plant in bonsai.

Principle 3:

Small twiggy growth will always remain small and twiggy.

This principle is not so obvious and is in fact frequently overlooked by bonsai folk. What I mean is that, when a small twiggy branch appears as the result of restricted growing conditions it will always keep this character, even if the plant is rejuvenated by repotting or planting in the earth. New growth will be coarse and vigorous with long internodes and large leaves, but the twiggy branch will be unaffected. Thus you can grow a bunch of nice small branches low on the tree, plant it in the ground and grow a sacrifice branch or leader to increase trunk size enormously, come back and cut off the sacrifice branch and have a big trunked tree with nice small branches. However this will only work if you do not allow buds from the small branches to break into a water sprout or coarse growth. The branch will remain twiggy but its diameter will increase until it is unusable.

And finally

These few principles may sound simplistic, but they are the entire basis for manipulating plant growth in bonsai. You will need to think about them carefully to be able to apply them effectively. To see how their application can work for leaf and stem reduction see the article "How to Reduce the Size of Leaves".

Aging of Container Soils

by Brent Walston

Introduction

There is a changing dynamic relationship between an individual plant and its soil in a container. We realize that plants grow and change, but we don't consider that soils 'age' as well. In container growing, it is important to match the life expectancy of the soil to the frequency of repotting. This insures that your bonsai and other container plants won't be forced to struggle in a 'collapsed' soil mix. As soils age, they tend to break down, reducing the particle size and retaining more water. This process can actually match the growth rate and water needs of the plant, if carefully balanced.

In the Beginning...

Newly repotted plants don't take up as much water (in general) as leafy established plants. This is because the root system is compromised when repotting is accompanied by root pruning and combing out of old soil. We take this into account by reducing transpiration by reducing the foliage, or repotting while the plant is dormant.
In establishing a new root network, aeration and fertility appear to be the prime factors, not water holding capacity. Of course there must be enough capacity to allow for transpiration, but the rate of transpiration is much less at the time of transplant (and shortly afterward) than after root establishment and consequent new growth.

But Then...

As the roots grow and new shoots and foliage develop, transpiration increases. This can be dealt with by pruning to reduce transpiration, increasing the watering, reducing sunlight, etc. But one of the really nice things about having an organic component in the soil is that it begins to break down about the time the plant is demanding more water. This effectively increases the water holding capacity. A well designed soil and proper plant maintenance will help keep the moisture content in balance.
Once soil is thoroughly root colonized it is not subject to collapse for most species unless the plant is subjected to poor treatment such as massive overwatering, not enough direct light, etc. The root network will tend to keep the soil aerated by creating a woody framework. In fact, fine particles will often be washed out of a healthy root network.

Peat Moss and Nursery Mixes

Peat moss, when used in reasonable quantities of less than ten percent, does not add sufficient small particle volume to affect either drainage or aeration. That is what I really like about it. It is so efficient at retaining water without using up space, that it makes an ideal amendment for this purpose. Its lightweight fibrous nature also keeps it in position in the soil mix, rather than washing it quickly to the bottom. The old "UC" mix developed by the University of CA (Davis, I believe) was 50% peat moss and 50% fine sand. This of course was a nursery container mix, not for bonsai. It was used for years until the cost of peat moss forced them to change it.

Soil 'Shelf Life'

What you must understand is that the UC mix and all the other nursery container mixes are designed for quick growth and short 'shelf life'. Ideally, typical nursery plants don't stay in the same pot for more than one or two years (five gallon and under sizes). Any longer than that usually results in soil collapse or root bound conditions. Bonsai mixes must last longer, and they must be more flexible and stable to account for pruning and training. Except for training pots, we don't allow full flat out growth. This means that we have to pay a lot closer attention to soil characteristics than general nurseries do.
I try to make my soil mixes last as long as possible, even those in training pots. I use fresh bark and stable inorganic amendments (lava rock and perlite). This soil will last many years before the bark completely breaks down. Usually by that time the roots are in need of pruning and attention anyhow. That is how a soil should be designed, to last as long as the plant needs to stay potted.

All Inorganic?

Using only stable inorganic components such as lava rock or pumice will create a soil that will last longer than it really needs to last. Using only unstable inorganic amendments such as clay baked to much less than vitrification, akadama, etc. creates a soil that may not last as long as it needs to for some plants, although it is usually fine for two or three years. I have used pure fir bark, and it worked beautifully for about four years, but now I am repotting those plants because it is now quickly breaking down. A combination of stable inorganic and fresh organic amendment (fir bark), works fine for me. I get the right breakdown curve for repotting practices, higher CEC (cation exchange capacity), good aeration and drainage.

And finally

I think, and this is just my opinion, that you will simply get a different set of problems no matter what you use for soil. If you use purely inorganic components, you will probably have to use organic fertilizers and their attendant problems such as removing the surface residue, smell, insects, etc. If you use more than an optimum amount of organic components then you will get problems related to water and aeration, quick breakdown and collapse which you will have to overcome with closer attention to watering practices. There is no 'best' soil, there are only soils that work well in a set of environmental conditions that include the species of plant, how it is manipulated, who is doing the manipulations, watering, the climate, fertilizer type and practice, light/shade. All these things are interrelated.

Soils for Containers and Bonsai

by Brent Walston

Introduction

Soils for container growing, and that includes bonsai, are very different from soils used in gardens and field growing. In the following article I will discuss the necessary parameters for container soils and how to use them to develop good container mixes.

Soil Basics

The study of soils is centered on the size and composition of the particles. Particle size varies directly with air retention, and inversely with water retention. This means that, as the particle size decreases, so does the amount of air retained at the saturation point. It also means that, as the particle size decreases, the amount of water retained increases
We all know this intuitively and actually work with this principle all the time in the garden. Clay soils retain more water and less air than sandy soils. Interestingly, we often use the same cure for both extremes, the introduction of organic matter. Organic matter is very good at water retention. Soil amendments, such as peat moss can hold many times their weight in water. It is obvious that this would help a sandy soil, but how does it help a clay soil?
As long as the organic matter particle size is not too small, and the amendment is thoroughly incorporated, clay soils are improved by 'aggregation'. That is, the inorganic and the organic soil particles tend to clump together to form larger aggregate particles that will trap air between them as well as water within them. This improves the aeration, or air retention of the soil. Stated differently, it improves the drainage.
Clay has a tremendous ability to retain nutrients, organic amendments have a lesser ability, and inorganic larger particles such as sand have very little. Container soils which contain little or no clay or native soil, must be fertilized regularly to overcome this deficiency. Also, soilless mixes (those that contain no native earth), will usually be deficient in trace elements, and these will have to be added in some manner.

Bonsai is a Nursery Container Practice

Bonsai is container gardening, and in that sense, is little different than general nursery work in the mechanics of soils and drainage. Soils for containers should drain 2 to 3 times faster than native soils. The earth acts like a huge suction pump and can literally pull water from heavy soils. But this same garden soil in a container is usually instant death for your plants. Because of the impermeable walls and bottom of the pot, this same soil will not drain properly. Container mixes should drain so fast that you can stand there and watch the water flow through.
Most nursery mixes these days are 'Soilless', that is, they contain no native soil, except possibly washed sand. They may contain compost. They are usually composed of three elements:

• Inorganic structural element: lava rock, perlite, sand, baked clay, decomposed granite, pumice, etc
• Organic structural element which can hold some water and nutrients: fir bark, pine bark, nitrolized redwood chips, etc.
• A water holding element (optional): compost, peat moss, or vermiculite

Water Retention and Drainage

Soils must drain quickly, but still retain a reservoir of water, 25% percent by volume is considered optimal. The air space after initial drainage (the saturation point) should also be about 25%. It is hard to beat peat moss for water retention without adversely affecting drainage. Peat moss is also effective at nutrient retention. Vermiculite after it begins to break down to its basic clay constituent is also very good water and nutrient retention, but must be used conservatively, because it is after all, clay.

Pathogens

Anytime you use compost (unless it has been pasteurized, or thoroughly composted) or native soil you run the risk of introducing pathogens, insects and other pests. If you are a believer in the necessity of these elements, pasteurize them them by putting them in an oven bag with enough water to moisten, and heat to 140F to 160F degrees (internally) for about half an hour.

Mixes Vary with Container Size and Species

I use several mixes for my nursery plants (including bonsai) according to the species of plant and the size of the container and also the cost of the constituents. For my liner (small starter) pots to about seven gallon size pots, I use extremely fast draining mixes because nothing is surer death to a seedling or rooted cutting than a dense soil. You should be able to easily 'see' big air spaces.
The formula is very simple: eight parts screened fir bark 1/4 to 3/8 inch size, eight parts screened perlite, one part peat moss, one half part vermiculite (optional). I also incorporate Osmocote time release fertilizer. Mixing in a cement mixer makes it very easy and pretty cheap. This is an excellent mix for bonsai, except for aesthetic reasons you may wish to replace the perlite with other inorganic structural elements such as decomposed granite, turface, pumice, etc.
This mix will dry out very quickly, but it will get you maximum root growth in the shortest amount of time.
The above mix works well up to seven to ten gallon size. The larger the container the denser the soil mix is the rule in nursery practice, until you are planting in the earth where the densest mix occurs. For ten to fifteen gallon size and larger, I substitute unscreened fir bark (3/8 inch minus it is called) for screened bark, reduce or eliminate the the peat moss and replace the perlite with 3/8 lava rock. This makes a denser, less expensive mixture that will not dry out as quickly, but since bark does not quickly decompose, it will not quickly collapse (more on this later).
This mix works especially well for slower growing species that need excessive aeration. Faster growing species will do better with a denser mix of more traditional nursery mix of nitrolized wood fiber, sand and compost. It is about 80% redwood shavings.
Most beginner gardeners and bonsai growers usually use a mix that is too dense, thinking that what grows them best in the earth ought to work just as well in a container. This is not the case.

Root Colonization and Soil Collapse

This next point, which I have discovered after years of throwing out dead nursery plants, is not obvious and will not be found in most books.

In a container there is a race going on between the growth of the roots and the decomposition of the soil.

If the organic soil elements compost or decompose before the roots can completely colonize the pot, the soil collapses and loses its drainage and air spaces, and the root growth stops. If the roots fill the container first, the soil will not collapse because the roots will form a structure that will support the plant and the soil, and drainage will be maintained.
I have even seen rootballs nearly devoid of soil because of erosion, but uncollapsed because of the root network. This is not as important in finished bonsai because we tend to use premium soils with few organic elements that can collapse, but it can happen if you use too high a proportion of compost or wood fiber, other than bark (which decomposes slowly). This is a very important point for growing out plants for bonsai, where we use larger containers and less expensive and denser soils.

In any soil mix, time, watering, weathering, chemical and biological decomposition tend to produce smaller particle size , loss of air space and drainage.

I call this process 'Soil Collapse'. For the organic elements of the soil, this can occur very quickly. For the inorganic elements, it is usually a slower process, but can be rapid for some volcanic soil amendments and decomposed granite. It is important to match the 'soil life' with the rate of root growth to make sure that root colonization occurs before soil collapse. The root colonization rate varies with the species, fertilizer, water, sunlight, and pruning practices. These are all interdependent forces. That is, plants will grow faster under optimum light, fertilizer and full foliage conditions, thereby decreasing the time needed for the roots to colonize the container. Plants grown in more shade than they like, suffer greatly from this effect because top and root growth slows and soils tend to stay wetter increasing the rate of organic decomposition.
In the nursery business, soil is a large part of the bottom line and premium materials are not affordable for all plants. For fast growing annuals, perennials and woody trees and shrubs, soils that contain a high percentage of wood fiber, as much as eighty to ninety percent, are perfectly acceptable. Slower growing plants require more sand, bark, perlite, etc. that will break down more slowly.
After years of trying to make my plants adapt to my soil mixes, I have realized that it is much better to adapt my soil mix to my plants. I grow less and less landscape material and more bonsai related material so my mixes are now all bark and perlite (or pumice) based. I find that the collapse theory works quite well in predicting what mixes will succeed with what plants.
For a more indepth treatment of this subject see the article Aging of Container Soils.

Premium Materials

In bonsai, there is so little soil involved that cost is rarely a factor. Premium soil mix components are a small part of the bottom line. However, some components work so well that we use them anyhow, such as peat moss. Peat moss is not a necessary component but its excellent water retention makes it a valuable component in very hot areas where watering only once a day is desireable. It is often the decomposed peat that one finds in the bottom of the pot.
There is always a tradeoff in soil mix components. I prefer porous volcanic materials such lava rock, pumice, and perlite (a sort of manmade volcanic rock) because they are light weight, hold water and air and are readily available. But they do break down, and after several years one finds evidence of this in the bottom of the pot. I also use DG, decomposed granite. You can screen it to get any particle size you want to get excellent drainage, but some grades break down rapidly. I have been very lucky to obtain a grade that has probably been mined from a source near water, so the weathering has been minimal and it is very stable. On the minus side, it is heavy and has no water retention.

And finally

After fifteen years of this, I am still playing with soil mix formulas and this seems to be true of most nurseryman, so there will probably never be an end to this thread.
For more soil mysteries explained, see the companion article Why the Earth Is Not Like a Pot.