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Obtaining Maximum Growth in Pre-Bonsai

by Brent Walston


This article is an exchange between John Powers and Brent Walston that took place on the Internet Bonsai Club in July 1996. John is asking a series of questions regarding how to obtain rapid growth for training bonsai.

John, you have posed an interesting set of questions, just the kind of stuff that I think about all the time. As you aluded below, maximum growth for bonsai does not exist in a vacuum, there is always a price to pay for maximum flat out growth, such as loss of trunk curvature and taper which I think is best achieved by growing out whips and then cutting them off. [For more on this see the article Developing Large Trunks for Bonsai]

John asks:
Will most trees grow best in the ground as opposed to large containers? Is there a certain point where a large container works as well or better than a ground planted tree?

Brent replies:
In general trees will grow fastest when planted in the ground. However, there are some caveats to be considered. It has been my experience that trees grow more slowly when first planted in the ground rather than shifted to a larger container. This effect seems to last one to two years for most species I have observed. I think what is going on is that the tree is sacrificing top growth for root growth, but it is also possible that the tree has more difficulty getting established in this (usually) denser medium.

This is most dramatically seen in California Oaks. In nature these trees spend the first five years of their lives sinking their tap and lateral roots in search of water, the tops grow hardly at all. At some point the root growth slows, the process reverses and suddenly the top growth explodes growing as much as four or five feet in a single season after growing only inches for several years. In pots I have watched them do the same thing, they grow very little until all the soil is colonized with roots then the tops grow very quickly.

So in the long run planting in the ground is the best solution, but only if you plan to leave your trees undisturbed for three or more years, otherwise it will be slower than container growing. This assumes of course that water, soil drainage, structure and fertility are adequate.

What type of soil do trees in the ground grow best in? I know roots need oxygen, moisture, and micro and macro nutrients. In a container these four things are best supplied by a coarse and fast draining mix. Would a similar soil work as well in the ground, or since the tree is in the ground with better drainage, would a finer mix with more organic compounds allow greater fuel for growth?

I used to believe the latter, that since you can use finer soils for ground growing, it was superior as well. Now I'm not so sure. I think the very same mixes that are used for containers when used for ground growing will achieve superior results. But you must pay the price of increased watering and fertilizing. All of this is anecdotal, but I'm a good observer, and hard headed, I like to do things for myself and see if they really work.

I have mounds of discarded potting soil and peat and perlite from cutting flats, hundreds of yards of it at one time. I didn't do much with it for a while and just sort of watched things grow in it. Now this stuff was not really soil, it contained almost no native soil and was so fluffy you could hardly walk in it. I have locust trees that have naturalized and the seed comes up everywhere. One seedling came up in the middle of the pile one year. That locust seedling grew over ten feet tall in a single season, yet the mix was so loose that I could still pull it out roots and all with a good yank, which I did. The root mass was incredible, it never did reach any substantial earth which was at least three feet below.

My second experience with this phenomenon was last year. I plowed all my old discarded potting soil from the dead plants into a bank below a new area for trees in pots. Rather than just waste this bank of soil mix which was as much as two feet deep in places, I used it to plant a 'Catlin' elm forest for future use. I figured to let them grow out, prune them, then dig them as I needed them. I planted over a hundred of my little liners, small plants with quarter inch trunks and about six inches tall. This was last spring. 'Catlin' grows slowly that's why I wanted to get some in the ground. I recently harvested a bunch of the stems for cuttings, so I got a good look at what has happened. In the deepest soil areas the whips are almost five feet tall and the trunk caliper is now about an inch and a half. Other 'Catlin's that I put in the ground about three years ago in creek silt have hardly grown at all.

Some of the best growth yet has come from the 'escape method' where I put five gallon cans on the ground and allow the tree roots to escape through the drainage holes and into the earth. I love this method because it is so efficient. You must continue to water through the can to keep the above ground roots alive. When you get the trunk you want, you simply sever the escaped roots at the holes and cut back the top of the tree at the same time or sometime before. I am getting massive 'Seiju' elms by this method. They are putting on more than an inch of caliper per year.

Can feeding gradually be built up to larger than normal levels? Overdosing a tree with large amounts of fert all at once would not be beneficial, but if you were growing a tree in the ground, and the soil was nutrient rich, could you gradually increase the amount of available nutrients? Can foliar feeding be increased as well. Is there a point you can over foliar feed? I know that with hydroponic growing, the plants are watered every time with a nutrient rich solution. Can anything like this be done with a tree in the ground?

The same principles of feeding apply whether you are growing in the ground or in containers. Container mixes do not contain enough clay or fines to hold nutrients well and must be fertilized heavily and frequently to make up for this. If you use this type of mix for ground growing you will still have to fertilize heavily. Most bonsai mixes have very poor nutrient holding capacity, that is why we fertilize so often or continuously. If you are growing in native sandy loam the need for fertilizer is greatly reduced, so is watering. It is very difficult to give a set formula because of the great number of variables involed. You must rely on the plants and become good at recognizing good nutrition by growth and leaf color.

I think the emphasis on foliar feeding is greatly overdone. Roots are the primary method that plants have devised to take up water and nutrients, and they are hundreds of times more efficient at it than leaves. The response may be faster when fertilizer is applied to leaves but in the long run it is the roots that will take up and store the far greater part of nutrients needed. Foliar feeding is fine as long as the runoff ends up on the roots, if it just escapes, it is a terrible waste. You would be hard pressed to over feed by applying fertilizer to the leaves unless you used too strong a concentration. In any case if the runoff is going to the roots as well how do you know that it is too much foliar feed or too much root feed?

Overfertilizing trees, no matter how you apply it, will result in salt burn, you will see it at the margins of the leaves, they will turn brown and dry out.

What types of fertilizer should be used to fuel super growth? What NPK ratios should be used and when? What do high nitrogen fertilizers do? I know high phosphorous ferts help in fruiting and flowering. What else do they help in. What is the Potassium used for? Would using only a very high Nitrogen fert during the growing season help.

Personally I don't think the type of fertilizer matters a bit as long as it has what the plant needs, which is NPK and minor nutrients such as iron, calcium, magnesium, manganese, zinc, copper, molybdenum, boron, cobalt, and nickel. You will pay a little more for a fertilizer that has these minors but it is well worth it, especially when growing on such a small scale as most of us are. Nitrogen is the primary nutrient for green growth. It will make leaves large, deep green and will create long internodes in good growing conditions and with adequate soil space. It is essential in spring when deciduous and evergreen plants are putting out new growth. It should be tapered off at the end of the season to stop forcing soft new growth that could be damaged by frost. Phosphorus is important in promoting root growth, flowering, strengthening cell walls and for fighting off diseases. Potassium aids in general cell functioning. A balanced fertilizer seems to work just fine under normal conditions (all the numbers for NPK are the same such as 10-10-10). Higher nitrogen percentages are suitable for spring growth and for acid conditions.

Are there methods of lengthening the growing season. Without a greenhouse it would be difficult to grow a tree during winter, but for container plants, you could bring some into the house and grow them under fluorescent lights. On average, how long do temperate trees need for a dormancy period.

Living where I do, I have an almost nine month growing season if you count root growth. This is where I think there is potential for simple extended season growth. It is very difficult to do anything about air temperature when growing outside, but soil temperatures can be changed or maintained by some rather simple methods.

Most of think trees go dormant when the leaves fall. Not true. There is no top growth but the roots continue to grow until the soil temperature falls below about sixty to sixty five degrees. This continued root growth occurs here in Northern California until about Christmas, root growth picks up again in late Feb depending on the weather. By extending root growth you set up the plant for increased top growth the following season. You can maintain fall temperatures longer by mulching with an insulating layer such as straw, etc, and you can raise spring temperatures sooner by mulching with black plastic to capture the sun's heat.

Grow bags for grounded trees restrict downward growth and allow upward growth. I know it is the lateral roots near the surface of the soil that feed the tree. The vertical and/or tap roots are used for anchorage. Would the use of grow bags help? Is there an optimum depth that at which a tree should be planted?

Last first, in general a tree should never be planting any deeper than it was growing, and this is usually the position of the first roots, although there is a better way to tell. If you bury the crown of a tree you stand a good chance of getting crown rot which will girdle the bark and cause the death of the tree. When trees germinate the seedlings exhibit root tissue and stem tissue and for most woody species there is an observable demarcation, usually a ring of tissue which is the crown. You can bury the tissue up to the crown, you can also leave the crown exposed if there are better roots down below. I use this phenomenon when root grafting pines. I pot the understock high so I can graft below the crown of the tree. After the graft takes I bury the exposed stem up to the graft union which will disappear into the nebari and make a clean undetectable graft for bonsai.

Taps roots are a phenomenon of seedlings. If you cut the tap root off a seedling, it will only form lateral roots and never grow another true tap root. That is why it is important to obtain plant material that has been properly handled in the early stages for bonsai. It will not have a tap root and instead there will be a nice dense shallow ball of lateral and hair roots.

I think grow bags are over-rated, they are designed for easy removal of trees not for achieving maximum root growth. Trees for bonsai should not be planted in the ground until they have had the preliminary root work done anyway such as removing the tap roots and straightening out the surface laterals to achieve good nebari. You can cut the surface roots once a year or every other year with a sharp spade to increase root density near the crown and make lifting out easy. This procedure should be superior to grow bags.

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