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Grow Boxes and Training Pots

By Will Heath, USA
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Common procedures for developing a bonsai from seedlings or young stock involve first ground or grow box growth during which the plant undergoes many trunk chops and cut backs of branching to encourage back budding and to add movement to the trunk as well as developing taper. Multiple sacrifice branches are encouraged in order to thicken the trunk. This period of allowing growth and then cutting it back is continued until the desired trunk size, taper, and movement are achieved.

Later, once the growers' goals are met concerning the trunk, taper, and movement, the plant is transferred to a training pot to define the nebari and create finer "feeder" roots while accustoming the plant to a container. During this period ramification and final branch placement are also begun. Finally, the plant is transferred into bonsai pot where it will spend the remainder of its lifetime.

Grow boxes are also used to place freshly collected stock into in order for the plant to recover and become accustomed to life out of the ground while waiting for it to become strong enough to begin training. The grow box is used instead of a training pot to accompany the larger root mass in need of fine feeder roots which is typical of collected trees.

Although grow boxes and training pots are often considered as the same thing, they serve two completely different purposes. In the following article I will separate the two and explain in detail the uses of both.

Grow Boxes

Growing or grow boxes can be defined as any container that will allow free, almost unrestricted growth of a plant in order to increase the size of the trunk, root mass, branch numbers, and/or height of the tree.

Photograph by Vance Wood

Grow boxes are typically slightly oversized pots or boxes used for the expressed purpose of obtaining faster growth, trunk and root development than would be unobtainable in a regular bonsai pot. Although, for simplicity and clarity, I have used the term "grow boxes" in this article, the term is used here to encompass all oversized containers that can be used for the purposes outlined.

These containers come in as many shapes and forms as there are bonsaists. The typical wooden box is most likely the most efficient and attractive, but I have seen dish washing pans, dresser drawers, small tires, large nursery pots, large bonsai pots, azalea pots, cooking pans, produce boxes, and Tupperware containers used for this purpose. The common denominator is using a container that will allow good drainage, root ramification, and that is reasonably affordable. Not to mention bringing growing conditions as close to actual ground growing as possible.

Growing in the ground will achieve these results quicker yet some people do not have the space to maintain a growing bed so grow boxes are a substitute and are the next best thing.

A common misconception is that the larger the container, the faster and better the growth, this is a myth and actually could be more detrimental in the long run due to the fact that the massive amount of excess soil surrounding the root ball may never get the chance to dry out. A grow box should be only a few inches wider at most than the root mass of the plant being put into it. Once the roots colonize the new soil, if more growth is required, the plant can be slipped potted into a larger container without distributing the roots or, if the desired results have been obtained, it can be moved into a training pot.

With collected trees this also holds true. As the roots are usually damaged slightly and feeder roots are low in numbers, it is important not to place them in a overly large soil mass. A grow box that is a few inches bigger than the root mass is all that is needed. In most cases the root mass will be worked down in size anyhow in the future so the goal should be to create feeder roots, not encourage a bigger root mass. Since collected trees usually take awhile before fully participating in the transfer of water out of the soil, having to big of a pot and thereby, too much soil, can lead to an overly wet environment.

If you think about how nurseries grow stock in pots this will make more sense. They start out seedlings in very small pots and then once the roots fill the container, they move it up into the next size pot and then once the roots have filled this, they move it one size up again. They do not simply place a seedling into a ten gallon pot and wait. Considering that nurseries make their profit selling plants and that they can sell bigger stock for more money than small stock, you must admit that they have to get stock to grow as quickly as humanly possible. There practice of up-potting then must be the most efficient and quickest way to develop stock.

I know that the common response to this practice is the question that if plants grow the best in the ground, wouldn't it best to try and duplicate that environment in a super sized pot with lots of excess soil. The answer is no because pots are just not like the ground, the drainage and water retention are not the same. For further reading on this subject I would recommend two of Brent Walstons' excellent articles, "Overpotting" and "Why the Earth Is Not Like a Pot". For more information on Brent Walston, who is certainly the leading authority on growing and developing stock for bonsai in the United States, you can see an in-depth interview at

As mentioned above, almost any container will work as a grow box as long as it is not too large, typically 2 to 3 inches of new soil around the root mass is plenty, any more and you risk the overall health of the tree.

I personally prefer using wooden boxes as shown above for this purpose as they are shallow, easily built to size from inexpensive materials, lightweight, and relatively attractive. Being shallow, they help to create the nebari we desire in bonsai. The wood for grow boxes can be purchased if necessary or as I do, you can recycle wood from projects, pallets or old produce crates. I recently built a deck around my pool using plastic lumber which saws, nails, and screws just like wood, but will never rot. The scraps from the deck have given me material for many grow boxes.

Making a grow box is a simple matter as illustrated here. I leave spaces between the bottom boards for drainage. Any type of screen that is bigger than window screen and will not rust can be used over the slats to prevent your soil mix from falling through. I personally use self adhesive drywall mesh for this purpose and it comes in a roll, can be easily cut to size, will not decompose, and it inexpensive.

Illustration by Will Heath

You can get as fancy as you wish; one club member cuts his end pieces taller and cuts ovals for handles on the top. I have seen the edges mitered, the upper lip routed, the outside of the box stained, and metal straps wrapped around the sides for strength, only your imagination and skill set the limits.

Whatever your material or design, keep a few boxes of varying sizes handy and material to create a custom sized box when the need arises. Spaces for drainage can vary and different board widths can be used on the bottom as the only object is to assure good drainage and enough strength to withstand moving or rotating the box from time to time.

Training Pots

Once the trunk thickness, nebari, and branching have been achieved in a grow box, it is time to more the tree into a training pot so that it can develop the fine root system it will need to survive life in a bonsai pot. Ramification and finer development can also be started while the tree is in a training pot.

Photograph by Vance Wood

Sometimes the development of the trunk and nebari in collected stock and nursery stock is already at an advanced state and the grow box step can be skipped altogether so that the tree can be placed directly into a training pot.

Finer root development can be obtained by using a pot with screened sides and bottom. These are available made exclusively for this purpose or you could also use pond baskets, which are not as attractive but work in basically the same way and are available at low prices at most pond stores and box stores in the pond department.

Photograph by Vance Wood

The first two pictures show screen sided redwood training pots. This design is so unique that Vance Wood was awarded a patent for it some years ago. Screen sided pots help in root development and they effectively prevent the circling of roots common in all other types of pots. The design combines excellent drainage with automated pruning of roots. The root tips that reach the light through the screen stop growing, which allows resources to be redirected to other roots deeper within the soil and closer to the trunk, creating excellent ramification of the roots and creating a mass of fine feeder roots. I have had excellent results with screen sided pots.

Another nice feature of Vance's pots is that the bottom is not fastened in and can be pushed upwards which makes removing the root mass a breeze.

Vance's Redwood pots are very attractive they look great around the yard and both his pots and pond baskets work better than any other training pots I have used. The fine root systems that result from the use of screen sided training pots has convinced me never to use anything else.

Photograph by Vance Wood

The Third picture is of a pond basket, sold at most garden centers to be used for plants growing in ponds or for filtering. These come in various sizes and are quite inexpensive; the 2-gallon size shown was purchased for under a dollar.

Pond baskets have all the advantages of the redwood pot except the appearance. They can be buried for winter if needed and like the wooden framed pot, has excellent drainage all around. These are quickly becoming very popular with bonsai enthusiasts everywhere.

I wire two long and narrow blocks to the bottom to act as feet so that air can circulate underneath the pot and so water can drain out of the screened bottom better. By wiring the blocks to the bottom they remain attached when I rotate or move the pot. The addition of feet are not necessary with Vance Wood's planters shown above, as the bottom screen is already designed to be off of the surface of the bench.

Many people use basic garden center plastic liners, larger bonsai pots and such for training pots however, these do little for fine root development and are not usually recommended.

I hope that I have successfully explained the differences between a grow box and a training pot as well as clearly explained the uses and advantages of both. Happy growing.

Will Heath, is an outdoorsman, archer, and amateur mycologist who first became enamored with bonsai by following his desire to capture pieces of nature he observed while in the forests of Michigan. He is an editor and co-founder of the Art of Bonsai project, an editor at Bonsai Today, and a board member of the Four Seasons Bonsai Club. His educational articles and humorous stories have been published in Bonsai Today, the ABS Journal and newsletters around the world.

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