Chapter 20 Professional Nursery Practices for Growing Pre-Bonsai

Ever wonder why all nursery pots have the same approximate dimensions, or why most trees for sale at big box stores are about the same height? It’s not by chance. AmericanHort (formerly the American Nursery and Landscaping Association (ANLA) publishes detailed guidelines for the horticulture industry called the American Standard for Nursery Stock. The 2014 edition is available for downloaded here.

“The Standards” are aimed mainly at industry professionals, providing growers and vendors with shared procedures and terms for measuring and describing stock, grading plants and trees for sale, and standardized containers (among other uses.) Even so we still can learn a great deal from them. For example, they help us understand why it is hard to find bonsai starter trees in most nurseries. The twisted trunks and odd shapes that we want most for bonsai are classified as defects by the mainstream trade; standard practice is to cull out any trees that do not conform to the species guidelines provided in the Standard.

Just how different are the standards for commercial nursery stock and bonsai stock? Table 1 is an excerpt from the 2004 Standards showing the normally expected relationships between root ball, height, and trunk caliper (the diameter 6 inches above soil line (12 inches for trees >4” dia.)).

Table 1: Excerpts of the ANLA Standards The last two columns in this table are my estimated values for nebari (2x trunk caliper, and final height of a finished bonsai (6x the nebari, or 12x the caliper). These numbers will vary with the style of the final tree.

Std. Trunk Caliper (in.) Std. Ht. (ft.) Max. Ht. (ft.) Root Ball (conventionally field grown) Root Ball (grown in fabric bag) Est. Size of Nebari (in.) Ht. of final bonsai (in.)
1.0 to 1.2 8-10’ 11’ 18” dia, 12” dp. 10” dia. 2-3” <12”
1.5 to 1.7 10-12’ 14’ 20” dia, 14” dp. 12” dia. 3-4” 18-24”
2.0 12-14’ 16’ 24” dia, 16” dp. 16” dia. 4-5” 24-30”
3.0 14-16’ 18’ 32” dia, 22” dp. 20” dia. 6-7” 36-42”
4.0 16-18’ 21’ 40” dia, 28” dp. 24” dia. 8-9” 48-54”

Table 2: Sizes of Field Grown Bonsai Stock Examples of “dig-your-own” tree sizes sold by Glenbrook Bonsai in the UK (now out of business).

Species Caliper Range (in.) Height Range (in.)
San Jose juniper 2-4” 24-30”
Jap. black pine 3” up to 32”
Chinese elm 2” 12-24”
Zelkova 2-4” up to 18”

A standard 3-inch caliper tree grown for the nursery trade should be 14-16 feet tall, growing in a nursery pot at least 32 inches in diameter and 22 inches deep. For bonsai, we would want that same 3-inch caliper tree to be around 3 feet tall, and assuming the nebari is ~2x trunk caliper, in a container no more than 6-8 inches deep.

Differences like these are not the only things we can learn from AmericanHort Standards though. Take a look at the expected size of the root ball of a field-grown 3-inch caliper tree (32” diameter, 22” depth). That is a very reasonable estimate of the size of the root ball we would need to dig out if we were field collecting a tree with a 3-inch trunk.

My point here is, there are many useful bits of information in the Standard that we can use, even if bonsai stock does not fit the Standard’s guidelines overall. We can learn how growers consistently produce large numbers of healthy well-proportioned trees for the landscape and home improvement markets. With some tinkering, many of their best methods can be applied on a small scale to growing and developing bonsai stock.

20.1 What Other Tips and Tricks Can We Adapt From the Trade?

20.1.1 Pot-in-Pot Systems

Growing a tree in the ground develops size and taper much faster than growing the same tree in a pot. It turns out that trees in pots CAN grow almost as rapidly if they are buried to insulate the roots from rapid temperature changes. Production nurseries do this using a pot-in-pot system. A grid of holes is dug into the ground and heavyweight plastic socket pots are inserted. Small stock trees are planted in slightly smaller, lighter pots that are slotted into the sockets. Trees are watered and fed regularly via drip irrigation lines, and the smaller pots are rotated regularly to help make growth more uniform. When trees are ready to sell the pots are lifted and a new tree put back into the socket. Trees grown this way develop at almost the same rate as if they were planted out in open ground.

Pot-in-pot nursery

Field-growing trees in a pot-in-pot nursery. In the first three rows in the foreground, every other socket is empty, awaiting a new insert. Link to original image.

The pots and sockets designed for commercial pot-in-pot installations are too deep for growing bonsai but the basic principles still apply. Say you are growing trees in kitchen dishpans. Buy two, drill drain holes, then bury one as a socket. Put ~1/2” of fine gravel in the bottom of the socket to provide added drainage and keep the two pans from sticking together. Place the second pan (with your tree) in the socket pan. The buried pan will insulate roots and slow water loss from summer heat, both of which will speed up growth. Drip irrigation lets you fine-tune watering so soil moisture stays nearly perfect.

20.1.2 Bag-Pots

In 2006 the High Caliper Company introduced a “pot” made of heavy-weight rough polyester fabric. As roots grow they hit fine woven fibers, not a solid plastic wall, and the roots branch rather than turn. Any roots that do pass through the fabric are exposed to air, and die back on their own. When it is time to plant a tree or pot it up to the next largest size of container, the bag is cut and pulled off. This tears the very tips of the feeder roots off, triggering a flush of new root growth and branching.

Fabric growing bags

Vegetables growing in fabric bags. Bags now are available with different lifespans, from 1-year bags used for annual crops to heavy geo-fabrics in which trees are supposedly able to live for up to 20 years. Link to original image.

A fabric grow bag containing a landscape tree. Link to original image, by Anthony LeBude, NC State University, Bugwood.org ..

These claims are not market hype; studies by state cooperative extension labs have confirmed bag pots work as claimed, and AmericanHort now has formal standards for them. These bags are prefect for bonsai, because a tree of a given caliper can be grown in a bag-pot that is 1/2 to 2/3 the size of the recommended plastic pot. Bag-pots can be kept above ground like a regular pot, sunk in sockets for bag-in-pot growing, or buried directly. When it comes time to dig a tree up, >90% of the root ball will be inside the bag.

High Caliper has a retail outlet for bag pots, which is listed under Sources. The bags are rot resistant, so can be reused several times; I still have yet to wear out bags I started using over a decade ago. They are not cheap, but are not outrageous either.

As fabric bags have gained popularity, other companies have introduced less expensive versions that are not quite so long lasting, but still last 3-5 years without degrading.

20.1.3 Using Air and Copper to Train and Prune Roots

Several companies now sell pots with mesh rather than solid bottom and sides. The advantage of mesh pots is that roots dehydrate and die shortly after emerging from the holes, which encourages branching back in the soil mass. These pots are more expensive than plain-sided pots, and tree roots are more susceptible to drying out than in fabric bags, but they can be useful for developing roots in rot-susceptible species.

A slotted-side pot (left-most of the three) designed to prevent roots from circling the container. Link to original image, by Matthew Chappell, University of Georgia, Bugwood.org ..

Another tool of the nursery trade is Spin Out, which stops roots from circling and encourages branching instead. Spin Out is basically 7% copper hydroxide in latex paint. It is painted on the inside of a pot and allowed to dry for at least 24 hours, then the tree is planted in the pot normally. The paint acts as a slow-release membrane for the water-soluble copper. When fine roots reach the pot wall, the copper stunts the growing tip, causing the root to branch further back. The advantage of this is that trees in containers develop much finer root systems.

Spin Out is seriously expensive ($300+), and only available in 2-gallon containers or larger. If you want to experiment with copper as a root inhibitor, there is a cheaper workaround solution. Some older agricultural fungicides are basically soluble copper, and can be mixed with whatever latex paint is handy. Look for Champion WP (77% copper hydroxide), Funguran-OH, Kocide (38% copper hydroxide) or KOP-Hydroxide. These chemicals are sold under the Fertilome and Hi-Yield brand names. Simply adjust how much latex paint and chemical you mix so the final concentration is 7% by weight copper hydroxide (or copper sulfate).

Spin Out has several drawbacks. First, trees left too long in a coated pot may have stunted roots; it is essential to repot as soon as the root mass begins to crowd the pot. Second, some trees are extremely sensitive to copper, and might die. Third, the runoff is toxic to aquatic organisms.

20.1.4 Cotton Twine and Masking Tape

Professional growers don’t always get the results they wanted the first time. They may need to bend a branch to a make a new leader, or “encourage” growth in a different direction. So why don’t we ever find scraps of training wire on nursery stock? I learned that nursery pros use plain white cotton twine and masking tape rather than wire to hold small branches. Both are tough enough to do the job initially, but rot before girdling or marking limbs. A folded over piece of masking tape beneath an emerging bud will direct it upward as it grows without risking wire damage.