Chapter 42 Journal Tree 2: A Boxwood

42.1 Mid-March, 2021

I purchased this Green Tower boxwood for $60 in a local garden center. I described how I selected this tree here. It is a good example of how I repot most of the nursery stock I find locally.

Usually I repot a new tree for the first time in mid-March. I pull it out of the pot and shake out or quickly hose off any loose soil. Then I soak the root mass in a bucket of water for 30 minutes to an hour. This loosens the root ball and softens the roots so fewer break.

Close-up view of the root ball of the Green Tower boxwood (Buxus sempervirens ’Monrue’). The rootball has completely filled the 5-gallon pot. The first step in cleaning and untangling these roots will be to cut off the bottom third of the root ball using a pruning saw. Original photo by Dan Johnson.

The next step is to untangle and remove some of the existing roots. Working with the root ball on a table I use a blunt pointed root hook, my fingers, and a sharpened pencil or dowel to untangle the roots and spread them out radially, like spokes on a wheel. If the roots are too tangled to separate, it may be necessary to cut off the bottom third of the root ball with a pruning saw. As needed I either soak the root ball or hose it off to eliminate the nursery soil and keep the roots moist.

Untangling the roots this way WILL break some of them off, and that is okay. I keep track of how many roots break off in the initial comb-out and adjust how much I trim off in a later step.

Once I have the roots more or less untangled, I look for any that are obviously dead and cut those off too. If I can afford to, I also cut back any larger roots to leave more space for feeder roots.

The partially untangled root ball. Left. I have washed out most of the old soil mix and combed out the roots with a root hook and my fingers. Also, I have cut off most of the larger woody roots. I left very few roots larger than 1/4" in diameter below the soil line. Right. The partially untangled roots viewed from the opposite side. I removed 1/3 of the roots when I cut off the bottom of the root ball. During the process of cleaning the roots to the point shown here I had to remove another 10-20% of the root ball. Original photo by Dan Johnson.

The root mass that originally filled a 5-gallon pot now is small enough to fit comfortably in a standard dishpan. I lifted up the tree, let the roots hang down, and trimmed off any long roots so the root ends were more even, and the roots could be spread out on the surface of the soil.

To balance top and bottom mass, I removed more than 80% of the above-ground mass. I looked first for branches that obviously would not be needed (for example the weaker of double branches) and removed or shortened those first. Next I shortened the longer branches and removed the growing tips of short branches that would not need to be thickened. Then I clipped the newest growth from the prior season, and left only older leaves. Finally I cut back the remaining branches as far as I could and still leave strong green growth.

Once I was satisfied with the balance in top and bottom mass, I was ready to repot.

42.1.1 Repotting

I filled the pan half-way with soil mix, smoothed that flat to form a base, then added a mound of soil mix sufficient to support the tree’s root mass 1-2 inches ABOVE the lip of the container. Next I bent most of the roots out laterally so they sat in the top half of the container, atop the moundedsoil.

To work soil into the basal roots I gently twisted the tree 1/8-turn each way a couple times helps to settle it on the mound, then I start working the soil in around the roots, again using my fingers and a wooden dowel.

Tip #1: Many growers like to wire trees into their training pots, but I do not unless the tree is too off-balance to support itself. My main reasons are:

• Wiring in place makes it harder to get roots settled into the soil;
• Small movements of the tree in the pot stimulate fine root formation;
• I like to be able to lift a tree and root mass out to check root health.

Tip #2: I always make sure I have twice as much soil mix on hand as the training container holds. That way I do not run out midway through potting up the tree.

Next I poured about 1/2 inch of soil on top of the roots and used my fingers and a dowel to work the soil into spaces between roots. When I thought the roots were well filled with soil, I lifted the tree about 1/2 inch and shook it GENTLY to straighten the roots and settle soil into the smaller spaces. This exposed new spaces needing soil. By the time I finish working soil in and around the roots, the extra 1-2 inches of height due to mounded soil was down to less than 1/2 inch above the desired final position of the tree. That last 1/2 inch disappeared when I watered the tree in.

The boxwood after replanting in a dishpan, with a soda can for scale. Original photo by Dan Johnson.

Cutting a 4-foot shrub down to ~12 inches tall might seem extreme, but it was a necessary design step. The nebari formed by the twin trunks is only about 3 inches wide in all, meaning the completed twin-trunk should be no taller than ~18 inches. Currently the tallest trunk is about 12 inches above the soil level, meaning I have ~6 inches of additional height in which to develop the foliage pads and branches. I would not have been so aggressive with most other trees, but I know that boxwoods accept aggressive pruning as long as there are some green leaves to keep drawing nutrients to the branch.

Ideally the root mat should be no thicker than the width of the tree at its base. In this case, the final tree will need to fit into a container 3-4 inches deep. It will take a couple rounds of repotting to reduce the root mat that far, so I do not over-prune roots to reach this point right away unless I know that the particular species I am working with can tolerate it.

42.2 May 20, 2021

The boxwood has multiple new green sprouts beginning extension growth.