April is not the best time to plant trees in Austin, since hot days may be just around the corner. But it is almost Arbor Day for most of the country, so it seems like a good time to discuss planting techniques. Although following traditional guidelines may work for most situations, I would like to discuss what's happening on the cutting edge of our industry in relation to transplanting container-grown trees.
If you saw my post on root crowns, you know a little bit about the problem we are trying to address. Trees grown in containers are very likely to have multiple problems with their root systems. Most of them stem from standard nursery practices. When a plant is grown in a container, roots will radiate outward until they hit the side of the container, then turn. Some turn downward, but most begin growing around the container until they encircle the root ball. This would not be a problem if it were addressed, but nurseries tend to pull the plants out of one pot and put them into a larger one without any notice of the circling roots. To the extent that the plant can still function, new roots reach the sides of the new container and the problem repeats. By the time a large container-grown tree reaches the consumer, it might have several internal rings marking each of the smaller containers that have housed the plant.Many of the transplanted trees we see today are performing poorly due to these root issues, and I expect that to be the case for many years to come.
Some research has shown that nurseries can easily correct the problem by shaving off the outer edge of the root ball each time the plant is potted up (for example, Ed Gilman in Florida and Brian Kempf in California). This removes the circling roots and starts the tree off right in the new pot. They found no negative affects on tree growth from this practice. Notice this is not the same as the tradition of slicing through the sides of the root ball vertically to break the circling roots--they found no difference between sliced root balls and those left untouched. This is removing the outer soil all the way around and off the bottom. How much to remove will vary based on container size--a one-gallon pot might require only a half-inch slice, while a 90-gallon pot might lose the outer two or three inches. The key is to remove circling roots and leave roots radiating out from the stem.
Unfortunately, most nurseries have not caught up to this research. It is rare in most markets to find a container-grown tree without circling roots at multiple container sizes. This is where the installer comes in. Gilman and others have had some success shaving the largest rootball at planting, but I am going to talk about a more aggressive approach. Some researchers have been turning tradition on its head by washing, dismantling, and reorganizing root systems when they are installed (Appleton, Bonnie Lee 2007. The BareRoot of the matter. American Nurseryman: Issue
10, Vol. 205: 41-46; Flott, Jim 2006. Don’t Plant Trees, Plant Roots. City Trees. Journal of the Society of Municipal Arborists. Vol. 42, No. 2: 32-36). While we used to learn that root balls are delicate and should not be disturbed more than necessary, Appleton has been instructing equipment operators to drop the trees from a few feet in the air so the root ball will be easier to dismantle. Though shocking to some, this practice seems to be leading to better results over the long term.
Flott and others have been soaking the root balls in tubs of water for a few hours, then reaching in to massage out the substrate (nursery-speak for the soil in the pot, which is not really soil but primarily organic matter). Appleton uses a high pressure hose or a pressure washer along with had massaging to work out the substrate. In each case, the goal is to remove as much of the substrate as possible to reveal the root system. Then circling roots are worked free of the ball so they can be spread outward at planting. Though, naturally, the goal is to avoid as much damage as possible, it is surprising how many roots can be lost without penalty to the tree after transplant.
The process from here is really as simple and basic as it sounds. Once the container is removed, we begin washing out the substrate. Our experience is mainly in the spray method rather than soaking, but in either case we get water into the edges of the ball and begin massaging with our hands to loosen and remove the substrate. We like to start on top of the ball, since most container-grown trees have been buried too deeply. Once we begin to define the root crown, we seek out and cut any stem-girdling roots. Only after we have established the proper grade of the plant do we begin working on the sides and bottom of the root system. It can be difficult to get started, but soon enough individual roots start to become visible and we begin tugging gently to see where they grow. Some roots will be broken in the process, but by carefully working around the perimeter, we can gradually dismantle the root ball. It is not unlike untying a big knot.
Once most of the substrate is removed, we generally have well defined flares at the base of the stem with large, loose roots. There is usually still a small ball at the center, but it is much smaller than the original container. Now we know more about how the hole should look. We only need to dig a deep enough hole for this center ball; beyond that point, we dig shallow trenches to create pathways for the loose roots. The tree is inserted into the hole and the native soil is worked back in with water (if we use the soaking method, we can use the tub of water to help work the soil into the ball). Sometimes called "mudding in," this technique helps to work the soil deep into the root ball. Add a little soil and a little water, then jiggle the tree and use your hands to push soil into the gaps. If air pockets are left under the tree, it may lead to root loss or settling that will adversely affect the tree's ability to adapt to the new site. Continue to add more soil and more water until you either run out of soil or the hole is full. Then follow standard mulching procedures (remember, mulch should be three inches thick on the soil surface but SHOULD NOT be piled against the base of the tree (see more here).
This is messy, muddy work. Depending on the site, it may be difficult to wash out the ball near the planting hole without excessive damage to nearby soil and turf. But if you dig the hole first, then wash the root ball out into the hole, the moisture merely helps keep roots hydrated to minimize loss and makes it easier to work the roots into the native soil. Still, it may be necessary to create a staging area for the washing, then bring fully washed trees to the planting sites (just make sure to be efficient--too much time exposed to air will allow more roots to dry out and die before the tree is planted). It is still a good idea to keep a tarp handy to collect the soil that is dug out, which helps to ensure there is enough to backfill the hole at the end. one big benefit to this method is that the hole becomes a wide, shallow dish (read: easy to dig) instead of a narrow, deep pit. Roots are immediately placed in contact with native soil, so transplant shock is minimal, as are problems with stem-girdling roots. Because the roots spread out farther, the tree is more stable and usually needs no staking.