Beginning in the late '60s, Carl Whitcomb, then professor of horticulture at Oklahoma State University, began a decade-long series of trials and studies involving hundreds of species of trees and shrubs. With the help of his students he grew, dug up, measured, replanted, and remeasured thousands of specimens, measuring root growth, trunk expansion, and shoot and leaf development. It involved a good deal of painstaking effort that ultimately paid off in some fascinating new knowledge about trees and shrubs and, in particular, their roots.
Dr. Whitcomb found, for instance, that trees and shrubs grow most of their new roots - better than 80 percent, in fact - in the fall of the year, which gives added gardening importance to the weeks that lie ahead when transplanting, watering, and even feeding can be advantageously undertaken.
It turns out that there is a pecking order for nutrients in woody plants that doesn't vary from year to year: Flowers and fruits get first choice, followed by the leaves, then the stem, and finally the roots. For instance, the trunk or stem does not increase in size until late summer.
``Then, all of a sudden, it puts on a burst of growth,'' says Whitcomb. After that, the bulk of all nutrients go into putting on new root growth.
Nearly all root growth takes place in the top 12 inches of soil, spreading out well beyond the circumference of the tree. Tree roots grow rapidly when soil temperatures are in the 50s and 60s degrees F., and continue growing down into the mid-30s.
Among other things, Whitcomb's trials also showed that there is no need to add amendments (compost, peat moss, etc.) to the planting holes when setting out new or transplanted trees. In heavy soils, this can even be a distinct disadvantage. If soil quality in the hole is markedly superior to the surrounding area, it encourages a type of ``pot culture'' in the ground.
Yet another controversial discovery: It is best not to prune either the top or the roots of a tree when it is being transplanted, other than to remove damaged material. There simply is no need to ``balance'' top with bottom - as his research showed when the unpruned trees grew more vigorously in the long run.
Because many of these discoveries flew in the face of accepted industry practice, Whitcomb was at first disbelieved. Several people set out to prove him wrong, but as the years passed no one was able to do so. Those who tried his methods found them effective, and in a recent interview Whitcomb confirmed that everything he has done since the period of intense research ``reinforces those earlier findings.''
Here are some of his thoughts and suggestions:
Planting: Fall is a good time to move trees or shrubs from one area of your garden to another.
Dig a hole a few inches wider than the root ball and no deeper, so that the stem is at the same soil level as it was in its previous position. Fill in with unamended soil from the hole you have dug and firm this by watering. Don't stamp down the soil with your feet, as this can create compaction in heavy soils.
Soil aeration: After planting, loosen the soil away from the hole so that the new roots can move readily into the surrounding area. Do this by pushing a spading fork into the soil every six to 12 inches and gently rocking it back and forth. Do the same thing with established trees if you suspect the soil is compacted around the outer perimeter of the tree. Some minor root damage may occur, but this will be quickly overcome in the fall period of rapid root growth.
Feeding: Late fall, after the top has gone dormant, is a good time to feed tree roots. The soil is still warm, so the roots readily absorb the nutrients and store them there, ready for release to the rest of the tree in the spring.
In contrast, cold soils retard the takeup of nutrients in the spring. But, Whitcomb says, do not feed the trees before they go dormant or you may encourage new top growth that will be killed off in the winter. Spread the fertilizer beyond the outer leaves up to a third of the tree's overall diameter.
Soil amendments: While these shouldn't be added directly into the planting hole, in Whitcomb's view they can be spread around the tree as a mulch where they are slowly assimilated by the top layer of soil where most of the roots thrive.
Watering: Be sure your trees and shrubs do not go into winter in dry ground. If adequate rain hasn't fallen, water them deeply on several occasions before the soil freezes.
For more information on Dr. Whitcomb, write Lacebark Publications, Rt. 5, Box 174, Stillwater, OK 74074.