August 1996
Trees, whether useful or ornamental, make a big impact on our landscapes. Here is some research from various horticultural journals to help you care for your trees.
Transplant size affects the tree's ability to survive and its growth rate after transplanting. Smaller trees will grow more quickly after transplanting than larger trees, and when mulched or cultivated to prevent competition from grass and weeds are likely to survive transplanting better. On the other hand, larger trees (comparing unbranched whips to two-inch caliper trees) survive better in situations where competition and chances of damage from maintenance equipment, vandals, or animals are high. Despite the faster growth rate exhibited by smaller trees, it may be some time before they catch up in size to larger transplants. You will have to weigh the various size advantages yourself and make a selection to suit your landscape needs and budget.
When transplanting container-grown trees, inspect the root ball carefully. If the trees have filled their pots with tangled roots, they are root bound and research has shown that slashing the sides of the ball stimulates new growth. If they are not root bound, research shows that slashing the root ball has no effect on the subsequent growth of the tree, and you can skip that step.
For balled and burlaped trees in wire baskets, traditional recommendations are to remove the basket. This may be unnecessary as studies of weeping willow roots show they grow around and engulf the wire, forming complete vascular unions. It's not a bad idea to go ahead and cut away as much of the wire basket as you can, but don't risk damaging the roots by wrestling a recalcitrant basket off the root ball.
Speaking of roots, it is important to know just how few of a tree's roots are dug when a sapling is moved. According to one study, for 6-year-old ash, poplar, locust, and probably other species, up to 95 percent of the root mass is left behind during transplanting. Roots extended up to 3 times the dripline radius from the tree. Root prune prior to transplanting to stimulate root growth. This will result in a higher percentage of total roots within the root ball. Most of the new growth is initiated closely behind the cut, so the ultimate root ball should be 4 to 6 inches larger than the root-pruning circle.
Whether or not to amend the soil that fills the planting hole has been a hotly debated question over the last few years. A recent study found that amended backfill produced no significant differences in plant growth over unamended backfill when the native soil had good drainage and tilth. However, when the native soil was compacted clay subsoil, amending the backfill produced either similar or LESS growth than using the native soil alone.
In the same study, mulching and irrigation produced dramatic increases in growth, and digging a hole twice the size of the control significantly increased growth as well. Home owners have frequently been told to dig a "five-dollar hole for a fifty-cent plant." At one time, it was recommended to spend the money on peat moss or other organic material to amend the planting hole. Today's five-dollar hole should consist of a large planting hole, a thick layer of mulch, and an investment in drip irrigation or other reliable methods to get water to the plant.
On the other hand, amendments that alter soil pH can have a positive effect. Comparing sulfur alone, peat moss alone, and sulfur and peat moss combined, researchers have found bare-root flowering dogwood had most growth when planted in a soil amended with both. The combination lowered the soil pH below that of either single amendment, which is probably what caused the improved growth. For best results, remember to test your soil before planting and adjust the pH to suit your desired species, or change species.
Most people select a species for planting based on size, form, and ornamental features. One more factor to consider is rate of wound closure. Choose quick healers for positions where trees are likely to incur damage from children, pets, lawn mowers, etc. There is not much data currently available, but one study reports the following ratings in order of fastest wound closure: green ash, Norway maple, American sweetgum, Higan cherry, southern magnolia, honeylocust, red maple, sycamore, rover birch, Bradford Callery pear, and silver maple.
Another area in which there is not a great deal of data, but which is an up-and-coming horticultural research area, is the use of mycorrhizal fungi in plant culture. One recent study showed that mycorrhizae (beneficial fungi associated with plant roots) can ameliorate the affects of air pollution on the growth of loblolly pine seedlings. Already there are commercial products available to the home gardener claiming to contain mycorrhizae; they may just work. However, there is little known about them as yet, so you might want to wait for more information before investing your dollars.
(Originally published as "Research Roundup: Trees in the Home Landscape," by Ellen M. Silva, Extension Technician, Department of Horticulture, Virginia Tech, in The Virginia Gardener Newsletter, Volume 9, Number 4.)