BOSTON — AS part of an ongoing attempt to educate gardeners, nursery owners and landscape services often mount ``fall is for planting'' campaigns this time of year. By that they don't mean planting only tulips, daffodils, and other spring-flowering bulbs. Fall, it has been established, is also a great time to plant trees and other woody perennials.
Research at Cornell University in Ithaca, N.Y., by George Good and Tom Correll brought this to light a little over a decade ago. Previously, fall planting was considered appropriate only for the warmer, more southerly latitudes but not for any region where the soil might freeze.
Then, in an experiment, the Cornell scientists planted a variety of woody perennials on the 21st day of August, September, October, and November one year, and again in May and June the following year.
Only the plants set out in late November suffered any winter injury. The rest of the fall-planted specimens grew more vigorously than those planted in the spring.
Apparently the contrast between soil and air temperatures during the cooler months makes fall an ideal transplanting time for perennials. While cooler air temperatures retard unwanted top growth like leaves that would be vulnerable to winter kill, the still-warm soil is ideal for root development.
In fact, it has since been found that the principal root growth of all perennials takes place in the fall.
The original Cornell study found that roots grow and develop as long as the soil remains above a temperature of 40 degrees F. and that woody plants can be set out up to four weeks before the soil drops to 40 degrees. As a rule of thumb, the cut-off date for planting is seven weeks after the first fall frost.
A mulch of organic materials spread over the root area of the newly planted tree or shrub will extend this root-growing period by slowing down the loss of soil heat to the much colder air.
The late November plantings that suffered winter damage in the Cornell trials went in too late to allow for new root development before the bitter cold set in. Even in the depths of winter, roots continue to supply small amounts of moisture to trees and shrubs, hence the need for an established root system before the deep freeze comes.
BUT there are exceptions to this fall-planting rule. Fruit trees, routinely planted at this time of year in the South, do not take readily to fall planting in the North nor do trees with deep-probing tap roots.
A cardinal rule for success with perennials is to see that they enter winter with adequate moisture in the soil. If generous rains haven't fallen, water well before the soil freezes.
Fall is also a good time to fertilize perennials, including trees, once the leaves have fallen. At this stage, all top growth has stopped so the still-active roots grab onto the fertilizer, storing it for use early the following spring.