''Fall is for planting.'' That was the highly visible slogan when US and Canadian nurserymen met here for their annual convention - and they weren't referring only to spring-flowering bulbs. For many trees and hardy shrubs, fall is also the best of all planting seasons.
Recent basic research by a team of Cornell university scientists confirms this fact. In consecutive seasons, George Good and Tom Corell planted a variety of woody plants and perennials on the 21st days of August, September, October, November, May, and June.
Only those plants set out in late November suffered any winter injury. For the rest, the fall-planted specimens grew more vigorously than those planted the following spring.
What makes fall such an ideal planting season? A principal reason is the contrast between soil and air temperatures at this time of year. While cool air temperatures retard unwanted new top growth that would be vulnerable to winter weather, warm soil boosts root development which is needed to support the plant through winter and set it growing vigorously in the spring.
The soil, in fact, acts like a great bank vault, but instead of storing money it stores up much of the excess heat of summer, holding it for many weeks after air temperatures have fallen.
The Cornell study found that roots grow as long as soil temperatures remain above 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 cutoff date for fall planting is seven weeks after the first fall frost.
Those late-November plantings that suffered winter damage at Cornell's Long Island trials went in a little too late to allow for good root development. Even in the depths of winter the roots continue to supply small amounts of moisture to trees and shrubs, hence the necessity for an established root system once the deep-freeze weather comes along.
Not every winter-hardy specimen can take fall planting. Fruit trees that can be fall-planted in the South should not be set out at this time of year in the North. Ann Reilly, speaking for the American Association of Nurserymen, notes that the magnolia, tulip tree, black gum, golden rain tree, and several species of oak do not transplant well in the fall in the North. Consult your local nurseryman or extension service on what specimens can be safely transplanted at this time of year in your region.
If you want to set out a new specimen in your garden this fall or perhaps transplant from one area to another, take these steps:
* Dig a planting hole three times the size of the root system of the tree or shrub that you wish to set out.
* Unpack the nursery-bought tree and soak the roots in a pail of water for 6 to 12 hours. If you are moving a tree from another area of your garden it will help a good deal if the tree is watered deeply the day before transplanting.
* Fill the hole one-third with sphagnum peat moss, a handful of ground limestone, and about half a cup of bone meal. Or simply add the same volume of compost or aged manure. Fill with water and let it soak away.
* Set the tree in the hole and partially fill with soil, tamping it down firmly around the roots. Add more water and let it soak away.
* Finish filling the hole, making sure that the tree is at the same depth in the soil as it stood in the nursery or wherever it previously grew. Firm the soil to leave a saucerlike depression around the tree, which will collect rainwater.
* Water again and add a 4-inch mulch of wood chips, bark, or shredded leaves. This not only helps retain soil moisture but also traps soil heat, giving the roots that little bit longer to become established.
* Protect the trunk of the tree with a wrapping of burlap or any one of the commercial products available at a nursery.
* Finally, water regularly once a week during the first year unless, of course, good rains have fallen.