A brisk north wind is blowing, straight off Hudson Bay by the feel of it, and Lewis Hill, a nurseryman, expects a late spring frost. He is used to late frosts. Often the weather station atop Mr. Washington is the only place in New England reporting colder weather than his Hillcrest Nursery here in the Green Mountains of Vermont. The question is: How will the blossomfilled trees in his orchards take it?
A heavy frost will damage the crop; a light one may actually help by taking out some of the blossoms. This brings Mr. Hill to a favorite topic: thinning fruit on heavily productive trees.
Why, he asks, is the gardener who so assiduously thins his carrots and beets so timid when it comes to removing some of the pea-size apples, pears, peaches, etc., from an otherwise overproducing tree? The practice, says the author of "Fruits and Berries for the Home Garden," "Practical Pruning," and other books, "has so much going for it."
These are some of the benefits of thinning:
* The tree puts less energy into seed production and more into fruit (edible flesh) production. While a properly thinned tree will give less fruit in numbers, it will produce more bushels of fruit.
* Fruit production is consistently good year after year (no heavy crop followed by a lean crop or a year of no fruit at all).
* Mature fruit hangs on the tree much better -- until it is fully ripe, in other words.
* Tree vigor is maintained year after year.
* Tree-life expectancy is also extended.
The first of the benefits explains all the others. Seed production requires a high input of energy on the part of the tree; by comparison, edible-fruit production requires only modest energy inputs. With apples, for instance, each fruit, large or small, has 10 seeds. By reducing the number of fruits on the bough, you drastically reduce the number of seeds the tree has to produce. This leaves the tree with energy to spare, which it can put into that part of the fruit we all love best -- the part we eat. The same applies to peaches, large plums, nectarines, pears, and the like.
Trees invariably drop fruit naturally in a year of heavy setting. This generally occurs when the fruit is from pea- to marble-size. Immediately after this natural drop is the time for the gardener to step in and continue the process. Don't wait a few weeks and let the tree put energy into fruit you are merely going to throw away.
With practice you will develop a "thinning style" of your own, Mr. Hill says.
Meanwhile, this is how he and his wife, Nancy, tackle the job: They leave only one fruit to a cluster and no more than one fruit every 6 inches or so on a branch. Obviously, says the tall Vermonter, "you will look out for misshapen or insect-damaged fruit and remove them first."
In the event of a very poor fruit set (a late frost, perhaps) these rules will be bent. Under such circumstances, fruit that has set may be left closer on the boughs, and more than one fruit to a cluster is tolerable.
Mr. Hill contends that it is the larger tree fruits that benefit most from thinning: apples, pears, peaches, and large plums.
"I would never bother to thin a crab apple," he says, as the natural drop seems adequate. The same applies to almost all small fruits and berries. On the other hand, he does recommend restricting mature grape vines to between 40 and 60 bunches.
One final point about thinning: Use it to revitalize an old tree. An old, previously uncared-for tree must be pruned lightly over several years to get it back to the desired shape. While drastic pruning might kill it, drastic thinning will have the opposite effect of channeling all the energy back into the tree itself.