Q: Fall is supposed to be good for planting trees, but November can be very cold here. How late can I plant, or does it truly not matter? — Leanne Seitz, Radnor, Pa.
A: Fall is an excellent time to plant trees because that’s when they’re programmed to do most of their root growing. Trees keep producing roots as long as the ground temperature is above 40 degrees — long after hard frosts arrive. If you want to maximize root growth, planting earlier in fall is certainly better than later, but the truth is you have a great deal of leeway in timing the job. Most trees that are dug — either balled-and-burlapped (with a large clump of soil around the roots) or bare-root — do much better when planted in fall rather than spring. When a tree is dug from the nursery — even with a very large root ball and by the most careful professionals — it loses more than 90 percent of its roots. A bare-root tree loses even more — up to 95 percent. These trees need to regrow a lot of roots to survive. In the spring, root growth slows down while the tree puts forth new shoots and leaves, but in the fall the tree’s energies are dedicated to root regeneration. So a fall-planted tree will have the beginnings of a healthy new root system in place before the buds open next spring.
There are a handful of exceptions: Oaks, tulip poplars, magnolias, birches, and sweet gums should be dug and moved only in spring. No matter how early in the fall these trees are transplanted, they seem to be unable to form new roots from broken tips. The roots — and thus the entire tree — make a better transition in spring, when the first buds begin to swell and open. If they’ve been grown in containers, however, fall planting is fine for these trees, too. (Container-grown trees don’t lose roots in transplanting, so you can plant them just about any time of year the ground isn’t frozen solid — no matter what the variety). Ideally, plan a fall planting so that a dug tree has at least one month to six weeks of root-growing weather — that is, before the ground temperature falls below 40 degrees. In most years that means you can plant around the first hard-frost date for your area. (Ask your nurseryman or county extension agent.) Another good rule of thumb is to plant when about half the forest trees have developed their fall color. Dig the hole deep enough so that the tree rests about 2 to 4 inches above ground level and wide enough that the roots can extend freely — about 2 to 3 times the root ball’s width. Cut away any burlap. Synthetic burlap won’t rot, and some natural fabrics are treated with a decay-resistant substance to prolong life in the selling yard. If you can’t remove it all, just let it lie flat so the roots can grow out and away.
As in any planting operation, it’s important to water the root zone thoroughly right away, both to settle the soil around the roots and to provide the moisture needed to form new roots. Spread a couple of inches of composted bark, shredded leaves, or any good organic mulch generously over the root area, which will slow the cooling of the soil and retain moisture.