Joe Lamp'l, host of "GardenSMART" on PBS, advises transplanting trees and shrubs in the fall.

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One of the most common questions I get relates to the best time to plant or transplant trees and shrubs. Any time, as long as the ground isn’t frozen. But you decrease your chances of success if you don’t do it in autumn.

Cooler air is kinder to plant foliage and reduces the chances of an energy-zapping chain reaction throughout the rest of the plant.

This is especially true for the ones that have lost a major portion of their roots from being dug up. In addition, soil temperatures are still warm, which creates an excellent environment for the production of new root growth.

In the fall many plants and trees are entering a period of dormancy. With no need to allocate resources into foliage, plants are transferring all their energy into roots and storing nutrients and resources for the cool months ahead.

By spring, the result should be a well-established root system. Here are a few additional tips to ensure the success of your fall transplants.

Make the hole two to three times wider than the current root ball, but don’t make it any deeper than the plant was growing in its previous environment.

Look for the flare of the trunk near the soil level. In the planting, don’t place the tree to a point where any part of that flare is ultimately covered with soil.

I place transplants in their new environment with up to one-third of the root ball higher than the surrounding soil level. I then taper soil up to cover all the roots and add a generous layer of mulch above that.

Newly disturbed soil has a tendency to settle, and plants growing below grade can easily succumb to root rot or disease. It’s always better to plant a tree or shrub slightly high and allow the area to drain than for a plant to sit in a bowl and collect excess water.

New research indicates you should not amend the hole with additional organic material; plant roots growing in amended soil rarely venture into harder native soil. The long-term effect is a smaller root system, reduced growth and a less hardy plant.

Simply break up the clumps in existing soil and remove the rocks and backfill. Studies show that plant roots growing in only the native soil actually did a better job at establishing and expanding beyond the original hole.

Be sure to lightly tamp or hand-pack the soil around plant roots to ensure good soil-to-root contact. I add water to the hole after backfilling it halfway. Not only does it provide needed moisture, but the extra water also helps eliminate air pockets that could otherwise result in dead roots.

I then water again thoroughly once all the soil is in place.

The final step is to mulch with 3 to 4 inches of organic matter such as shredded leaves, ground bark or straw. Mulch helps retain moisture and keeps soil temperatures moderate.

Winter conditions can be very dry, so water throughout the winter to prevent soil conditions from becoming too arid. Roots are still growing and soil moisture is essential for proper establishment.

Joe Lamp’l, host of “GardenSMART” on PBS, is a Master Gardener and author.