Pruning stimulates growth of roots, too

Several years ago, while working at the University of Massachusetts, I would take my horticulture classes to Stewart’s Nursery & Garden Center in Turners Falls, Mass.

On one of these visits we watched from the edge of a field as one of the workers dug 6-inch caliper (trunk diameter) maple trees with a tree spade.

From the seat of a tractor, the operator manipulated the three large spade-shaped blades to encircle a trunk and then drove the blades into the soft soil.

With the push of another lever, the tree was lifted out of the ground in a 40-inch-wide ball of soil. But instead of lowering the tree to the ground next to the hole for wrapping in burlap, the operator put it back into the hole, moving on to the next tree in the row.

We did not understand what was going on and asked our host, Jim Stewart.

“Root pruning,” replied Jim. “This is a lot quicker than digging a trench around each tree and just as effective in producing a root ball full of roots. We will leave these trees in their holes for another year to allow the roots to grow, then harvest them for sale.”

I explained to my students that a tree’s roots grow at least twice as far from the trunk as the drip line of the canopy. Even with a mechanical tree spade that can dig and lift a 40-inch ball, the root system of such a large tree would be cut in half by the digging.

But the pruning would stimulate branching of the roots that remained in the ball and, over the course of the next year, the ball would fill with new roots. The process we were watching would produce a tree with a much better chance of survival after planting than if it had been sold right after digging.

When transplanting trees in the home landscape, a similar root pruning process can make the difference between an attractive, healthy plant and an unsightly, declining tree. This is particularly true with those species that are considered difficult to transplant, such as paper birch and buckeye.

Root prune this spring, transplant next year. Using a sharp spade, cut a narrow trench completely around the plant, creating a large soil ball. The soil ball for a 1-inch-diameter tree should be at least 16 inches in diameter while 24 inches is adequate for a tree with a trunk 2 inches in diameter.

Larger trees are best left to the professionals and their tree spades. The trench does not need to be more than a few inches wide, but it should be at least 15 to 24 inches deep in order to severe most of the roots.

Trees 1 inch or less in diameter can be dug the following year by hand. Working from the trench dug the previous year, use the spade to undercut the soil ball at the same depth as the trench, at least 15 to 24 inches.

Once the root ball is dug, work a burlap sheet under the ball by gently rocking it back and forth in the hole. Wrap the burlap around the ball to help hold the soil together as it is lifted and moved.

Keep in mind that even a 16-inch soil ball is very heavy; it may take several strong helpers to lift and carry the plant to its new site. And always lift and carry a tree by the root ball, never by the trunk.

Send queries to Gardening Questions, P.O. Box 418, Ellsworth 04605, or to reesermanley@shead.org. Include name, address and telephone number.