Volume 14, Issue 47 ~ November 23 - November 29, 2006

The Bay Gardener

By Dr. Frank Gouin

Plant In Ground or Freeze Your Roots Off

Popular shrubs don’t fare well during cold weather in above-ground planters

Gardeners who plant trees and shrubs in above-ground containers in the spring shouldn’t be surprised to find them dead the following spring. That’s because roots of plants are less cold hardy than stems, branches and leaves.

If you grow trees and shrubs in above-ground planters, choose species with roots that can tolerate your region’s minimum winter temperatures. Pine trees, most junipers and spruce trees can tolerate temperatures down to negative 14 degrees, while the roots of boxwood, Japanese holly, Chinese holly, magnolia and privet can only tolerate temperatures of approximately 20 degrees. Do not assume that these species’ popularity in Maryland landscape means that they can survive in above-ground planters.

It’s the temperature of the soil that makes all the difference in survival between plants growing in the ground and those planted in raised containers. In raised planters, the temperature in the rooting medium typically matches the ambient air temperature by two to three o’clock in the morning. If the soil is dry, this equilibrium will occur much faster than if the soil is moist. The smaller the container, the faster the change in temperature, which kills the roots sooner if the roots of the plants are not sufficiently cold hardy.

In a landscape, soil temperature seldom drops below 28 degrees, measured six inches below the soil’s surface. Underground temperatures stay warmer because of earth’s limited amount of exposed surface area. The earth is constantly generating heat from below.

In raised planters, on the other hand, both the top and walls of the container are exposed to changes in temperature. Only the bottom of the container remains in contact with the ground, which isn’t warm enough at the surface to supply heat to the planter. Come early-to-mid-winter, the planter’s soil will become as cold as the outside air.

Creative gardeners have tried packing insulation around above-ground containers without success, because perfect insulation doesn’t exist. The only way to over-winter root-sensitive plants growing in planters is to line the inner walls with heating cables to prevent the soil from freezing. Such planters also must stay irrigated during the winter months, because dry soil freezes faster than wet or moist soil. Water warms the soil up because as it freezes it generates heat, called latent heat of fusion, which is absorbed by the soil.

On Oak Transplanting and Topping

Q We have two descendants of the Wye Oak in our Cedarhurst neighborhood and a couple of questions.

One was planted as a seedling and is now about 15 feet tall. It blew over to about a 45-degree angle during Ernesto but did not uproot. Its owner tried to right it within 24 hours, when the ground was still soggy, but it wouldn’t budge. She has asked tree experts for help but they didn’t give her a good answer.

The second tree was also a mail-order seedling. It was planted in a yard and grew to be about eight feet before the house changed hands. The new owners did not want that potentially big of a tree in the front yard, and they paid to have it moved to the community’s property. The tree seems to be doing well, overall, but it has a few dead branches on one side in particular. Should we give it a haircut?

—Pam Foster, Cedarhurst

A It is nearly impossible to upright a tree that has been pushed or blown over. For some reason or other, root development on that side of the tree will never be the same, even if you were able to bring the plant upright. My recommendation would be to either leave it as it is or cut it down.

With regards to the tree that was transplanted and has dead wood, I recommend that you only prune the dead branches. Never give trees a haircut. It not only ruins their appearance but it makes trees less wind resistant, increasing their chances of being blown over.

Ask Dr. Gouin your questions at frgouin@erols.com. All questions will appear in Bay Weekly. Please include your name and address.

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