Trees Stress, too

Every fall the older, interior needles on evergreens yellow and fall. This occurs on pines, spruces, arborvitae and junipers, and it's perfectly normal. Each year these plants grow a new set of needles in the spring, and drop their oldest set in the fall.

Needle drop usually occurs in mid-fall. This year the needles on many hemlocks and white pine began yellowing early, in September. Environmental stress caused this early, heavier than normal needle. Stressed plants lose more needles, but as long as the terminal, or current year's growth, is fresh and green, the life of the plant isn't in danger.

Why is it so early and so severe this year? Trees and shrubs generally respond slowly to changes in their environment. They recover slowly, too. Last year's drought (more than eight weeks with no appreciable precipitation in some locations) was very hard on plant material.

Because hemlocks and white pine are shallow-rooted trees, their small, absorptive roots are particularly susceptible to drying and injury during drought periods. Roots injured and or killed by drought stress can take up to two years to regenerate.

This year's weather exacerbated last year's drought stress. In the spring, when soil temperatures reach 45 degrees Fahrenheit, the root systems of most tress and shrubs become active and begin to grow. This period of growth lasts until the plant begins to put out its new leaves. Because of our late winter, this period of spring growth was shorter than normal.

In addition, the long, cool wet spring enabled many plants to put out lots and lots of new leaves. When hot weather finally came, with no warning, many plants had a difficult time sustaining their large leaf canopies. This resulted in more stress.

If you look around, you will see the lingering effects of stress on many trees. Drought followed by a severe winter, a late spring, excess moisture and a short, but intense heat wave has left its mark on many plants.

Evergreens are losing their needles earlier. The leaves on many landscape plants have brown leaf edges. Because of moisture stress during late summer and our short-lived heat waves, some leaves have patches of dead, light-brown leaf tissue running through them.

The outer leaves on many oaks turned brown in August. This was cause by a fungus called Botryosphaeria quercum, which typically shows up on oaks after a period of drought.

What does this mean for the future health of your trees and shrubs? In the grand scheme of things, not a whole lot. Plants for the most part are very resilient. In times of extreme stress, they will die back a bit from the top to protect the remainder of the plant. As conditions become more favorable, they slowly recover.

What can you do to help your plants recover? If we do get a long, dry fall (almost unimaginable, isn't it?) keep them watered until the ground freezes. Wait until next spring, or even the spring after that, to fertilize. Fertilizing stressed plants generally does more harm than good. And hope for a normal growing season next year. That's really what would do our plant the most good.

By Cheryl Smith (Plant Health Specialist) and Margaret Hagen (Extension Educator) University of New Hampshire Cooperative Extension