When valuable evergreen shrubs turn brown, don't panic. Many evergreens drop needles naturally in fall and require no treatment. The browning or loss of needles during spring or summer, however, might indicate a serious problem.
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In fall, healthy evergreens drop older, inner needles. During unusually warm and dry summers, the condition may be more noticeable. Andy Beck, horticulture extension educator at Penn State University, suggests leaving the needles in place as mulch.
Drought and Winter Burn
Because evergreens retain their needles all winter, the plants continue to lose moisture through the needles. The frozen ground does not supply enough water to compensate and the plant dehydrates. To prevent winter dehydration, water shrubs until the first hard frost and as soon as soil temperatures reach 40 degrees Fahrenheit.
The hemlock woolly adelgid is an introduced insect that attacks hemlocks in the northeastern United States. It sucks fluids from the tree and causes needle loss and dieback. Look for white, cottony egg sacks. If you find them, treat the tree with a horticultural oil or an insecticidal soap immediately.