Like sick toddlers, sick trees can’t tell you what’s wrong. When your Arizona cypress’ (Cupressus arizonica) silver-green sheen starts fading to dull brown, it could be the entirely natural process of older needles dying as new, light-blocking growth interferes with their photosynthesis. Or, your tree may be battling a serious problem. Across its growing range in U.S. Department of Agriculture plant hardiness zones 7 though 11, Arizona cypress may face needle-browning drought, disease or insect infestation.
As drought-tolerant as Arizona cypress is, prolonged dry weather browns its needles. Water the tree three times monthly between April and September during drought, supplying 10 gallons of water for each 1 inch of its trunk’s circumference measured at knee height. The Colorado State University Extension estimates that five minutes of hand watering with a hose and soft-spray attachment set at medium provides 10 gallons of water. Soaking the top 1 foot of soil 3 to 5 inches beyond the drip line with this method takes between five and 40 minutes for a 1- to 8-inch tree. If you’re short on time, substitute a timer-equipped soaker hose coiled around the drip line. During dry winters, water once or twice a month. Adequate watering stimulates new needle growth.
Juniper tip blight (Phomopsis juniperovora) threatens Arizona cypresses during wet springs. If the lower interior branches’ spring growth yellows and browns, suspect blight. Gray cankers on the infected twigs harbor the fungus over the winter and release infectious spores during rainy spring weather. Splashing water or contaminated tools carry the spores to healthy shoots. The disease rarely affects older needles.
To manage blight, prune the browning shoots back to 3 inches below the discolored needles and dispose of them. Rinse your pruning shears in a solution of 1 part household bleach to 9 parts water between cuts so you don't inadvertently spread the disease. Preventive measures include using drip irrigation to keep the foliage dry and pruning trees that are crowding the cypress. Juniper blight's mostly cosmetic damage seldom warrants fungicide control.
Arizona cypresses west of the Rocky Mountains may experience invasions of tiny, black-headed, needle-tunneling green caterpillars. White-mottled, gray or brown cypress tip moths scatter their eggs over the trees' needles in spring. After hatching between April and June, the larvae wriggle into the needles to feast on soft tissue through the winter. What you see is a tree with brown, dried needles scattered among its healthy green ones. Diagnose an infestation by breaking off a brown tip and slicing it open to look for the caterpillars.
To protect your trees from infestation, treat them with malathion insecticide when the adult moths appear in spring. Mix a tank-sprayer solution with 1 tablespoon -- or the label’s specified amount -- of malathion concentrate per gallon of water. Wearing protective clothing, eyewear, socks and shoes, saturate both sides of the needles. Keep the kids and pets away until the tree dries, and repeat the spray every 10 days until the moths are gone. While a young cypress may succumb to heavy tip miner feeding, an established tree replaces the damaged tips after they're pruned.