DEAR JESSICA: Two of my beautiful Emerald Green Arborvitaes have turned yellow, and we think it’s spider mites. We have a row of about 15, so we’re trying to quickly come up with a plan to defend the others. My dad, who lives in New Hyde Park, said he read a great article you wrote about the topic. I’ve searched online but can’t locate it. Can you help?
— Susan Reische,
DEAR SUSAN: There could be a few reasons your trees are turning yellow. Inspect them after reading my descriptions below, and treat them accordingly. The good news is that regardless of which is to blame, the condition in most cases should be reversible.
Healthy arborvitaes experience yellowing on their inner branches in autumn before those needles turn brown and eventually fall off. This is normal behavior. One telltale sign is that the yellowing and browning occur evenly on upper and lower branches. If this is the case, especially if trees are in their second year, there’s no need to act.
Yellowing also could be due to other causes, such as leaf scorch, nutrient deficiencies, scale or leafminer attack, as well as spider mites.
Leaf scorch, caused by hot, dry weather during summer, is easily remedied with regular, deep watering and a 3- to 4-inch layer of mulch to retain soil moisture. In this case, you would have noticed the yellowing begin at branch tips and move inward.
If needles are yellow and stunted, your trees might be suffering a nitrogen deficiency; if tips turn reddish brown and die after yellowing, soil might be low on potassium. But there’s no way to know either without a soil test. I recommend you call the Cooperative Extension to arrange for soil analysis.
Fletcher scale, also known as arborvitae soft scale, is made up of flat, oval, yellowish crawling insects that mature to a beige-tan. They threaten the health of the plant because they suck chlorophyll out of needles and excrete a sticky substance called honeydew, upon which sooty mold grows. The mold can attract ants, which often are mistaken as culprits in the plant’s decline. Scale should be treated with applications of dormant oil in early spring before new growth begins but after the danger of frost has passed. Follow package directions closely.
The arborvitae leafminer starts life as a caterpillar before developing into a small tan moth that lays its eggs between leaf scales. Larvae overwinter in the plant and then tunnel, or “mine,” into foliage for feeding. Then the cycle starts all over again. The resulting damage presents as brown leaf tips, first noticed in late winter or early spring. If that’s what you’re seeing, the good news is that affected trees can be saved as long as at least 20 percent of its foliage has survived. If you’re sure trees are infested, apply acephate or spinosad twice: in mid-May and again in August. It would be best to get a diagnosis confirmation from your local Extension office before proceeding, however.
Spruce spider mites also suck juices out of plants, which results in yellowing branches. Infestations aren’t as common, but when hit hard, trees can die as a result. They’re most active at the beginning and end of the season, from May to June and again from September through frost. Mites are easily knocked off trees with a hose-end sprayer, but effectiveness is related to timing: To check if mites are active, hold a white sheet of paper under branches and tap them to loosen any insects. If present, you’ll see moving pinhead dots on your paper. A few aren’t cause for concern, but if they are larger in numbers, get the hose. A covered paper implies a serious infestation, for which you should apply Neem oil, horticultural oil or insecticidal soap, all of which are very effective natural alternatives to chemicals.
DEAR JESSICA: I just read your calendar entry about pruning my roses, but when I ran out to do it, my roses had buds already! Do you want me to cut down the roses with all the new buds on them? My roses start blooming in June and throughout the summer and fall. Please let me know what to do; I’m confused.
— Ellen Hribok,
DEAR ELLEN: You don’t say what type of roses you have, but since they bloom all summer, that’s an indication they are the type that continually produce buds during the growing season. You wouldn’t need to prune as a matter of course, but if they’re overgrown, overcrowded or have crisscrossed canes, it would be fine to remove or cut them back, even if they have buds on them. Overall, the plant will be healthier and more attractive, and will bloom on the remaining wood.
DEAR JESSICA: My husband would like to know when he should plant his dahlia bulbs, which he has stored all winter.
— Sandy Lauper,
DEAR SANDY: Your husband should plant his dahlias outside around Memorial Day, in a spot that gets at least six hours of sunlight daily. To give them a head start, he can start them in pots indoors, kept by a sunny window, about a month before transplanting time.