Q: My mugo pine gets eaten every spring. It ends up losing all its old needles from some kind of caterpillar. What can I do to keep this from happening this year?
A: The pest eating your pine’s needles each spring is the European pine sawfly. While this critter looks like a caterpillar, it is more closely related to bees and wasps. This pest overwinters as eggs, laid in needles last year. The needles have yellow bands where eggs have been laid, and can be removed if noticed in winter.
Once the eggs hatch (around the time flowering pear is in full bloom), the young larvae begin to feed on needles. They are unable to eat all the way through the needle, and leave brown, curly strips of uneaten tissue. These brown needles, just under this year’s new growth, help to detect the presence of this pest. Look for brown needles on mugo pines in the next few weeks to locate sawfly populations.
European pine sawflies mass together to eat, and will raise their bodies in unison when disturbed. Otherwise they blend in with the plant, making them difficult to see. Larvae will completely strip the plant’s older needles, leaving new needles untouched, giving the plant a “poodle cut” look. Sawflies are easily killed with horticultural oils, insecticidal soaps, or traditional insecticides, or simply knock them into a bucket of soapy water.
Q: My neighbor grows a low ground cover with small white flowers. It grows well in shade, and spreads nicely in her garden. I’ve enclosed a picture. What is it?
A: The shade-loving ground cover your neighbor is growing is sweet woodruff (Galium odoratum). Sweet woodruff is a shade-loving ground cover native to Europe that has been growing in North American gardens for over 100 years. Beloved because of its carefree nature and reliable spring bloom, sweet woodruff spreads to form drifts of light green leaves topped by airy sprays of white flowers in spring. In shady, moist sites, this vigorous plant will spread to form a dense ground cover, but it is easily removed if it oversteps its boundaries. Sweet woodruff is also tolerant of dry shade, once established.
Q: I have a tree peony with growth coming up from the base. What is it, and what should I do with it? Can you tell me more about how to grow this plant?
A: Tree peonies are woody plants that are grafted onto herbaceous peony rootstock. Oftentimes, the rootstock sends up shoots; these will not be woody, and will divert energy from the grafted top portion of the plant. Prune out any shoots coming up from the base.
Tree peonies are highly valued for their large, showy spring blooms in shades of white, pink, red, purple and yellow. They perform best in deep soils high in organic matter, in areas with high dappled shade. Plant tree peonies in fall, with the bud union about ¾ inch underground. Mulch well before winter. Use tree peonies as specimen plants in shady perennial borders.
Q: I have a little pine shrub in my front yard with little white specks on the leaves. I noticed a few last year, and this year I see dozens of these white specks. What kind of disease is this, and what should I do?
A: Scale is an insect pest that damages plants by removing plant sap from its host. Because scale insects do not move once settled, some gardeners confuse this pest with a plant disease. Pine needle scale spends the winter as eggs, protected underneath dead females. Juvenile scale, called crawlers, are the active stage of this pest. At the time when common lilacs are in full bloom, the crawlers hatch from eggs.
Scale can spread from plant to plant in the crawler stage, moving by wind or sometimes birds or mammals. Once in their new location, the crawlers settle, insert their mouthparts, and begin to form protective armor over their bodies. This armor protects scale from most insecticides.
Pine needle scale can cause significant damage over time by removing plant sap from the needle tissue. A second and sometimes third generation can be produced in only one season. To manage scale infestations, the best option is to control the crawlers. They are easy to kill with insecticidal soaps or horticultural oils. Scout for pine needle scale crawlers when lilacs are in full bloom, then wait about 10 days to apply oil or soaps. (Do not use oils on conifers with blue needles, because the oil will remove the waxy hue.)
Denise Ellsworth directs the honeybee and native pollinator education program for the Ohio State University. If you have questions about caring for your garden, contact her at 330-263-3700 or click on the Ask Denise link on her blog at www.osugarden.com.