Hemlock often under-utilized in landscaping
The Canadian or Eastern hemlock (Tsuga canadensis) is, in my opinion, an under-utilized landscape tree. Perhaps they are spurned as too common, or rejected because they get too big. But they have some traits that should endear them to homeowners: they will grow in full sun or full shade; they have handsome foliage all year round; they are easy to transplant and are relatively quick to grow. Their sole liability is their susceptibility to an insect pest that is common in southern New England, the woolly adelgid. More on that later.
Canadian hemlocks are tough. They will survive minus 35 degrees, grow on rocky hillsides, and require little in the way of fertility in the soil. They do best in slightly moist acidic soils, but are tolerant of somewhat alkaline soils. They are not city trees, nor do they like hot, dry conditions. But for most of northern New England, they are happy most anywhere.
Hemlocks make good screens. Plant a row of four-foot trees 3-6 feet apart, and in 10 years or less they will fill in nicely, blocking the view of your neighbor's yard and eventually the house, too. The taller you want your hedge or screen, the farther apart you should plant your trees.
Tree books say hemlocks make good hedges, but in reality they often get away and grow too tall. Yes, if you are willing to prune them every July, hemlocks make good hedges. But since they are trees, not shrubs, and their natural size approaches 50 feet in 20 or 30 years, you must keep them in bounds by annual pruning. But skip a year or two, and — oops — they get bigger than the six or eight feet that is manageable for pruning.
Birds are fond of hemlocks, too. According to "Trees, Shrubs and Vines for Attracting Birds by Richard M. DeGraaf, Canadian hemlock is used by more than 25 species of birds for food, nesting or cover. Ruffed grouse eat the needles, while the seeds are the preferred food of pine siskins, American goldfinches and boreal chickadees. Nearly 20 kinds of birds nest in them, including red shouldered hawks and four kinds of warblers. Their dense foliage provides good places to hide from predators and to break cold winter winds. For people who like weeping trees, there is a variety of hemlock that fits the bill. 'Sargentii' is a named variety that has been reproduced by grafting from a tree that was discovered in 1870 near Fishkill Mountain in New York state. Sargent hemlocks get to be 10 to 15 feet tall, and can spread to twice that width. Trees grown from the seeds of these weepers will vary considerably, so buy a grafted one if you want to be sure it will meet your expectations.
"The woolly adelgids are coming! The woolly adelgids are coming!" That could be the cry of a modern Paul Revere — and with good reason. Woolly adelgids can kill a hemlock tree in just a year or two, starting with desiccation and yellowing of branches the first year. They are already a major problem in southern New England, and have appeared in small numbers in New Hampshire, Vermont and Maine. Since hemlocks are prominent in our landscape, foresters, nurserymen and landscapers have been highly vigilant and have been able to keep major infestations from occurring up here, though our climate may help to control them, too. Now is the time of year to be on the lookout for these critters.
These culprits are tiny guys — roughly the size of aphids. They suck sap from the trees at the base of the needles, particularly on young branches, and may introduce a toxin from their saliva. They secrete a white, woolly substance which is visible at this time of year, particularly on the underside of branches. Females overwintering on hemlocks deposit about 50 eggs each in white woolly masses. These eggs hatch in early spring and can be spread by wind or on the feet of birds.
Control of woolly adelgids is critical. According to Doug Cygan, the Invasive Species Coordinator for the N.H. Department of Agriculture, woolly adelgids need not be present in large numbers to be a problem. Sometimes only one small tuft is present on a tree, but the adelgids can spread if not killed. If you see what you think might be a woolly adelgid infestation, Cygan advises calling for help. Your local Cooperative Extension office is a good place to begin. He warns do not touch the egg mass or insect, as you might destroy it before a proper identification can be made.
In most cases, a tree will need to be cut down and burned if adelgids are found, but small landscape trees may be treated with horticultural oils by a professional. Cygan noted that spider egg nests can resemble woolly adelgid egg masses, so don't panic if you see something you think is an adelgid infestation.
If I were to compare our nearby forests to a symphony orchestra, the hemlocks would be the woodwinds: beautiful and essential, but rarely featured. I like them best as background, and hope those pesky adelgids can be kept at bay.
Henry Homeyer's new book is "The New Hampshire Gardener's Companion: An Insider's Guide to Gardening in the Granite State." Write him at gardening.guyvalley.net or P.O. Box 364, Cornish Flat, N.H. 03746.