Hemlock tree is one of the most useful and beautiful of our evergreen trees, both in the garden and the forest. Among its many virtues is the grace of its soft foliage, rich color, and ability to be sheared as a hedge. Its shade tolerance is shared only by arborvitae, a much less elegant tree.
Landscaping with Hemlocks
Hemlock trees can be grown easily if you consider their various needs when planting them. It’s not so important to know how to plant a Hemlock tree, but where to place them. Hemlocks evolved in the shade of large landscape trees.
Therefore, you will need to find a protected area, particularly against winter winds and summer heat. Although hemlocks are tolerant of various lighting conditions, they won’t tolerate compacted or dry soils.
Many hemlocks are available for gardeners to choose from in USDA plant hardiness zones 3, 7, and 8. However, some cultivars do not thrive in this range. Would you mind reading the nursery tag before you bring your hemlock home?
Taking Care of Hemlock Trees
Hemlocks are easy to care for once they have been established. However, it cannot be easy to establish them. They require acidic soil that is moist but not too wet and frequent waterings. Hemlocks are riverbank trees like willows.
If your site is high and dry, you might need to place a thick mulch ring over the root zone. You may also wish to install a drip irrigation system.
High winds can cause hemlock to fall easily due to its shallow roots. If this happens, you should consider a stabilizing tree stake while the tree is still young.
Hemlock Tree Pests
We have watched with dismay the invasion of a tiny sucking insect called the woolly adelgid (pronounced a-DEL-jid) that threatens this valuable tree for the past decade.
March is a good time to inspect for the white egg masses that look like tiny cotton balls and, if found, to make decisions for both short-term and long-term care, or no care, for that is also an option.
Let’s start with the bottom line. You can spray with horticultural oil to eliminate the current insects, repeating it as the trees become reinfested. You can use imidacloprid, a soil-applied systemic insecticide (Merit is the trade name).
It is very slow-acting but may last a year or so. You can try to maintain the tree’s health, so it can withstand the insects by watering, mulching, controlling other insects, not shearing, and not applying nitrogen fertilizers.
You may do some or all of these for very valuable or specimen trees, but it’s not practical to treat the whole forest.
More Information On Woolly Adelgid Insect
Now some details for those of you who want to know more than these bare basics. The woolly adelgid, originally from Japan, was first discovered in Virginia in the 1950s.
By 1985 it had reached Connecticut in its spread along the East Coast; in the 1990s, the Massachusetts border was breached, and today most of New England and much of the South is its domain.
The wee black bug is almost invisible without a magnifying glass. Still, its woolly egg cases are easily seen in late winter and early spring on the undersides of the young foliage at the ends of hemlock branches.
Though the insects’ life cycle is complex, they feed and lay eggs over the winter and then eat and grow until about July, when they become dormant until November.
Control is by spraying with horticultural oil, covering both the undersides and tops of branches. (The oil concentration is lower in spring and summer when there is new soft growth than when plants are dormant.) Except for small specimens, it’s a job better left to tree professionals. And therein lies the rub.
It can get expensive for the repeated sprayings that may be needed over time. Soap sprays work too, but not as well as oil.
The new systemic insecticide, imidacloprid, holds some promise to control many sucking insects and some small chewing ones, the adelgid included.
To apply, follow the directions on the bag and use adequate quantities. Work it very lightly into the soil’s surface (hemlocks are shallow-rooted), then water it in very well and mulch to keep it and the roots moist.
Hemlock Tree Infestation Study
Less toxic than many insecticides, Merit’s drawback is that it has to be absorbed into the needles to be effective, and that takes weeks, maybe months. However, its effects may last through next winter.
For many plants, the best way to use imidacloprid is still being tested. My own suspicion is that it might be good to apply for hemlock adelgids right now, so it can become effective before the end of June.
Then keep it well watered throughout the summer to catch the renewed November feeding.
Normally when a tree is attacked, it takes four or five years until it dies.
First, the crown thins, needles turn yellow, and eventually, its only use is lumber or firewood.
However, the 10-year study found that many infected trees did not die, and the reasons give us clues as to how to provide some protection for our own trees.
The forest hemlocks that survived were on the east- or north-facing slopes, which are cooler and moister than hot south or west-facing exposures, were in good lowland soil as opposed to high, thin, bony ridges, and were not weakened by other environmental insults or insects like the hemlock looper.
What this means at home is you should keep hemlocks well-watered during summer. Perhaps mist them during dry heat waves.
Expect those in a shaded location in good soil to do better than those in full sun.
If planting new ones, carefully choose the best location or choose a different tree.
We know that nitrogen only makes for healthier adelgids that lay more eggs, so eschew the fertilizer.
And since these pesty foreigners are spread by wind and birds, keep the bird feeders well away from your hemlocks.