On Canadian Hemlocks

April 24, 2021

Canadian Hemlocks have always been one of my favorite conifers for numerous reasons, despite its many drawbacks. More shade adaptable than most evergreens, it tolerates most soil types except extreme wet feet, has a fairly rapid growth rate, and nice feathery foliage, all making this a nice choice for an evergreen tree in landscapes. In fact, Mike Dirr goes as far as to say, “If I was forced to select but one conifer for my garden it would certainly be Tsuga canadensis.” A pretty strong statement, in my opinion.

Hemlock, unfortunately, have taken a major hit in recent years due to Hemlock Woolly Adelgid (HWA) killing huge numbers of this tree in the northeastern and mid-Atlantic regions of the U.S. In landscapes, control of HWA is not terribly difficult if caught at the proper time, but is certainly a high risk. In naturalized areas, where treatment is next to impossible, thousands of acres are being killed. As a result of HWA, far fewer hemlocks are grown in nurseries and planted than 20-30 years ago. Both the most widely used Canadian Hemlock and the less used, but attractive Carolina Hemlock, are susceptible, plus other species not used in New England. At one point it was thought that northern New England was too cold for HWA, but the insect was able to acclimate to our climate.

Now present in 17 states and southeast Nova Scotia, since HWA arrival in 1951, hemlocks have been virtually eliminated as a marketable ornamental tree in the eastern U.S. HWA was first discovered in Maine in 1999 on nursery stock shipped from Connecticut. In 2000 it was found in New Hampshire. Quarantines have existed in both states for several years. Both growers and landscapers have shied away from hemlock as they should have.

The USDA began research around 2000 to develop a resistant variety. After crossing Tsuga chinensis and Tsuga carolina, the Traveler Hemlock hybrid was introduced and has now been in the testing stage for 20 years. Traveler Hemlock has been grown at the U.S. National Arboretum and no signs of HWA have shown up, and it is now ready to be marketed to the trade. In 2020 a Plant Patent was Applied For (PPAF), so Traveler can only be propagated and grown by licensed growers. Also this hemlock must be grown from cuttings because it doesn’t produce seeds. As a result of these two factors, Traveler Hemlock can’t be produced quite as quickly as Canadian Hemlock and it will take a few years to get production off the ground, but the future looks very bright.
Hopefully, in the not-too-distant future, Hemlock will once again become a practical evergreen tree that we can see planted in the landscape.

by Phil Caldwell

— Phil Caldwell is a past president of NHLA (1989) who now lives and works in Maine.