Hemlock T.L.C.

You’ve heard it before: Save the Trees! But hear me out, hemlocks are particularly important and particularly in danger. A few years ago I talked about the hopeful hemlock and its fight for survival. Now I’ll explain a little more about how you can treat them.

All About Hemlocks: A Refresher

If you didn’t know, hemlocks are an integral part of the eastern United States’ forest ecosystem. They are coniferous evergreens, meaning they produce pine cones and foliage that flourishes year-round. In the winter, the eastern hemlock provides shelter for birds and other animals to hide and nest. They’re also beautiful. Hemlocks make ideal Christmas trees with their dark-green needles, pyramidal shape, and widely spaced, swooping branches. They can grow up to 40-70 feet tall and have an abundance of small brown cones that dangle like delicate, perfectly crafted ornaments. Even more, their roots prevent land erosion, they contribute to air quality, privacy for homes, shade, ecotourism, local economy… the list goes on.

If you’ve ever hiked through North Georgia, you have most likely enjoyed the comfort and beauty of this keystone species.

The Villain

These forest giants have one major enemy: the hemlock wooly adelgid (HWA). This parasitic non-native insect has been killing hemlocks for years and at an alarming rate. Since its introduction in as early as 1951, the U.S. Forest Service estimates that the HWA has killed millions of native hemlocks across North America. Because the HWA is an invasive species that has no natural predators in the U.S., the eastern hemlock is defenseless and weak without our help. Imagine, for a moment, a winter forest with nothing but barren deciduous trees. Imagine how different your leisurely hike would look without the beauty of these evergreens. That’s what would happen, among other disastrous environmental issues, if we left these poor trees to fend for themselves.

Here is a close-up of a hemlock’s needles infected by HWA.

So… let’s save the trees! 

Many colleges, universities, research labs, and organizations have come together in their fight to preserve and maintain the health of the hemlock tree. You may have even heard of HemlockFest, an annual 3 day music festival in North Georgia, thats proceeds go to the research and development of saving hemlocks.

But you may be asking yourself, how can I help? For one, you can always plant hemlocks. See my previous blog about planting trees. Once established, they’re easy to maintain and will add plenty of property value to your home.

Another way to help would be to learn how to treat hemlocks if infected by HWA. Though you can certainly do it yourself, I would recommend consulting companies or arborists, like myself, who specialize in hemlock treatments. I have a Georgia Commercial Pesticide Applicator License which allows me to buy, apply, or supervise the application of a restricted-use pesticide (RUP) without a fee. Although typically the insecticides used to help hemlocks are not RUPs and can be found at your local hardware store, the use of these chemicals are also toxic to bees, so, much care should be taken. The following methods should only be applied to the residential hemlocks on your own property. If you want to help with the bigger picture, volunteering at HemlockFest, donating, or participating in service projects with the U.S. Forest Service are all great ways to join in the effort to save our hemlocks.

Here is a heavily diseased hemlock branch.


There are many proposed ways to treat hemlocks infected by HWA but I have found that combining techniques (soil drench and soil injection) and combining chemicals (Imidacloprid 750 WSP -Merit or any generic brand- 4pk and Dinotefuran -Safari, no generics- 12 oz bottle) yield the best results. It does depend on the level of distress the tree is in. Dinotefuran packs a faster punch (results in weeks) but its effects don’t seem to last as long (1-2 years). Imidacloprid is slower to reach the crown of the tree (2-3 months), but does have longer-lasting action (5-6 years).

Here is a heavily diseased hemlock.

Let’s Get to it! 

  1. Mix master batches of each insecticide, per instructions on the containers.
  2. Measure the diameter of the tree at breast height (4.5’) on the upper side of the slope, rounding to the nearest inch.
  3. Use the chart provided with instructions to determine how many ounces of the master batch you need, based on the diameter of the tree.
  4. Clear leaves from 1-2 foot area around the trunk.
  5. Soil drench in one of the following ways:
  6. Pour the prescribed amount into a measuring cup and distribute the mixture in the measuring cup evenly around the base of the tree.
  7. Use a hammer and rebar to make small individual holes (one hold for each inch of trunk diameter) only 3-4” deep within 12” of the trunk, evenly distributed around the tree. Pour chemical mix gently into the holes, press holes shut.
  8. If the tree is on a slope, focus chemical application more on the up-side of the slope.
  9. Replace the leaf litter around the base of the tree.

Other Tips

  1. Don’t apply right after it’s rained or when rain is expected within the next 24 hours.  Be careful with areas near streams and where soil is rocky.
  2. Don’t apply during a summer drought.
  3. Do not fertilize diseased trees.
  4. The best time to treat is in March or April
  5. For large trees that cannot be completely sprayed, I recommend soil drenches.The insecticide is applied to the soil surrounding the roots of the tree. The tree roots take up the product and move it into the foliage where the insect is killed. Soil drenches must be applied when there is adequate soil moisture in either the spring or fall.
Here is a healthy hemlock branch.
…and a healthy hemlock tree! We can do this!

More sources:

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