Hemlock yellowing

Asked September 29, 2020, 12:49 AM EDT

Recent needle yellowing—noticeably more than usual—in copse of 6 large Hemlocks. I don’t see any scale. What might cause this?

Howard County Maryland

1 Response

We do not see signs of woolly adelgid (a common and very damaging hemlock pest) or scale on the stems or foliage, though it is difficult to tell from the photos. We recommend having the trees inspected/assessed by a certified arborist, especially since any pesticide intervention recommended (if there are scale or adelgids) will require treatment by a professional pesticide applicator. You can use the second and third links on this page to search for an arborist: https://extension.umd.edu/hgic/topics/how-do-you-decide-when-remove-tree

Hemlocks are best-adapted to cool, mountainous conditions with soil that drains very well and evening temperatures that cool off well. In suburbia and in central MD, they struggle both with heavier clay-based soils, warmer temperatures overall (especially when impervious surfaces radiate and reflect heat day and night), high humidity, and poorer air quality (they are known to be intolerant of air pollution, which comprises not only car exhaust but also ground-level ozone, the same traits used in determining air-quality ratings for humans in weather reports). Drought will also stress them if supplemental watering (on an as-needed basis) isn't provided. This year's July being one of the hottest on record probably added to their chronic stress levels; unfortunately, we are receiving queries regarding struggling hemlocks throughout the state.

Miri