Protecting Our Hemlocks: Slowing the Spread of Hemlock Woolly Adelgid

By Tys Theijsmeijer, Senior Director of Ecosystem Stewardship Programs and Policy

The work is supported in part by the Invasive Species Centre and also forms a key component of RBG’s current fundraising initiatives. Donations to this effort support the Tree Canopy and Forest Restoration Fund (Donate Link).

Mallory Peirce, RBGs Senior Terrestrial Ecologist “The injected trees from last year certainly have fewer woolly sacs, but we haven’t done a technical assessment on their status yet. Last fall, we noticed that the trees we injected in the fall of 2024 the HWA populations were significantly reduced! On one treated tree, it was difficult to find woolly sacs to show a working group I was touring around”.

Getting a full handle on the Hemlock Woolly Adelgid (HWA) situation across RBG’s natural lands is a major undertaking—requiring the inspection of each individual hemlock tree. These detailed surveys will continue through the rest of 2025, with the goal of completing a comprehensive review of all trees.

At RBG, native Eastern Hemlocks are found primarily on the shaded, cool, north-facing slopes of ravines. In total, it’s estimated that approximately 1,200 Eastern Hemlocks of varying ages are spread across thirteen key forest stands, covering roughly 16 hectares. Additionally, 102 hemlock trees of various species have been planted in cultivated areas, with the highest concentration located in the Rock Garden.

The largest known Eastern Hemlock on RBG property—measuring 91 cm in diameter—stands alone among oak trees in the interior forest on the north side of Cootes Paradise. It’s a living remnant of a forest community type that no longer exists here due to the impacts of a warming climate.

Mallory Peirce, RBG’s Senior Terrestrial Ecologist, shared encouraging news from this year’s inspection efforts: “We’re about two-thirds of the way through the inventory of Hendrie’s Hemlocks for this season, and so far, we’ve seen no signs of HWA—thankfully. The trees are looking really healthy.”

Looking ahead, climate change is expected to shift forest composition, introducing tree species that currently thrive further south. As part of our forest restoration planning, RBG is preparing for this transition by evaluating species better suited to future conditions. These include trees such as Hackberry, Shellbark Hickory, Chestnut Oak, and native fruit species like American Persimmon and Pawpaw. We are also exploring renewed efforts to reintroduce rare and at-risk species—including American Chestnut, Butternut, and Red Mulberry, also more suited to the warmer conditions projected for 2050 and beyond.

RBG’s complete list of future-compatible species will be published later this year. We are grateful for the guidance provided by Natural Resources Canada, whose climate-adaptation tools support this important planning work.