Just over a century ago, a gardener in Richmond, Virginia established a Japanese garden, importing plants from all over the world. Her name was Sallie Dookey. She died and her garden was left to the city.

In 1951, an entymologist noticed a new alien Asian insect, the hemlock woolly adelgid, on a Eastern Hemlock (Tsuga canadensis) on a nearby property. It was thought to have come from Sallie Dookey’s garden. The Asian hemlocks and spruces that this parasite was known to feed on have resistance to the pest.

The North American hemlock does not.

Nothing much happened for a few decades, and local experts advised anyone who found the adelgids on their hemlocks (at their southern range in Virginia) to spray them annually with a pesticide or oil. This seemed to work.

Then in the late 1980s, an intimation of disaster. A stand of old hemlocks (they can live for 600 years) 40 miles from the city was discovered, dead. It had been infested by the woolly adelgids. And the pest was found in the Shenandoah National Park in Northern Virginia, thought to have been carried by migrating birds. “The insect multiplied with explosive speed,” wrote Richard Preston in a shocking 2007 New Yorker article.

Shocking because who among us gardeners has not been a Sallie Dookey? Working from morn to night to create an eye-popping display, a perfect collection, a showcase for the unusual and dramatic? But thanks to Sallie Dookey’s diligence, hemlocks disappeared from Shenandoah. The woolly adelgids spread northward and have now reached New Hampshire.

They move at a rate of 30 kilometres a year, hitching a ride on the wind, on birds and forest animals. People contribute through nursery, logging, and recreational activities. The hemlock is doomed, through most of its range. Will the cold temperatures that protect Ontario from many pests, killing them off in the winter, preserve our hemlocks? Time will tell. 

A hemlock stand is a spiritual place – dark, damp and vital. This is a beautiful tree - slow-growing and shade tolerant, with delicate needles and an elegant arching form. “Hemlocks create deep shade and cover the ground with beds of needles, altering the temperature, moisture and chemistry of the soil around them, creating a distinctive habitat for certain animals and plants,” Richard Preston wrote. Locally, one place you can get a sense of the hemlock’s majesty is Grant’s Woods near Orillia.

The moral of the story? Check whether a plant you're considering is on an unwanted list. I have two lists on this website – the first is of invasives you are likely to find on offer in a garden centre, the second is of those that will arrive uninvited. Do the research before you buy. And don’t be seduced by a plant’s good looks into being the first on your street to play host to a devastating pest.

Don’t be a Sallie Dookey.