Weeds or what

If it's a wonderful plant here, it might just be a weed somewhere else-that's the wacky and worrisome world of gardening.

Several weeks ago I wrote about the tamarix, a shrubby tree with feathery pink flowers. Recently via email, I heard from Mimi Yanus of Boulder Colorado. She was reading my column on the Toronto Star website and wrote, "This plant is a terror to the US southwest, consuming literally millions of gallons of water every day, and displacing native plants along river banks and riparian areas. When native plants are displaced, so are native animals. This plant is a problem across the American west, and, I assume, western Canada."

Mimi makes an interesting point. Though garden writers might write for a "local" audience, their words can travel far beyond on websites. Tarmarix is listed in many reputable books as a worthy ornamental plant, and in fact I could not find any references to it being an invasive species in Ontario, where I write, but let it loose in a warmer climate and it makes the top 100 noxious plant lists.

Writers have a responsibility not to recommend invasive aliens, but gardeners have a responsibility to learn about their local world and which plants are terrorizing natural areas.

Various grasses, vines and ground covers which are well behaved in cold climates, are out of control in warmer habitats. There is a wealth of information on the web, including lists and links, like the Canadian Botanical Conservation Network's Invasive Alien Plants section at www.rbg.ca/cbcn/.

Beyond invasive species there is another world of plant behavior based on the type of garden you have. On my dry hillside garden, goldenrod wants to take over, but other typically rambunctious customers like cutleaf sumac are rather well mannered.

I've also noticed, that very few plants reseed in my garden, the one exception being Russian sage which is fine with me.

A furious reseeder in many gardens is the annual Verbena bonariensis. I love it but have always had a hard time getting it established. I saw a grand patch of it at the Royal Botanical Gardens Rock Garden, where it seemed to be regarded with some disdain, probably because it came back from seed too easily. But this annual, with its small, purple flowers on top of tall wiry stems mixes in gracefully in the perennial border. (Note, it's listed as invasive in the American South.)

The Chicago Botanic Garden puts an interesting twist on this problem of invasive species. They have put together a list called Plant Conservation, Invasive Plant Alternatives at www.chicagobotanic.org.

They offer up the weird and wonderful purple smokebush for instance, (Cotinus coggygria) as an alternative to the vigorous Japanese barberry. Purple smokebush looks good with or without the smoke. It demands full sun or it will lean and grow asymmetrically. The leaves are a rich, jewel like purple, and the smoky flowers have to be seen to be believed. Some gardeners like to prune this shrub back hard in the spring to promote new, vigorous growth. Purple smokebush can also tolerate life on the lean side, with just average soil, and a minimum amount of watering.

Invasive plants, and destructive insects do not respect borders, gardeners need to do their part to be informed about the latest threats.