Story last updated at 9/1/2010 - 12:10 pm
A decade ago there were no thistles growing along the logging road where I hiked almost daily. Their appearance a few years later was surprising, but I took little notice other than to admire their striking purple flowers and fearsome stature, daring anyone to come near the profusion of prickly stems and leaves. I knew they were considered invasive but I wasn't aware of the serious extent of their insidious aggression or probable impact on other vegetation.
The sprawl of these thistles far up the hill and across the road has finally prompted my research which clearly indicates that if we don't want thistles growing thicker and more widespread, measures better be taken to eradicate them, plain and simple.
The good news is that so far I've found only bull thistle in this location, which is more easily managed than the other common species, Canada thistle. Both are invasive, non-native plants that may cause harm, but Canada thistle, designated by law a noxious weed in Alaska, wins the prize for most pervasive. Despite the name, Canada thistle is not native to Canada but was brought to North America from Europe - so I guess we can't blame the Canadians, eh?
According to a very informative USDA brochure I picked up at the Forest Service office entitled "Invasive Thistles in Alaska," only one species of thistle, the edible thistle, is native to Alaska and, although it could be here on Prince of Wales Island, it is apparently rare - worth reporting to Forest Service botanists if discovered.
My initial efforts at thistle identification were slow to unfold until I realized the plants were all the same species in that area. I didn't have Canada thistle to compare to but recognized the so-called spiny wings that grow on the stem of the bull thistle as the most obvious proof of identification.
Despite their similar appearance, the differences between Canada and bull thistles are many. Canada thistle is a perennial, living year after year and spreading into huge colonies of underground rhizomes as well as pumping out as many as 5,000 seeds per plant! Once established, the rhizome root network is extremely difficult to eradicate since even a half inch piece of root has the potential of becoming a new plant.
Bull thistle, on the other hand, is a biennial plant, completing its life cycle in two years. It lacks the widespread rhizome system of roots and has instead a sturdy taproot which can often be pulled up for eradication and it reproduces by seed alone. In the first year the bull thistle appears as a rosette of flat leaves on the ground, then grows tall in the second year, developing purple flower heads capable of producing up to 4,000 seeds a year. Seeds can be transported far and wide by the wind and reportedly can remain viable in the soil for 21 years!
Measures should taken early on to control thistle populations or the difficulty of the task will increase as patches grow, further depriving native plants of nutrients, water and space. The Cooperative Extension Service's Alaska Committee for Noxious and Invasive Plants Management website states that Alaska is still in a unique position to stop the spread of undesirable plants. By detecting untoward species as they are establishing and eradicating them with the help of interested citizens and agencies, invasive plants might be held at bay in Alaska.
A great little booklet called "Selected Invasive Plants of Alaska" is available for free from the Cooperative Extension Service with photos and information about a couple dozen threatening, invasive plants. It is amazing how fast these weeds overtake habitats but hopefully a more informed public will lend a hand to the cause and our native plants will prevail.
Carla Petersen is a freelance artist and writer on Prince of Wales Island. She may be reached at whalepassoriginals@gmail.com.
