Observant readers may have noticed the prickly-leaved weed sidling up to ‘Rozella’ in my last post. That dastardly villain is Canada thistle, aka Cirsium arvense, and it is one nasty customer. According to the University of California Cooperative Extension,
Once established, Canada thistle spreads rapidly by horizontal roots, up to several meters per year. The extensive horizontal root system assures long-term persistence and spread by vegetative means. A segment of root as small as 1/8 to 3/8 inch (3-6 mm) in length and 1/16 inch (1 mm) in diameter is able to propagate a new plant. . . . Once established, Canada thistle is a fierce competitor for nutrients and water needed by crops or native plants. It produces allelopathic chemicals that assist in displacing competing plant species
Okay. It’s obvious this mess didn’t happen overnight. This peony bed was created in 2002, and my weeding practices have been, at best, inconsistent over the years. I’m really not sure what year the Canada thistle showed up, but since I wasn’t aware of how extensive its roots could get, I didn’t give this weed the immediate attention it apparently so richly deserved. I’m sure many times I looked at those spines and thought to myself, I’ll pull that the next time I have gloves handy. And even if I had gone to get gloves, the odds that I would have been distracted from my task by some other domestic duty or child-induced problem would have been pretty high. I’m not saying I never pulled the Canada thistles, I’m just saying I never pulled them frequently enough.
This year, I am trying to reform. I am making weeding my top priority in the garden. The peony bed was the very first thing I weeded in early spring, and I removed every single thistle plant as far down into the soil as I could manage. That might have been a mistake. According to the Colorado State Cooperative Extension,
Canada thistle allocates most of its reproductive energy into vegetative propagation. New shoots and roots can form almost anywhere along the root system of established plants. Tillage segments roots and stimulates new plants to develop. Shoots emerge from root and shoot pieces about 15 days after disturbance by tillage. Small root pieces, 0.25 inch long by 0.125 inch in diameter, have enough stored energy to develop new plants. Also, these small roots can survive at least 100 days without nutrient replenishment from photosynthesis.
As Purdue University explains further,
Removal of shoots and severe damage to established plants stimulate new growth from underground buds. It is the buds on the creeping roots of established Canada thistle plants which largely account for re-establishment after attempts at control. Buds on creeping roots can generate new shoots a year or more after top-growth has been destroyed.
So I think by pulling and digging, I actually encouraged the thistles to make more shoots and more roots. It’s one devil of a weed, I’m telling you. Those roots have to be deprived of nutrition over and over again to eradicate it.
In established plants of Canada thistle, carbohydrates move from the root system up to the newly forming shoots as growth starts in spring. As leaves on the shoots develop, photosynthates start moving to newly developing roots and flowerheads. The developing flowerheads take more and more of the energy (photosynthates) produced by the leafy stems and stored in the roots. Carbohydrates in the root system are at their lowest when the plant begins flowering.
Ideally, you’d remove each stem when it was in flower, so that the roots had the least amount of reserves. The largest shoots in this infestation were budding up, and so I decided to remove all the shoots today. I wore leather gloves, but I didn’t pull the shoots out. I cut them off at soil level, and threw them straight into the wheelbarrow:
After the research I’ve recently done on Canada thistle, I know this is just the first round in a long fight. The peony bed looks a lot better at the moment, but I have no illusions about the future.