Diffuse Knapweed, Centaurea diffusa is a relative of the cottage garden flower, Bachelor’s Button and a native of southern Europe and the Ukraine. I used to think that Bindweed was a bad weed, but that was before Diffuse Knapweed moved into north Boulder and has taken over several acres all around me. It is classified as a biennial that can be a short-lived perennial. I am beginning to think that if it is mowed and flowers little, it is perennial, but if it grows to a mature plant producing thousands of seed, then it is a biennial or annual.
The plant begins as an attractive rosette with finely divided leaves to about 6” in diameter. The second year it sends up a branched stalk one to two feet tall which flowers small white, sometimes purple blooms. The bracts are tipped with sharp spines that are unfriendly to animals and humans alike. The foxes do not bother my neighbor’s poultry once the knapweed is in bloom; it is that dense and prickly. Once the plant dies it becomes a tumbleweed, snapping off at ground level and tumbling until it gets lodged in a fence or shrub.When a strong wind caught my neighbor’s fence packed with knapweed, it snapped off a whole row of steel fence posts.
Both personal experience and reports from others indicate that Diffuse Knapweed has some level of toxicity. Pulling it with bare hands can result in tasting it and I have heard of people getting sick from pulling it. I always use leather gloves and long sleeves. Some livestock and geese will eat it when it is young. Apparently there are some herbicides, which can control it, but the weed will return if other management techniques are not used. Some insects are being evaluated for biological control and the Colorado Department of Agriculture has made many releases of two seedhead flies. Burning works to destroy the seeds, but I would pity the tree or building with knapweed piled against it that was caught in a fire. My experience with trying to control it on two acres without herbicides is that I cannot dig, burn or hire help enough to keep up with this weed. I think one Colorado Lottery prize should be reserved for whoever can come up with an effective ecological control for Diffuse Knapweed; five million dollars would be cheap.
Purple Loosestrife, Lythrum salicaria, is an invasive ornamental native to Europe and Asia. It is a long-lived perennial having upright stalks from 2’-7’ tall. The leaves are lance-shaped and are arranged in pairs up the stem alternating at right angles. The flowers are rose-purple with five or six petals and are arranged in attractive racemes (spikes). A single mature plant can produce three million seeds or can regrow from cut or broken pieces. It requires a moist or wet environment and so has become a big problem in riparian areas and irrigation ditches. It grows densely and can fill up areas and crowd out wildlife, restrict the passage of irrigation water and prevent the movements of boats.
Supposedly safe sterile varieties are said to cross with fertile plants to produce seeds so it is best to just say no to Purple Loosestrife, though there is a native species, Lythrum alatum which is smaller with fewer flowers. Control is difficult since it usually grows around water making herbicides more dangerous. Flowers can be cut before they open to prevent seeding but this does not kill the plant. Plants can be dug or pulled by hand. If you see a Purple Loosestrife population, you can report it to Dave Weber, Division of Wildlife, 303-291-7231. Epilobium danielsii, the native Fireweed, looks similar so be sure to know the difference.
Yellow Toadflax or Butter and Eggs, Linaria vulgaris, is an invasive ornamental from Europe. It is a perennial that reproduces from seed and from underground rootstocks. The leaves are pale green and narrow, about 2 ½” long, and the plants are 8”-24” tall. The flowers are snapdragon-like, 1” long, yellow with a bearded orange throat, blooming in clusters at the ends of the stems. It is tolerant of moist or dry conditions and many soil types. Years ago I unknowingly planted it as a drought tolerant ornamental. The first year I loved it. The second year it was sending up shoots several feet away and lots of them. When I tried to dig it up, I discovered its cancerous root system. For two years I looked for it weekly and dug out every piece I could find before it was truly gone. If it had been watered, it might have taken four years. You don’t have to be an eco-warrior to shun this noxious weed. Little else can compete with its hungry root system once firmly established.
Mediterranean (Ethiopian) Sage, Salvia aethopsis is a native of the Mediterranean and North Africa. It is a biennial member of the mint family growing two to three feet tall. The first year it is a rosette with large, aromatic grayish woolly leaves. The second year it sends up a branched flowering stalk with many showy white flowers. Each plant can produce thousands of seeds. It has spread over 400 acres in rangeland in northern Boulder County.