Invasive Plants and Weed Profiles

Last month we raised the issue of invasive introduced plants and the idea of being responsible gardeners who respect our native habitat and make an effort to not release invasive plants into the wild. We began with obvious noxious weeds that are having strong negative impacts on our native populations of plants and animals. This month again I am profiling weeds which have been declared as noxious, but this time a question is beginning to arise about under what circumstances are some of the less aggressive weeds invasive? In other words, is it possible that we might find a middle way that guides us strictly with the truly harmful invasives and more leniently with the less aggressive and more useful weedy plants? These are questions I am asking myself and will be asking in future interviews. We also welcome your views on this issue.

Spotted Knapweed, Centaurea maculosa is a biennial or short-lived perennial that is a native of central Europe, arriving in a shipment of alfalfa and clover seed. It forms a rosette of deeply lobed leaves covered with fine hair, and sends down a deep taproot. Multiple shoots can arise to 1’-4’ high. At the tips of these shoots, the solitary 1” flowers form; they are pink-lavender to purple and bloom in summer and into fall. Bracts under the flowers have dark spots (hence the name “spotted knapweed”) tipped with fringe. The fruits are 1/8” long with bristles. It generally grows in dry meadows, roadsides and gravelly flood plains of creeks and rivers.

         Spotted Knapweed is the number one weed problem on rangeland in western Montana, covering 4.5 million acres. It outcompetes native plants for moisture and nutrients and like most knapweeds, it is believed to exude chemicals in their surrounding area which inhibits the growth of other plants. Each mature plant can produce more than a thousand seeds which establish particularly well in areas disturbed by logging, grazing, flooding or fire. Dense populations reduce forage for wildlife, grasses for nesting and cover and increase soil erosion.

Field Bindweed, Convulvulus arvensis is a weed almost everyone in Colorado is familiar with. It is a creeping perennial member of the morning glory family with thin, smooth stems spreading thickly over the ground or climbing, twining and binding everything up to 4’ high. The leaves are somewhat arrow-shaped and quite variable. The flowers are attractive and bell-shaped, approximately ¾”-1 ½” in diameter and blooming in white or pink. The seeds are numerous, in little round capsules and have famed viability: “one year seeding, 50 years weeding.” But it is the extensive horizontal roots and taproot that make it so impossible to eradicate. Here is the proverbial “roots to China” weed that rototilling only spreads.

         For years I have diligently dug out my bindweed with a deep knife weeder, and tried to make sure it did not flower and make seed, but two busy years with lax maintenance and my bindweed jungle “is baack.” Having rocks for their white roots to dive under and hide behind make removal even more impossible. This is a noxious weed and everybody wants to know of a method for getting rid of it. I have heard planting alfalfa in fields can smother it, and I have had some success in using White Moss Sedum as a smother crop, slicing right through the tolerant sedum to remove the bindweed. I have heard varying reports of the success and lack of success of Roundup to kill more than the tops of the bindweed. We would love to hear of successful solutions to bindweed than any of you may have found.

Common Tansy, Tanacetum vulgare is an escaped herb-ornamental. It is an aromatic perennial 1’-4’ tall and spreading. The leaves have a pungent aroma; they are serrated and finely divided giving a ferny appearance, and the stems are often purplish-red. The flowers are yellow, small and button-like, 1/4”-1/2” across formed in flat-topped dense clusters. Tansy reproduces by seed and by rhizomes (short horizontal underground stems). It grows along roadsides, in pastures and along streambanks and irrigation ditches where it is said to restrict water flow. Tansy is now widely established in the wild on the western slope of Colorado.

         I know this plant from our herb garden only, where it has spread widely in twelve years but has not become a problem. Perhaps it is in riparian areas where it is problematic. I can think of numerous herbs: mints, artemesias, comfrey, horseradish, feverfew and costmary which are more weedy and aggressive than tansy. Also tansy is highly thought of as a medicinal herb. It is also supposed to repel ants so when a retired client was being bothered by an ant colony against her house, I dug a hole in it and quickly planted a tansy. She said the ants left and never returned.

         Now nurseries are forbidden to sell it. I’m not saying it shouldn’t be classified as a weed, but perhaps there might be room for some flexibility.

Dame’s Rocket, Hesperis matronalis is an attractive ornamental that has escaped cultivation and is spreading in the Boulder foothills and the Roaring Fork Valley of western Colorado. Originally from western and central Asia it is now naturalized throughout Europe and the US. It is a biennial or perennial generally 2’-3’ high with white, mostly pink or purple four-petalled flowers in simple racemes resembling Summer Phlox and blooming May, June and July with sweetly scented flowers. Apparently it is most troublesome in riparian and wetland habitat.

         I knew this plant from my Grandma Daisy’s garden where she called it “Sweet Rocket”. In my Boulder garden, it appeared unexpectedly, bloomed gloriously for 2 or 3 years, then disappeared. Dame’s Rocket won’t be considered a noxious weed until January 1, 2001.