Musk thistle is an ever-increasing problem in Colorado. It's a noxious weed prevalent in open spaces, vacant lots, pastures, ditch banks and along roadsides. Musk thistle is a biennial, which means it lives only two years. In the first year, it germinates from seed and grows into a low-growing rosette of leaves. The leaves are very prickly-spiny, and often have a white margin. In the plant's second growing season, it sends up multiple flowering stems. At this stage, the plant may grow two to six feet tall. As many as 100 flowers per plant develop in mid-summer through fall. Flowers are attractive purple-pink, about the size of a silver dollar. Just beneath the flowers are some very spiny bracts. After flowering and seed dispersal, the musk thistle plant dies.
The key to musk thistle control is to halt seed production. To accomplish this in the first year of the plant's life, sever the root below ground with a shovel or hoe when the plant is in the rosette stage. This can also be accomplished in the second year by severing the root before flowers develop. Mowing musk thistle in its second year is effective if it's done before any flowers mature. Herbicides are most effective when used on first year rosettes, especially in late summer or fall. Second-year plants that are developing flowering stems are much less susceptible to herbicides. Homeowners can use glyphosate or 2,4-D herbicides, when temperatures are 60 to 85 degrees Fahrenheit, and when winds are less than five miles per hour. Don't allow spray to come in contact with any desirable vegetation. As with any pesticide, read herbicide label directions carefully and follow them explicitly.
Alternatively, an introduced insect called the musk thistle seed head weevil feeds on developing seeds and can reduce the number of viable seeds produced by fifty percent or more.
Aspen Trees
Lots of preparation, patience and understanding are needed to grow aspen in the urban landscape. Aspen prefer light soils that are acidic and drain well, but these conditions are rarely found in urban areas. Aspen will do best on the north and east sides of buildings.
If the soil where an aspen tree is to be planted has a high content of clay, build a berm of sandy loam 18 to 24 inches high. A berm is a mound or wall of earth. The berm should be mulched and several plants should be planted in the bed.
Nearly all aspen available for sale are collected, meaning they were dug out of the wild with little of their root system. Even after careful preparation and care, aspen still only have a life expectancy of about 25 years in the home landscape.
Expect insects and diseases, some of which have no controls. Blackened leaves towards the late summer and fall are caused by aphid secretion or a leaf spot disease. The leaf spot disease is known as Marssonina leaf spot or aspen leaf spot. The best control is to pick up the leaves, which are the source of reinfection, in the fall and cut away trees and branches to increase air circulation and reduce humidity throughout the aspens.
Orange pimples on the bark indicate the presence of cytospora canker. Cankered branches usually occur on weakened trees and need to be removed.
Oystershell scale is an insect about 1/8” long, brown or gray in color and is in the shape of an oyster shell. The insect feeds on the trunk and branches of the aspen, resulting in a weakened tree. The stage most familiar to the homeowner is the covering of the full-grown female. The covering remains attached to the tree for some time, often, with the overwintering eggs protected beneath the old scale. Overwintering scale can be gently scrubbed away with a plastic covered sponge. Oystershell scale can be difficult to control due to the waxy covering on the insect. Once an individual tree in an aspen grove starts developing deadwood and a thin crown, it should be removed to allow new shoots to develop.
Summit County, CO.