   Winter Moisture Propels Cool Season Weed Growth - February 23, 2005 Jeff Schalau, County Director, Agent, Agriculture & Natural Resources Arizona Cooperative Extension, Yavapai County I may be wrong, but I don’t think we should expect another year of winter precipitation like the winter of 2004/05 anytime soon. In most areas, the soil is so saturated that a light sprinkle will produce surface runoff. Aside from property damage and erosion caused by the flooding, we are already seeing explosive cool season weed growth. This growth will also produce an extraordinary seed crop and, in the case of perennial weeds, significant expansion of invasive root systems. In this column, I will focus on a few cool season annual weeds and some control strategies. London rocket (Sisymbrium irio) is one of the most abundant and competitive cool season annual weeds in north central Arizona. It is in the mustard family and looks very similar to a dandelion when young. As it matures, it bolts upward bearing long, tubular seed pods that have a small cluster of yellow flowers at the tip. The plants usually grow to about 2 feet (60 cm) tall. Annual brome grasses are also common cool season weeds. These include downy brome (Bromus tectorum), red brome (Bromus rubens), and Japanese brome (Bromus japonicus) are all present in north central Arizona. It is a common roadside weed that is also shade tolerant and often grows beneath trees and shrubs. When young, these species have soft green blades and are palatable to livestock. However, as they mature these grasses become unpalatable due to the seed’s long, sharp awns. These awns cause the seeds to become stuck in clothing and animal’s fur. The seeds can also cause injury to pets and livestock. These species are also known for their ability to burn and can easily carry fire from the ground into woody plant canopies. Burned areas are also invaded by annual bromes. Two more serious annual cool season weeds are present in the Verde Valley: Yellow starthistle (Centaurea solstitialis) and Malta starthistle (Centaurea melitensis). Both species are considered noxious weeds (although Malta starthistle is not officially listed in Arizona). Both species start out looking similar to dandelion (rosette form). As they mature, a flower stalk grows bearing yellow flowers that are surrounded by sharp spines. Yellow starthistle is more heavily armed having up to 1 inch long, rigid spines. Yellow starthistle is also poisonous to horses (although they rarely eat it unless other feed is not available). Saharan mustard (Brassica tournefortii) is a relative newcomer to the Verde Valley. It is a relative of London rocket, and looks similar when young. It can be easily distinguished as it produces a much stouter, many branched flower stalk with inconspicuous yellow flowers. Saharan mustard is currently a roadside weed, but appears to be advancing into natural areas. Like annual bromes, Saharan mustard also carries fire and reseeds in the burned over areas. Control of annual weeds is relatively easy in small areas such as landscapes and gardens. Hand pulling prior to seed ripening is an effective control for small populations. Contact herbicides are also effective including the new least-toxic products having the active ingredients acetic acid or pelargonic acid. I have effectively controlled annual brome in my yard through the use of chickens. They eat and scratch up the plants when young. Pre-emergent herbicides are also effective when applied in the fall prior to germination. Always follow label directions when using any pesticide. Weeds that are well-adapted to fire pose a significant threat to Sonoran desert native plant communities. Saguaro cactus and other Sonoran desert plant species did not evolve with fire as a major disturbance factor. When fires burn these weeds, native plants are slow to recolonize and non-native plants have a distinct advantage. It is relatively easy to control annual weeds in your landscape and garden, but much more difficult to stop their spread across natural areas. The University of Arizona Cooperative Extension has publications and information on gardening and pest management. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 ext. 14 or E-mail us at mgardener@verdeonline.com and be sure to include your address and phone number. Find past Backyard Gardener columns or submit column ideas at the Backyard Gardener web site: http://cals.arizona.edu/yavapai/anr/hort/byg/. |