I remember when I first moved to my new place in Cedaredge that I greatly appreciated the white flowering plants blanketing a slope on my property. Only after I was chastised by a neighbor for my insistence to perpetuate this beautiful sight did I come to realize these plants were in fact the much hated weed, whitetop.
Whitetop, more correctly known as hoary cress, is an aggressive noxious weed that has invaded our western U.S. area. This weed that can give us much trouble is also despised by ranchers. It came from SW Asia by way of a contaminated shipment of alfalfa seed.
While our species is Cardaria draba, there are two other species as well. This deep-rooted weed is a member of the mustard family and can attain heights of up to two feet. Leaves are blue-green and lance-shaped. The white flowers have four petals and are arranged in an inflorescence that gives it a flat top appearance. Seeds are borne in heart-shaped capsules containing two seeds each.
While new plants do arise from seed germinating in the fall, most recruitment for this perennial comes from shoots from its extensive underground root system. Roots can extend twelve feet horizontally and up to fifteen feet deep. These roots can produce as many as 400 shoots per year!
Only a quarter of the plant's total biomass is above ground. The extensive root system stores large quantities of water and nutrients, a key to its survival. This weed is common in alkaline soils, disturbed areas, salty soils, sub irrigated areas, roadsides and ditches. Because of its root system, it can survive wildfires that are prevalent in cheatgrass-infested areas. Whitetop then crowds out the more desirable native plants.
While whitetop is not poisonous, it is low in nutrients, especially as it matures, and generally unpalatable to cattle. Sheep are more likely to graze on this weed but by no means are they an effective control agent.
Cultural control including mulching, mowing, hand-weeding, and cultivation is not very effective. Roots can stay alive for a year following top growth removal during which time many new shoots are being pushed to the soil surface. Plowing or hoeing once-a-month for two to four years can provide significant control but is all that effort worth it?
The use of an herbicide is by far the most effective control method. But it too might need to be used more than once. Suggested herbicides include 2,4-D, glysophate and Telar®. The first herbicide will take multiple treatments over a few years. The gysophate treatment can be enhanced with ammonium sulfate fertilizer. Telar® can be more effective with the addition of 2,4-D for quicker burn down. This last treatment is the most effective herbicide but is persistent, preventing you from planting anything new in the area for several years.
So there you have it, the fourth weed in our series of 10. Next time I'll discuss myths associated with trees and then I will be back to weeds again.
Jim Leser retired to Cedaredge in 2007 after a career with Texas A&M University Extension in entomology. He is a member of the Cedaredge Tree Board and a Colorado Master Gardener.