|Iowa State University|
Weeds in the News
by Bob Hartzler
July 16, 1999 - There have been three articles in the journal Science in recent months that have addressed weed topics. An article describing the effects of potassium fertilizer on dandelion appeared in the May 21 issue (Science 284:1249). The research was conducted by David Tilman (University of Minnesota) and two scientists in England. They reported that dandelion has a high requirement for potassium, and that fertilization practices significantly impact dandelion populations. Tilman developed this theory while viewing plots at the Rothamsted Experiment Station (England) and observing that plots with high potassium applications had very high dandelion infestations, whereas other plots were relatively free of dandelions. Subsequent research found that dandelion had much higher concentrations of potassium than several grass species. In a survey of lawns that had not been treated with herbicides, they found that the density of dandelions was correlated with the tissue concentration of potassium. This provides further evidence that high soil potassium levels favor dandelion. They suggested that reducing potassium inputs could be a means of controlling dandelions in lawns. Unfortunately, they found that Kentucky bluegrass (the dominant turf species in Iowa) requires nearly as much potassium as dandelion.
A second article in the same issue of Science (284:1255) described a dilemna facing weed scientists and ecologists. Saltcedar (tamarisk) was introduced in the western US from Asia to help reduce erosion. As with many imported plant species, the plant ran wild and now forms dense thickets along many waterways. Saltcedar has crowded out many of the native species of the region. Research has been conducted to locate biological control agents that could be released to help manage the problem. A leaf-eating beetle from China is believed to have potential for controlling saltcedar and is scheduled to be released this summer.
However, as so often happens, there are potential drawbacks to any tactic that alters the vegetation of an area. Some people are concerned that controlling the saltcedar will eliminate nesting sites for the willow flycatcher. The flycatcher was classified as an endangered species in 1995. Ironically, the spread of saltcedar has been cited as one reason for the decline in the flycatcher population. Due to these concerns, the plans for initial release of the beetle have been scaled back until the risks are better understood.
The final article I will summarize appeared in the June 25 issue (Science 284:2083). This article described the fate of a supposedly extinct plant in Oregon. The vernal pool monkey flower (Mimulus tricolor) was classified as extinct in 1991. However, the plant was found growing in a former rye grass field outside of Corvalis this spring. While this story really doesn't have anything to do with weeds, it does illustrate the nature of the soil seed bank. Due to seed dormancy, it is highly unlikely that a weed that is firmly established in a field will ever be completely eradicated. It should be pointed out that many native plants are not nearly as resilient as weedy species. Because of this, we shouldn't take the decline in population of endangered plants lightly.
Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University
more information contact:
ISU Extension Agronomy
2104 Agronomy Hall
Ames, Iowa 50011-1010
Voice: (515) 294-1923
Fax: (515) 294-9985
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