Spring is here and our lawns look like they have the horticultural counterpart of the measles, but the spots are yellow, not pink. This leads to the stratification of gardeners into three groups. There are those who reach for an herbicide to get rid of dandelions, then the more environmentally concerned who pinch off seed pods and dig up what they can – and lastly those who simply accept things as they are since after all, it’s only Taraxacum officinale, alias dandelion. The first group of gardeners may receive some relief as a result of what American ecologists saw at the Rothamsted Experimental Station in England. They noticed there was a mosaic of plots in this grassland. Some plots, yellow with dandelion blooms, were bordered by plots where dandelions were almost absent despite the heavy rain of dandelion seed received annually. They found that the dandelion population was related to whether potassium had been applied.
Following tip on this observation the Minnesota group studied the effects of potassium on dandelions and five lawn grasses in a greenhouse environment and the relationship between the amount of potassium in plant tissue and the density of dandelions of lawns in Minnesota. In the greenhouse study potassium was highest in dandelion tissue, suggesting dandelion has a hearty appetite for this element. They also found that in pots given a low potassium fertilizer the dry weight of dandelion, fescues and orchard grass all decreased BUT dandelion suffered the greatest loss, down to 81% of its weight in pots given a complete mineral supplement.
The lawns selected for study were restricted to those that had not been fertilized, treated with herbicide or hand-weeded for several years. The amount of cover as well as the density of dandelions correlated with the tissue potassium content. When the plant tissue level of potassium was low, the dandelion density and cover in the lawns was low. Based on these results it is suggested that one step gardeners could take right now is to use a fertilizer of ammonium sulfate or ammonium phosphate, However, this would not be good for lawns of Kentucky bluegrass which is almost as greedy for potassium as dandelions.
At this time it is unclear how widely applicable the strategy of controlling weeds through nutrient limitations will turn out to be. In the opinion of agricultural and weed scientists, it appears to be an area that is worthwhile to follow up, for the relationship between soil characteristics and weeds is only beginning to be understood.