From: Nick Christians, Iowa State University.
I have had a number of calls lately on white clover. There seems to be more of it this year than people have seen for awhile. That is not your imagination. Clover loves wet weather, particularly early in the summer. In 1993, the wettest year on record for Iowa, clover took over many golf course fairways, lawns and sports fields in Iowa.
Below are a couple of pictures from a driving range in central Iowa taken on the 28th of June, 2010. This is typical of what we are seeing this summer.
What can be done? Clover is surprisingly tolerant of 2,4-D, our old standby for broad leaf weed control. It is not uncommon for areas treated with 2,4-D alone to have the clover curl up, but then recover in a few weeks.
The best herbicides for clover control are those that contain MCPP, or one of the pyridine compounds: tyiclopyr, chlopyralid, and fluroxypyr. Of the three pyridines, triclopyr seems to be the best in my experience. You can find these compounds in several of the combination products for broadleaf weed control. Remember that any of the materials present a risk to dicotyledenous plants in the landscape such as trees and shrubs. They are also deadly to tomatoes and most flowering plants.