Purple "clover-like" weeds
All summer long my lawn has been infested with purple clover-like weeds that seem to grow in clumps. I've used Weed B Gone ( sprayed selectively on the patches ) with limited to no success. The weeds seem to dry up, and about the time I get excited, the purple weeds come back with vengeance! What kind of weed killer will eradicate this stuff?
I forgot to add that these weeds produce little yellow flowers...
Arapahoe County Colorado weed issues
Without photos, it is difficult to confirm the identity of this weed, but it sounds like it is probably Oxalis corniculata, often called yellow woodsorrel or oxalis. It is considered a creeping perennial weed, sometimes mistaken for clover. It has yellow flowers and the mature fruit can explode ejecting seeds up to 10 feet.
It is a difficult weed to control in home lawns. It prefers lawns that are fertile and well-watered, and the appearance of this weed generally indicates that the lawn is not very vigorous and is likely being mowed too short. Incidents of the weed may be reduced by mowing the lawn no shorter than 3 inches high and also by watering less frequently, but for a longer duration.
An herbicide containing triclopyr (such as Weed-Be Gone Chickweed, Spurge and Oxalis killer) can help to remove this weed from lawns. Make sure the herbicide contains triclopyr; and read and follow all label directions when using pesticides.
Outside the lawn, dig this weed out as soon as the clover-like leaves appear, or spray with glyphosate (Roundup and other trade names). It will work best if the target weeds are not drought-stressed, so consider watering the area a day before spraying them with glyphosate.
A pre-emergent herbicide applied correctly and at the proper time should also help provide control. Optimum seed germination occurs around 60°F so a pre-emergent should be applied before it warms up in the spring. To be effective in controlling seedlings of perennial weeds, a pre-emergent must be used prior to germination of the weed seeds.
Here are some helpful fact sheets: