I have a dreadful weed in my garden. It has an underground runner making it nearly impossible to stop. The leaves are round and grow about 6 inches high. In the early spring it gets small yellow flowers. It comes up through my perennials – everywhere in a small garden. Is there a chemical I could safely use to get rid of this pest? And what might it be.
Weeds such as these can be very difficult to stop without significant effort. It can be a daunting task to identify a weed without a picture, but it sounds like you could be describing the Common Yellow Wood-Sorrel (Oxalis stricta).
This weed spreads primarily from rhizomes in the ground where its roots grow from. The rhizome is the underground plant part where energy is stored. Potatoes and “ginger root” are examples of rhizomes. More plants will grow out of the rhizome each year. The best way of controlling this weed is to dig out as much of the roots and rhizomes as possible, so that it can no longer spread. Further cutting and pulling out of any re-growth may be needed to rid your garden of this pest. In many cases, 100% irradication may be difficult so you may have to live with controlling it as best as you can.
Most herbicides will not be effective against this type of weed as it may kill some of the plants, but not the actual rhizome. Also, in Ontario, we have strict regulations regarding pesticide/herbicide use, so below you will find a link to a short guide on organic weed control that we developed in partnership with the City of Toronto: