I live in north Toronto. This particular weed (photo attached) has spread throughout my large naturalized garden and also the lawn. It seems to thrive on shade and hiding amongst perennials. It starts as a low whorl of stems with 3-lobed leaves. In this photo is it about 6 inches across the plant. Then it shoots up a few tall flower stalk with very small, yellow flowers. Those are appearing now, early June. Once they go to seed, the seed head is appears like a burr and is about 1/4 inch diameter. I have been unable to make headway killing it with either the lawn friendly or round-up like products. Pulling it is not practical due to the 1/2 acre size of the garden. Any suggests as to what it is and how to get rid of it?
Thank you for contacting the Toronto Master Gardeners with your weed identification question.
The weed in your picture is a rough cinquefoil or Potentilla norvegica L. It is a member of the rose family and is native to North America. It is prevalent in Ontario, mostly in open areas such as fields, pastures and road sides but will also show up in open, sandy, dry locations in a garden. Depending on its location, it will act like an annual and die in the fall or act as a tender perennial and return in the spring if it survives the winter.
This weed will have small, 5 petaled yellow flowers then go to seed in June and July. It reproduces by seed with each plant producing up to 13,000 seeds.
The best control method for this weed it to remove it before it goes to seed. Mowing can take a flowering top off but the plant will then grow back but be shorter and may cause spreading roots. Pulling out the whole plant before flowering or at least before seeds spread will result in less work in the future. If you consider that the plant has a short lifespan of 2 years max, then be very diligent to keep mowing or lopping off flower heads, thus preventing them from going to seed. This way within two seasons , you will allow nature to do the rest through natural attrition.