Weed-opedia: Yellow Woodsorrel
Yellow Woodsorrel (Oxalis stricta)
What does it look like?: Heart-shaped green leaves attached at the tips in groups of three (similar to shamrocks) and small yellow 5 petaled flowers
What causes it?: It reproduces by seed, and tends to germinate later than many other weeds, and often escapes spring herbicide applications. It also has stolons, and can reproduce by rhizomes.
When does it show up?: Summer through fall.
Best treatment plan: Repeat pre-emergent treatments throughout the growing season. Post-emergents can help also. Call a professional for a free quote.
How long will it take to get rid of it?: Probably a couple of growing seasons. This one is tenacious, and is a perennial.