Oxalis, also referred to as Yellow Wood Sorrel, is a Summer Annual Weed. As a summer annual, it germinates sometime in the late spring, grows well during the heat of summer, then dies off with the cold weather. Oxalis is sometimes referred to as “yellow clover” because it looks like clover but has a yellow flower.
A little oxalis is normal. It is an annual and will fill in bare areas during the heat of summer. A lot of oxalis indicates that either you're mowing too short in the spring and summer; or that your lawn is just not thick enough or healthy enough to compete with hit.
Since Oxalis will die soon anyway, the bulk of your effort should go into improving your lawn to prevent it next year. Here are a few things you can do to eliminate it right away:
The first thing you can do is simply pick it. You don’t need to worry about getting too much of the roots, just get the main part of the plant.
The second option is to use a spot chemical weed treatment which will have some success but can also cause stress on the existing grasses so it should be used sparingly.
Oxalis and other annual weeds will emerge where your lawn is not competing. It could be that your just mowing your lawn too short and keeping your lawn from competing.
It could also be that the grasses you have are weaker older grasses that just don’t fight that well. If you have older poor grasses, a total renovation might be the best choice. If you don’t want to go that far, simply superseding what you already have may really help. Contact us to learn more or to get our opinion on what would be best for your situation.
Good Nature Blog: How to Kill Clover Without Chemicals