Black medic (Medicago lupulina)
Often confused with white clover, black medic has 3 leaflet, clover-like leaves. Low growing with trailing stems. Clusters of small, bright yellow flowers appear in late spring or early summer. See botanical illustration.
An annual that can persist as a perennial, black medic is common in lawns from May through September. It grows in dry, high phosphorus, compact soils and enjoys full sun. It will take over and thrive in areas with weak, patchy grass.
Encourage optimum turf growing conditions. Maintain adequate nitrogen levels with low to medium phosphorus levels. Keep turf adequately watered. Pull or dig shallow tap-roots when they appear. Aerate to relieve compaction.
The best long-term control of any weed problem is to look at the whole picture and improve growing conditions for the lawn: a thick, healthy lawn stands a better chance of competing with weeds.
1. Black medic is easily removed by hand digging. It’s best to get on top of this as soon as the weeds show up. Ensure the soil is moist to facilitate easier removal.
2. Don’t be too vigorous about dethatching, which may expose more bare soil surface to weed seeds. Always overseed with a desirable blend of grass seed, such as perennial ryegrass, after dethatching.
3. Improve growing conditions to keep the grass thicker and prevent black medic from getting established.
4. Set the mower blade higher during summer months so you remove no more than one-third of the grass blade; longer grass will not dry out as fast.
5. Cut more frequently and mulch the grass clippings to improve soil conditions.
6. Use a balanced organic fertilizer.