Clover taking over

The mystery weed is most likely yellow clover, also called black medick (Medicago lupulina). It tends to hug the ground and has clover-like leaves on long wiry stems and bright yellow flowers. It can be an annual or biennial, or sometimes a short-lived perennial that reproduces prolifically by seed. It spreads quickly through sparse lawns under stress - drought, low fertility or high levels of phosphorus, potassium or sulphur.

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.

CONTROL
1. Yellow clover 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 yellow clover 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.