Two low-growing weeds tend to spread through lawns in early summer just as the grass slows after its spring rush: speedwell and self-heal. Both share a frustrating trick, they grow and even flower below the height most people mow at, so cutting the lawn does nothing to stop them and can actually help them spread. The fastest route to control is to thicken the grass with a summer feed, spot-treat with a selective lawn weedkiller while the weeds are actively growing, and resist mowing for a few days afterwards. Here is how to tell the two apart and beat each one.
How to Tell Speedwell and Self-Heal Apart
Speedwell (Veronica species) is the one most people notice first because of its flowers. They are small, usually four-petalled, and range from sky blue to white, sitting just above a mat of small, rounded, slightly scalloped leaves. After flowering it produces distinctive heart-shaped seed pods. The most troublesome type on lawns is slender speedwell, which spreads by creeping stems above ground and by underground runners, rooting wherever a node touches soil. That habit is the reason it forms spreading patches rather than single plants, and it is happy growing at the lowest mowing heights.
Self-heal (Prunella vulgaris) is a tougher, more upright perennial from the mint family. Its leaves are oval and grow in opposite pairs, and in summer it throws up short spikes of purple, hooded flowers. On an uncut lawn those spikes can reach 20cm or more, but on a regularly mown lawn the plant adapts and will flower on spikes as low as 3cm (just over an inch) to make sure it sets seed before the next cut. That ability to flower below the blade is exactly why mowing alone never clears it.
One useful clue is the company each weed keeps. Self-heal tends to colonise nutrient-poor, slightly compacted soils and patchy, undernourished lawns, so a thriving self-heal population is often a sign the grass around it is hungry. Slender speedwell will grow almost anywhere but spreads fastest in lawns that are mown short and infrequently fed, where the thin sward gives its creeping stems room to run.
It helps to know what you are not dealing with, because several lawn plants look similar at a glance. Speedwell is sometimes confused with creeping clover, but clover has three rounded leaflets and white or pink ball-shaped flowers rather than speedwell’s single rounded leaves and flat blue flowers. Self-heal is occasionally mistaken for bugle or ground ivy, both of which also carry purple flowers, but self-heal’s tidy oval leaves in opposite pairs and its squat, club-shaped flower head set it apart once you look closely. Getting the identification right tells you which control approach will work and saves you treating a weed that needs a different method.
Why Mowing Makes the Problem Worse
Both weeds have learned to live underneath the mower, but slender speedwell takes it a step further. Because it spreads from stem fragments that root at the nodes, every pass of the mower can chop the creeping stems into pieces and scatter them across the lawn, where many of them take root and start new plants. In effect, mowing a speedwell-infested lawn can sow it more thickly. This is also why you should never add clippings from a speedwell lawn to the compost heap or spread them as mulch, because the fragments survive and travel with the clippings.
The biology behind both weeds points to the same fix. They thrive in the gaps, the thin, bare or weak areas where grass is not dense enough to crowd them out. A thick, well-fed sward shades the soil surface so weed seeds cannot get the light they need to germinate, and it leaves no open ground for speedwell stems to colonise. Self-heal, which prefers poor soil, is gradually outcompeted as the grass around it grows stronger. So the single most effective long-term move against both is not a chemical at all, it is feeding and thickening the grass.
The Treatment That Actually Works
For self-heal in a small lawn, hand removal works well because it has shallow roots. Rake the area first to lift the creeping stems upright, then use a hand fork or weeding tool to dig out the whole plant including the roots, taking care to remove any pieces left behind. Speedwell is harder to pull cleanly because of its rooting runners, so for anything more than the odd plant a selective weedkiller is usually the practical choice.
Selective lawn weedkillers work by mimicking a plant growth hormone, throwing broadleaf weeds into uncontrolled growth that kills them while leaving the grass, which processes the chemical differently, unharmed. Speedwell has a reputation as one of the hardest lawn weeds to kill this way, and the active ingredient with the best record against it is fluroxypyr. A widely available product is Weedol Lawn Weedkiller, which combines fluroxypyr with MCPA and clopyralid: a 500ml concentrate costs around £18/$23 and treats up to 330 square metres. Apply it between April and September while the weeds are growing strongly, and do not mow for three days before or after so the plant takes up and moves the chemical down to its roots. Self-heal usually responds to the same broadleaf treatments, and products combining triclopyr and dicamba are also effective.
Expect to treat more than once. Both weeds are perennials with established root systems, and a single application often knocks them back without finishing them off, so a follow-up after four to six weeks catches the survivors and any seedlings. You will find selective lawn weedkillers at B&Q, Wickes, Home Depot, Amazon and most garden centres, sold both as ready-to-use sprays for spot treatment and as concentrates for whole-lawn coverage.
Keeping Them From Coming Back
Once you have cleared the weeds, close the door behind them. Feed the lawn through summer to keep the grass dense, raise your mowing height a little so the grass shades the soil and the weeds get less light, and overseed any thin or bare patches in autumn before speedwell or self-heal can move back into the gaps. If self-heal keeps returning, treat it as a soil message: the grass is probably underfed or the ground is compacted, so a feed and an autumn aeration will do more than repeated spraying.
For gardeners who would rather avoid chemicals altogether, the organic route is slower but steady. Repeated hand-weeding of self-heal, combined with a spring and summer feeding programme and an autumn overseed, gradually tips the balance toward the grass. A 1.5kg bag of hard-wearing lawn seed costs around £15/$19 and covers roughly 50 square metres at overseeding rates, which is usually plenty for patching the gaps left after weeds are removed. Iron-based lawn sand can also weaken broadleaf weeds and green up the grass at the same time, though it works best as part of a feeding routine rather than as a one-off cure.
Get the timing wrong and you can spend a whole season chasing these weeds. Mowing a speedwell lawn without treating it first simply spreads it, spraying in a dry spell when the weeds are not actively growing wastes the chemical, and ignoring a hungry, thin lawn lets self-heal reseed itself again and again. Treat while they are growing, feed the grass that should be crowding them out, and both weeds become far easier to keep out of the lawn for good.
Timing the treatment around the weather pays off too. Selective weedkillers move through a plant fastest when it is growing actively and the soil is moist, so the best window is a mild, settled spell after rain rather than the middle of a hot, dry heatwave when the weeds shut down to conserve water. Spraying in drought conditions wastes the product and risks scorching grass already under stress. If a dry spell is forecast, water the lawn a day or two before treating, then choose a still, dry morning to apply so the chemical is not blown onto borders or washed off by rain before it can work.
