Weed prevention by thickening the lawn
A thick lawn is the best weed control. Fill thin spots, mow a touch higher and feed, so grass crowds out weeds before they start.
- When
- Mar 1 – Apr 15 (northern hemisphere) · Sep 1 – Oct 15 (southern)
- For
- Cool-season lawns (ryegrass, fescue, bluegrass)
- Rule
- sp-pre-emergent · Hessayon; RHS; Mellor
A thick lawn is the best weed control. In early spring overseed thin spots, mow a touch higher and feed so the grass crowds out broadleaf weeds and annual meadow-grass (Poa annua) before they get going. A pre-emergent product is optional and secondary — density comes first.
Why density works
Most lawn weeds, including dandelions, clover, crabgrass and annual meadow-grass (Poa annua), establish from seed that needs light and space at the soil surface. In a dense lawn there is neither. Every thin patch, by contrast, is an open invitation. Weedkillers treat the result; density removes the cause.
What to do in early spring
- Fill the gaps: overseed thin and bare patches as soon as the soil warms; see spring overseeding.
- Mow a little higher: an extra centimeter of leaf shades the soil and keeps weed seed from germinating.
- Feed: a spring feed drives the new shoots that thicken the sward.
- Fix the cause of thin areas: shade, compaction, poor drainage or acidic soil.
Where a pre-emergent fits
A pre-emergent herbicide is optional and secondary. It stops weed seed, but also grass seed, so it cannot be combined with overseeding the same spring. Use it only where a specific annual weed, such as crabgrass, was a real problem; see crabgrass pre-emergent timing.
Sources
- D.G. Hessayon — The Lawn Experthessayon
- Royal Horticultural Society — lawn guidancerhs
- David Mellor — The Lawn Biblemellor