Yarrow (Achillea millefolium)
Yarrow is a tough, deep-rooted perennial that often sneaks its way into New Zealand lawns. Once it’s in, it’s hard to shift. It spreads through underground rhizomes rather than seed, forming dense patches that compete with desirable lawn grasses.
Originally introduced as a drought-tolerant pasture plant, yarrow thrives in dry, compacted soils and survives close mowing with ease. That’s why it often shows up in tired or neglected lawns, especially through summer when grass growth slows.
Identification
Yarrow’s feathery, finely divided leaves give it a distinctive, fern-like appearance. Each leaf has 15–20 narrow leaflets along each side, far more than the 5–8 leaflets you’d see on Onehunga weed. When it’s allowed to flower (usually in garden edges or unmown areas), yarrow produces flat-topped clusters of small white blooms on upright stems.
If you crush a leaf, it doesn’t give off the strong odour typical of weeds like stinking mayweed. Underground, yarrow forms an extensive network of creeping rhizomes that help it regrow even after mowing or spraying.
Why It’s a Problem in Lawns
Yarrow competes aggressively with grass, particularly in thin or under-fertilised lawns. Because it grows from rhizomes rather than seed, it can survive most mowing regimes and quickly fill bare patches. Its fine foliage and upright stems also stand out visually, making lawns look uneven in texture and colour.
Control in Home Lawns
Yarrow is one of the more persistent lawn weeds to control. Simply mowing or hand-pulling rarely works, as new shoots regrow from underground rhizomes.
Best control methods:
- Cultural approach: Improve lawn density and soil health. Regular fertilising, aeration and correct watering help desirable grass species outcompete yarrow over time.
- Physical removal: In small infestations, dig out the entire plant, ensuring you remove as much of the rhizome system as possible. Leave any fragments and they’ll reshoot.
- Chemical control: Use a selective broadleaf herbicide that contains actives capable of moving into the root system, such as clopyralid (NZLA BWC) or combinations containing MCPA and dicamba. Glyphosate can be used for spot-spraying, but full eradication may need repeat treatments.
Prevention Tips
Healthy, dense lawns are your best defence. Overseed bare areas in autumn or spring, maintain balanced nutrition, and avoid over-watering. Yarrow prefers dry, compacted soils—fixing those conditions will make your lawn far less inviting to it.