Giant Buttercup (Ranunculus acris)
Giant buttercup is a deep-rooted perennial weed that thrives in damp, compacted, or poorly drained areas. While it’s best known as a problem in dairy pastures, it can also creep into lawns that stay wet through winter or have poor soil structure.
It produces bright yellow, glossy flowers and forms dense clumps of jagged leaves that smother surrounding grass. Cows and other livestock avoid eating it because the foliage contains bitter-tasting chemicals, which gives the plant a strong advantage in paddocks and pastures. In lawns, its coarse growth and shiny leaves stand out sharply against finer grasses, giving the lawn a patchy appearance.
Identification
Giant buttercup grows in upright clumps rather than spreading by stolons, unlike the creeping buttercup (Ranunculus repens). Leaves form a basal rosette and are deeply lobed and jagged, with three main sections that are less clearly divided than those of creeping buttercup.
The flowers are bright yellow, 10–20 mm wide, and held on tall stalks through spring and summer. Each bloom has five shiny petals and a small green centre that later forms seed heads. The stems are upright, and the plant can reach 30–60 cm in height under favourable conditions.
It prefers moist, shaded, or low-lying areas, and lawns with poor drainage are especially at risk. Over time, it can form dense patches that resist mowing and crowd out finer grasses.
Why It’s a Problem in Lawns
Giant buttercup competes strongly for moisture and nutrients, especially where the grass is weak or soil is compacted. Because it grows upright and flowers freely, it can quickly take over wetter lawns and garden edges. Its deep roots make it difficult to pull by hand, and the plant can regrow from crown fragments left behind after mowing or partial removal.
Once mature, its seeds can spread easily in soil or by lawnmowers, leading to new infestations nearby.
Control in Home Lawns
Controlling giant buttercup in lawns requires both herbicide treatment and better drainage or aeration to prevent regrowth. Small patches can be managed manually, but established infestations often need repeat spraying.
Best control methods:
- Physical removal: Dig out isolated plants, ensuring the full root crown is removed. Dispose of material away from compost.
- Chemical control: Use selective turf herbicides containing dicamba, MCPA, or picloram/triclopyr (NZLA Gold). These active ingredients target buttercup species effectively without damaging most common lawn grasses.
- Repeat applications: Giant buttercup may require follow-up spraying a few weeks later to catch new growth from surviving roots or seed germination.
- Drainage improvement: Aerate compacted soil and reduce standing water. Buttercup thrives in soggy, oxygen-poor soil, so improving drainage will help the lawn recover naturally.
Prevention Tips
- Aerate heavy or compacted lawns each spring.
- Avoid overwatering and ensure surface water can drain away freely.
- Maintain dense grass cover with regular feeding to reduce open soil.
- Mow at a moderate height (25–35 mm) to promote strong grass growth.
- Spot-treat any small regrowth early before flowering occurs.