Onehunga Weed (Soliva sessilis)
Onehunga weed, also known as lawn burweed, is one of the most disliked lawn weeds in New Zealand. It’s the prickly culprit that ruins barefoot walks across the lawn each summer. The tiny spined seed heads are painful, spread easily, and make this weed a persistent pest in home lawns, sports fields, and parks.
It’s a winter annual, meaning it germinates in autumn, grows through winter and spring, and dies off as the soil dries in summer—leaving behind the spiny seed heads that lodge in feet, jandals, and pet paws. Once those seed heads appear, the damage is done, so prevention and early control are key.
Originally from South America, Onehunga weed was first recorded in the Auckland suburb of Onehunga—hence the name.
Identification
Onehunga weed forms small, flat rosettes that often grow below mower height, allowing it to survive repeated mowing. The finely divided leaves resemble small ferns or carrot tops and are a dull green colour. Each plant develops a shallow fibrous root system, unlike similar-looking perennials such as yarrow which spread via underground rhizomes.
In late spring, the plant produces clusters of small, spined burrs near the base of the foliage rather than on stalks. These burrs contain sharp seeds that dry and harden through summer, remaining on the ground even after the plant dies off.
It’s often confused with:
- Yarrow (Achillea millefolium): has a rhizome system and finer, denser leaves.
- Soldier’s button (Cotula australis): grows small brown seed heads on stalks rather than burrs at ground level.
Why It’s a Problem
In lawns and turf, Onehunga weed spoils surface quality, produces sharp burrs that injure bare feet, and spreads rapidly through mowing and foot traffic. It thrives in thin, open turf and poorly maintained lawns where there’s bare soil for seedlings to establish in autumn.
Because it grows below mowing height, mechanical control is ineffective, and once the burrs form, chemical control won’t remove the prickles left behind.
Management and Prevention
Cultural control methods:
- Maintain a dense, healthy lawn: Fertilise, irrigate, and overseed thin patches to prevent open soil.
- Mow regularly but not too short: Over-scalping weakens grass and encourages Onehunga weed to establish.
- Irrigate during dry periods: Helps lawn grasses outcompete this winter-germinating weed.
- Apply pre-emergent herbicide in autumn: To prevent new seedlings from germinating.
- Remove burrs before they dry: Once hardened, they persist even after the plant dies.
Chemical Control:
Onehunga weed can be stubborn, showing tolerance or resistance to several common lawn herbicides. The most effective results come from combination herbicides that include multiple active ingredients.
Effective control options:
- MCPA/mecoprop/dicamba mixtures (sold under several turf brands) – good control of both normal and herbicide-resistant strains.
- Clopyralid (NZLA BWC) or triclopyr/picloram mixtures (NZLA Gold) – very effective but overuse has led to resistant strains in parts of the North Island.
- Mecoprop/ioxynil/bromoxynil (Image) or bentazone (Basagran) – effective when plants are young and actively growing.
- For resistant strains, avoid relying on clopyralid or picloram year after year. Rotate herbicide types and apply while plants are still small and green, before seed burrs form.
Key Tip
Once burrs have developed, even if you kill the plant, the prickles will remain. The best approach is to spray in early spring, long before flowering and seed formation begin.