Onion Grass (Romulea spp).
Onion Grass is also known as Guildford Grass, and is a small perennial herb that grows from a white bulb (corms). It has a yellow base and annual leaves with small pink, white, purple or yellow flowers.
Onion Weed has several shiny dark green leaves that grow from the base of the plant. If present the stem leaves are smaller.
Seeds of Onion Grass are spread mainly by mowing, but it also spreads by the corms, in the intestinal tract of grazing animals, soil and by water. All this makes Onion Grass difficult to remove once it establishes.
After you finish reading this, you will be able to:
- Identify Onion Grass, Guildford Grass or Onion Weed.
- Know what conditions favour Onion Grass.
- Know the best cultural and chemical options to control Onion Weed.
More information on common lawn weeds is in our weed ID chart. Onion Grass indicates low soil P. For more about what weeds reveal about soil conditions see our blog on indicator weeds.
How to Identify Onion Grass.
Category: Perennial herb.
Flower: The flowers are 2-3 cm in width and are small pink, white, purple or yellow. It has six petals and flowers in late winter and spring.
Height: Onion Grass grows up to 25 cm high.
Leaf length: The leaves are up to 18mm in length.
Reproduction: Every year the corm renews, and Guildford Grass reproduces by seed and corm. Seed of Onion Grass germinates in the Autumn to Winter, grows over the Winter and flowers from August to November.
The flowers develop in the second year and the seedbank persists for up to 5 years. This weed needs warm temperatures to germinate, and so favours bare ground in direct sunlight. Although a perennial, Onion Grass behaves like an annual and dies back at the end of the Summer.
Comments: Flowers typically open at noon and stay open only until 1 or 2 pm.
Habitat: Guildford Grass is a common weed of lawn, pasture, and sports fields.
How to remove Onion Grass from your lawn.
You can best control Guildford Grass with herbicides.
Cultural control.
Physical removal of Onion Grass is difficult as the corms occur to a depth of 200 mm. You can dig or pull out small populations of Guildford Grass. If you are intending to hand-pull this weed, be careful to remove the entire corm, as any left behind will resprout. This process has to be ongoing for several years to make sure that you remove all the remaining seeds and corms.
Research shows that Guildford Grass does not respond to increases in soil fertility so if you feed your turf areas it will help to control this weed by enhancing competition from your desirable turf grass species.
There are mixed reports in relation to the use of mowing to control this weed.
The general belief is that this does not work but work in Victoria shows that Onion grass is in fact very sensitive to close mowing.
If you mow at a height of 1 cm at three- to five-week intervals, it reduces the corm mass by 70%, seed pod density by 100% and plant density by 60% in comparison to not mowing at all. At a 5 cm height of cut, this also reduces onion grass corm mass by 58%.
Chemical control.
6-8 weeks later Onion Grass emerges, there is a short window when herbicides are at their most effective. This is because the old corm is exhausted and the new corm is only just starting to develop.
If you treat with a herbicide at flowering it gets rid of the flowers but not the corms.
If broadleaf weeds such as Dandelion or Bittercress are present, you can mix the herbicides below with Warhead Trio to control both types of weeds.
The sulfonylurea herbicide, Iodosulfuron controls Onion Grass. This is in the product ProForce Duke 100WG from Indigo Specialty Products.
Quali-Pro Negate also controls Guildford Grass but uou cannot use this on Buffalo grass turf.
