Onion Grass (Romulea rosea).

Onion Grass (Romulea rosea) is also known as Guildford Grass, and is a highly invasive 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 Grass has several very tough, shiny dark green leaves that grow from the base of the plant. If there are stem leaves present these are smaller. The flattened leaves, have a groove running lengthwise.

Seeds of Onion Grass are spread mainly when you mow, 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.

It is considered to be an environmental weed in much of Australia.

After you read this, you will be able to:

• Identify Onion Grass, or Guildford Grass.
• Know what conditions favour Onion Grass.
• Know the best cultural and chemical options to control Guildford Grass.

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.

The map of Onion Grass distribution is courtesy of The Atlas of Living Australia.

Category: Perennial herb.

Photosynthetic Pathway. Onion Grass is a C4 weed.

Flower: The flowers of Onion Grass are 2 to 3 cm wide, 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 18 mm longh.

Reproduction: Every year the corm renews, and Guildford Grass reproduces by seed and corm. Seeds germinate in the Autumn to Winter, grow over the Winter, and then flower from August to November.

The flowers develop in the second year and the seedbank persists for up to 5 years. Onion Grass needs warm temperatures to germinate, and so it favours bare ground in direct sunlight. Although a perennial, Guildford 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.

You can best control Guildford Grass with herbicides.

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, but If hand-pull this weed, be careful to remove the entire corm, as any left behind will re-grow. This process has to be ongoing for several years to make sure that you remove all the remaining seeds and corms.

Research shows that Onion Grass does not respond to increases in soil fertility. This means that if you feed your turf it will help to control this weed as it enhances competition from desirable turf grass species.

There are mixed reports in relation to the use of mowing to control Romulea rosea.

The general belief is that this does not work but work in Victoria, work 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%. This also reduces seed pod density by 100% and plant density by 60% in comparison to not mowing at all. A 5 cm height of cut, reduces Onion Grass corm mass by 58%.