Onion Grass (Romulea rosea)

Photo: (c) Harry Rose cc-by-2.0.

Family: Iridaceae

Growth Habit: Onion grass is a small perennial, with tough grass like leaves up to 30 cm high and 1-2 mm wide, with blund edges and prominent midrib. It produces a small brown underground corm about 1 cm in diameter. Flowering occurs in Spring each year.

Type of Plant: Autumn-winter germinating herbaceous perennial.

Photo: (c) Andrew massyn CC BY-SA 3.0

Flowers: Pink-violet flowers with a yellow throat and 6 equal spreading petals about 20 mm across.  One to four flowers are produced per plant, singularly at the end of a short stout stalk.

Fruit/Seed: Fruit a wrinkled broadly oblong capsule, 6 – 10 mm long containing many small oval seeds is produced soon after flowering, and as seeds mature, the capsule angles down to the soil surface. Eventually the capsule dries and splits open, releasing the seeds.

Dispersal: Soil disturbance and movement is the main form of dispersal, both of seed and corms. Water may also transport seeds small distances.

Distribution: Onion grass is a widespread weed throughout Tasmania. It most commonly occurs in run-down areas or pastures and turf areas such as sports fields, and on roadsides.

Status: Romulea rosea is not a declared weed under the Noxious Weeds Act 1964.

Weed Impact:

  • Onion weed is extremely difficult to remove once an infestation has become established. The tough leathery leaf blades readily blunt mower blades, and animals rarely graze the plant.

Control Methods:

  • Physical removal is difficult as the corms are usually very deep in the soil (up to 200 mm). Pulling on the leaf blades usually snaps them off at the top of the corm, leaving the corm in the soil.
  • Cultivation is seldom useful in destroying plants and may in fact assist their dispersal by breaking up and distributing the corm. Limited success may be obtained where the corms can be brought to the soil surface to dry out in summer.
  • Herbicides registered for use in Tasmania include metsulfuron methyl and glyphosate.

N.B. Always check the herbicide label before use.