Grass weeds is a term broadly used to describe a variety of grass-like weeds, including nutgrass, guinea grass, and Rhodes grass. They're commonly referred to as grass weeds due to their habit and appearance. They are typically considered weeds in lawns, garden beds, verges or pastures and are well-adapted to growing in our climate. Grass weeds can spread quickly, making them difficult to control. It's important to treat quickly and regularly to help eradicate them. Here's a list of common grass weeds and how to control them.
Nutgrass is a common weed in lawns, garden beds, parks and pastures. This perennial weed grows between 20-50cm tall with narrow, light green linear leaves that arise from a tuft at the base of the stem. Its tough and invasive nature can be attributed to its creeping underground stems or ‘rhizomes’, which aggressively creep into surrounding areas. Purple egg-shaped tubers or ‘nuts’ usually 1-2cm long form on the rhizomes and are the energy store from which new shoots and rhizomes are produced.
Flowers are reddish to purple flattened spikes and appear from summer to autumn. Seeds are ovoid, 1mm long and can be coloured black, brown or dull green.
There are various options to control nutgrass in lawns, garden beds, paths and rockeries. These options are non-selective, so you will need to take care when spraying or applying near plants you don’t want killed. If the infestation of nutgrass in your area is high, multiple applications of the product may be required to get it under control. Once the weeds have died, dig out as much of the tubers and rhizomes as possible. Observe the area closely as this disturbance can cause new shoots to arise – if there is any regrowth, treat as soon as possible.
Guinea grass, also known as panic grass, is a highly invasive weed species of pastures that has spread to nature strips, vacant land sites and roadsides. It was introduced as stock feed or fodder for cattle sheep. Due to its fast-growing nature, high drought tolerance and ease of seed dispersal, it has quickly become a major threat to our biodiversity.
This perennial weed forms a large clump of tufted grass with short underground rhizomes. It typically grows 1.5m tall and wide, although some can grow to 3m tall. The leaves are narrow with a prominent mid-rib and pointed tips.
The inflorescences are finely branched, with small ovoid flower spikelets (3mm long) which can be green, purplish or red. Seeds are oblong and are often purple.
Guinea grass reproduces from seed, so it’s best to treat early when plants are actively growing and flowering. Apply a suitable weedkiller in late spring to early autumn. Repeat applications may be required for regrowth. Once clumps have been killed, dig out with mattock or similar.
Rhodes grass, also known as feathertop Rhodes grass, is a large clumping perennial or annual weed that grows up to 1m tall. It is considered a major pest and is widespread along eastern parts of Australia. It forms small clumps of bluish-green stems and leaf blades between 5-25cm long. The stems are semi-prostrate, which means they grow close to the ground and can produce roots and stems at the nodes (stoloniferous), making it an incredibly invasive weed.
The flower heads appear during summer and autumn and are clustered on the ends of stems. They start off greenish to red-brown and as they mature, darken to brown or straw colour. The seeds are 1.5mm long and are slightly flattened ovoids.
Plants can spread into neighbouring areas via their creeping stems or stolons. Actively growing plants should be sprayed with a suitable weedkiller. Rhodes grass also easily spread via seeds, so seed heads must be stopped to help break the lifecycle. Apply a weedkiller when seed heads begin to form and repeat applications for any regrowth.
Annual ryegrass (Lolium rigidum) and perennial ryegrass (Lolium perenne) are both commonly grown lawn grasses, however, they can also be considered weeds of warm season grass lawns, garden beds, and crops and pastures. Both species are upright clumping grasses made up of dark green glossy leaves that are tinged purple at the base. They flourish in cool conditions and can grow 50-90cm in height, although they are often shorter in lawn situations. In warmer climates, their growth can be restricted.
The flowering stems produce flat spikes along the tip of the stalk and appear in winter and early spring. Seeds follow in spring to early summer and are 4-6mm long and flat. A high number of seeds are produced per plant and are quick to germinate after a rain event, making it difficult to control.
As seeds are produced in spring and summer, the best time to control is autumn and winter, prior to weed seed setting. There is no selective control for ryegrass growing in lawn and turf, so be careful when applying or spraying weedkiller.
There are two common weed species of barnyard grass: barnyard grass (Echinochloa crus-galli) and awnless barnyard grass (Echinochloa colona). Barnyard grass is a tufted annual grass with flat light green leaf blades that are 30cm long. The flower spikes are green or purple and crowded along the tip of the stalk. It has a short awns or hair-like bristles (up to 50mm) on the ends of its seeds. Awnless barnyard grass is a low-growing tufted weed that grows up to 1m tall. As its name suggests, the seeds are awnless.
Both species are prolific seeders, producing more than 40,000 seeds per plant which can persist in the soil for up to 12 months. Seeds germinate after heavy rains in late spring and summer.
It’s best to control weeds before they set seed. Spray with a suitable weedkiller when the plants are actively growing and reapply if there is any regrowth.
Brome grass is said to be the most competitive of all weed grasses. There are several species, but the three most common weed species include: great brome (Bromus diandrus), rigid brome (Bromus rigidus) and prairie grass (Bromus catharticus syn. Bromus wildenowii). They are all densely tufted annuals or short-lived perennials with dull hairy green stems and leaves that can grow up to 1m. Brome grasses all look very similar, but their flower and seed heads differ slightly in colour and shape.
Clusters of purplish-brown or straw-coloured flowers appear on the ends of stalks during spring and summer, followed by small oval seeds from autumn to spring. Brome grasses persists so well in landscapes due to their prolific seed-bearing nature, producing an average of 1000/m2. They’re also well adapted to low rainfall and because of their aggressive root systems, they can outcompete other crops and vegetation for water and nutrients. The seeds can also remain viable in the soil for up to three years.
As brome grasses spread by seeds, it’s important to prevent seed set and control early in the season with a suitable weedkiller. Spray when weeds are actively growing in spring and summer.