Nutgrass! A disruption in the soil can bring this noxious weed back.

Nutgrass (as know as Cyperus rotundas) is the scourge of green thumbs the world over. There’s a reason it’s often referred to as the world’s worst weed (which is a big call) – why? Well, because nutgrass is a difficult nut to crack (pun intended) being known to even shoot up through asphalt and even pierce though pool liners and sometimes there just seems no stopping it.

One of the best ways to identify nutgrass is by looking for the reddish-purple colour of the flower spikelets. Of course this only helps if the weed is left to grow to a size where it produces spikelets. Since it loves to sprout out of newly laid lawn you can identify it from its rapid growth and stiffer, stronger upright blades. Nutgrass will typically grow faster than the rest of the turf and will be obviously taller.

Nutgrass nuts can stay inactive or dormant in soil for up to a decade – and some have even been found 30 cms below the surface in heavy clay soils. Disruption in the soil from general earthworks or vehicle movement can cause the previously dormant/inactive nuts to shoot. The weight of a vehicle above causes the earth to crack and shift allowing oxygen and water deeper into the soil and to the nuts.

Many other weeds and larger plants can eventually take over nutgrass but they will rarely completely suppress it. If not adequately controlled – which is tricky considering its complex interconnected network of rhizomes (roots) – it will persist even though its growth is restricted by shade. If in full sun, nutgrass will grow at a rapid rate, especially if it doesn’t have competition – which can occur when annual weeds are removed with herbicide or manually.

Infestations of nutgrass are often discovered when we undertake lawn care tasks such as cultivating or improving the soil, bringing in new soil, weeding, fertilising, removing old turf and heavy watering – all of these things can disturb nuts and encourage them to shoot.

Most soil blends these days are carefully screened, aged and heat treated so the odds of getting nutgrass delivered to your place in a commercially produced planting material are small.

It’s a case of prevention is better than cure. The best approach is to prevent the establishment of the nutgrass in the first place since, once established, it can be difficult to control. Nutgrass establishment can be prevented through both chemical and manual control methods.

• Remove small plants before they develop tubers;
• Eliminate the wet conditions that promote growth;
• Use fabric mulch (geo fabric) in garden beds;
• Shade and dry the lawn. 

If these methods are not possible then control can be achieved by using properly timed applications of specialty herbicides.

Despite yielding mixed results when used on nutgrass infestations, applications of Glyphosate* has been found to translocate within the sprayed nutgrass plant and attached tubers. This translocation means that the Glyphosate can kill the plant it is sprayed on as well as tubers and the nuts through the underground rhizomes. The best results seem to be on fully established nuts that are already displayed the coloured spikelets (usually over one month old).

Just remember that Glyphosate is not a selective herbicide meaning that it will poison everything it directly touches.

Another option is to use a herbicide with the active ingredient Halosulfuron-methyl (more commonly known as Sempra) since this inhibits acetolactate synthase – a key enzyme in the nutgrass’ metabolic pathway – and so stops the plant growth. Death should occur in 2-3 weeks from the initial application. Sempra does only have a half-life of around 30 days so it does not persist for very long in the soil. Multiple applications might be needed for particularly advanced nutgrass infestations.

*Glyphosate is the common active ingredient in many readily available herbicides (often branded as Roundup or Zero).

Our best tips for staying on top of the worst weed in the world:

• Buy your turf, soil and top-dressings from a reputable source;
• Deal with an infestation as soon as it’s discovered;
• Stay vigilant – especially if you undertake tasks which may cause the soil to shift.

If you have further questions about nutgrass or other lawn care issues, just contact the team at Daleys Turf – we’re here to support you for the life of your lawn.