Cause and symptoms
The hydrophobic (non-wettable) nature of many Western Australian soils, particularly the sandier types, means that once they dry out, they are very difficult to re-wet thoroughly. During the hot months, waxy substances that emerge from the soil’s organic matter form a coating on the surface and this can prevent water from penetrating to the root zone.
The result is that dry patch develops on the lawn. Water droplets falling onto it simply run off to the side, and the water begins to establish a preferred pathway — so bordering areas of the lawn receive more of a drink and become greener just as the dry patch becomes browner. Too often, dry patch is blamed on African black beetles when few or none are present.
Remedying the water repellence of the surface is not quite as simple as applying a soil wetting agent, because the wetting agent will immediately run off in the same way the water does.
Before application aerate the surface, either with a mechanical or manual aerator or with a garden fork. Then soak the area with water and then apply the wetting agent. Read the directions on the label and follow up with another application after the recommended interval to prevent repellence building up again.
In the meantime it is worth checking to see whether faulty reticulation or a broken sprinkler was responsible for the dry patch forming in the first place. Even if all the sprinklers are working, a strong easterly wind may be diverting the spray, especially if it is in the form of a fine mist.
African black beetle
The main difference between dry patch and lawn heavily infested by African black beetle (Heteronychus arator) is that, in the latter case, the damaged turf can be rolled back like carpet. It is the grubs, not the adult beetles, which are the more voracious feeders, literally cutting the grass off at its roots.
Products containing cyfluthrin and bifenthrin, synthetic pyrethroids or chlorphyrifos control both the beetle and grubs.
Imidacloprid, sometimes combined with small quantities of cyfluthrin, is a low toxicity grub killer, as is neem extract. Some lawn fertilisers also contain insecticides like bifenthrin.
Pesticide should be used only for severe infestations where lawn damage is unquestionably due to African black beetle. There is some debate about how many beetles per square metre constitute a severe infestation, but it is possible to have a large number of beetles living in the lawn without seeing any damage.
To test for the presence of African black beetle pour a bucket of soapy water onto the affected areas to flush the beetles and their larvae to the surface.
Adult beetles will congregate under a moistened hessian bag or an old piece of carpet placed on the lawn at the end of the day. When the matting is lifted next morning, the beetles can be collected, put into a sealed bag and thrown in the bin. The adult beetles are nocturnal and are often drawn to garden lights where they are easy targets for collection.
Cause and symptoms
A fungal disease causes dollar spot, particularly in Queensland blue couch or saltene lawns, both of which have quite high watering needs. The disease can develop rapidly in humid weather, and is most prevalent from November to March. Soils deficient in nitrogen favour the development of the fungus.
Although each individual brown spot rarely exceeds the size of a 50 cent piece (or a US dollar, hence the common name), often the spots can merge to create larger patches, and the fungus bites deeply into the mat of turf. The markings on the grass blades are tan coloured with a brown border. Another feature of the disease is fine webbing, which may be seen early in the morning when the lawn is covered in dew.
Fungicide is best used as a preventative rather than as a cure. Sometimes fungicides need to be alternated because the fungus can develop resistance to the chemicals in the first product used. Home garden fungicides include mancozeb with sulphur, chlorothalon, iprodione and tebuconazole with trifloxystrobin.
Prevention also relies on other factors such as thatch control, moderate fertilising and watering in the morning so the grass dries out during the day.
Control is time consuming and it may be easier to replant with grasses more suited to Western Australian conditions.
Mullumbimby couch and other sedges
Common names are often misleading. Mullumbimby couch (Cyperus brevifolius) is not a couch, and other members of the Cyperus genus commonly called nut-grasses are neither grasses nor nut-bearing plants. In fact, all these plants are sedges, which invade stealthily because their fine foliage makes it hard for gardeners to detect them until they have gained a major foothold.
Most other lawn weeds are annuals and therefore appear as seasonal problems, but Cyperus species are perennial. They are so vigorous that they can rob the lawn of water and nutrients, and eventually they will take over completely. Being prolific seed producers, they also spread quickly to other areas of the garden and travel using underground rhizomes.
Once Mullumbimby couch and other sedges are well established they are difficult to eradicate. Hand-weeding or spot-painting with certain systemic herbicides can halt a small invasion — but it is important to note that not all herbicides are compatible with all lawn types
Suitable herbicides contain bentazone, halosulfuron and MSMA but check with garden centre staff as the wrong chemical can seriously harm a lawn. Always read the product label and apply as directed.
Prevention is better than a cure and a robust lawn is less susceptible to invasion by any weed, including Cyperus species.
General lawn care
Extensive preparation should be undertaken before planting lawn runners or turf to ensure the lawn is able to withstand drought, weeds and pests.
On a new site, once the ground has been cleared and levelled, spray herbicide to kill existing and opportunistic weeds. A repeat treatment may be necessary a few weeks later, to kill any seeds that have germinated in the interim.
An organic soil improver or compost, which will help retain moisture, should be dug into the soil and augmented with a lawn starter food. Sandy soils should be improved with an application of bentonite or kaolin clay, available from garden centres. Apply at a rate of 0.5 to 2kg per square metre after sprinkling onto dry soil and digging in to a depth of 30cm. This will help the soil retain water and nutrients.
If the area is heavily compacted cultivate with a corer or rotary hoe to improve drainage and reduce compaction.
When new lawn is needed to replace an old one, the steps are the same except that any remnant runners or stolons belonging to the previous lawn should also be removed, otherwise they will sprout afresh and contaminate the new turf.
A wetting agent should be applied twice a year to break down water repellence which creates dry spots in the lawn. This is caused by a waxy coating on soil grains. The wetting agent breaks down the wax and allows water to be distributed through the soil more easily.
To maintain the vigour of an established lawn, water it regularly within mandatory watering restrictions, and feed using a specialised lawn fertiliser. Additional watering with a hand held hose may be necessary in very hot weather.