Brown patches in a Cairns lawn: what is causing them, season by season, and what mowing can fix

The quickest way to narrow down a brown patch in Cairns is to match it to the time of year, then run one check. Bureau of Meteorology averages for Cairns Aero show how far apart the seasons are: January, February and March each average more than 400 mm of rain, while every month from June to October averages under 50 mm.

• Dry season, June to October: dry or water-repellent soil. Check: tug the brown turf. If it stays anchored, root-eating grubs are less likely.
• Straight after a mow, any month: scalping. Check: the patches sit on humps or high spots, and the grass looks stemmy.
• Warmer, wetter months: armyworm or curl grubs. Check: birds working the lawn early or late in the day, or turf that lifts like a carpet.
• Wet, mild spells in low or shaded spots: a fungus. Check: a circle that returns to the same place.
• Any month, where a dog goes: urine. Check: a dead centre ringed by darker green grass.

More than one cause can be at work at once. If a patch fits none of these, get the cause identified before buying any treatment.

In the Cairns dry season, check for a water problem before suspecting a pest. Rainfall at Cairns Aero averages 33.2 mm in July and 26.5 mm in August, so an unwatered lawn lives on what the soil holds. Turf Queensland lists drought and water-repellent soils among the causes of lawn dieback that get mistaken for grub damage. Lawn Solutions Australia suggests aerating dry spots with a garden fork, then using a wetting agent so the soil absorbs water better.

Watering a dry patch is fine within Cairns Regional Council's permanent water conservation measures:

• Sprinklers: odd-numbered houses Tuesday, Thursday and Saturday, even-numbered houses Wednesday, Friday and Sunday, 5am to 9am and 5pm to 9pm only. None on Mondays.
• Hand-held hoses with a trigger nozzle, watering cans and buckets: any time.

Our guide to Cairns water restrictions for lawns and gardens covers the higher levels. Kansas State University also advises against mowing turf in extreme drought stress, because wheel tracks take a long time to recover.

Brown patches that only start to show after mowing likely mean the lawn was scalped. Kansas State University describes scalping as the brown, stemmy look turf takes on when too much leaf blade comes off in one mow, and sets the limit at a third of the leaf. Usual causes are a mower out of adjustment, dull blades, or too long between cuts. Uneven ground is the other cause: the deck drops onto humps and cuts them lower than the rest.

1. 1Measure the real cutting height: stand the mower on a flat, hard floor and measure up to the blade's cutting edge.
2. 2Look at where the patches sit. On humps or high spots, the ground is the problem. Lawn Solutions Australia's fix is top dressing with soil or sand, no more than 5 to 10 mm thick at a time.
3. 3Check the blade, since dull blades scalp on their own.

If the grass is already long, bring it down over three or four mows. Our guide to how often to mow in Cairns sets the interval month by month.

A brown patch that grows from one day to the next in the warmer months can point to insects. Turf Queensland notes that two different pests get called lawn grubs.

• Armyworm: a moth caterpillar that feeds on turf leaves at night and arrives in large numbers. Signs include birds feeding on the lawn in the early morning or late afternoon, red-brown wasp-like insects, and green-yellow pellet droppings. The Queensland Government's field crop pest page sets the control threshold for lawn armyworm at a rapidly expanding area with more than 60 per cent leaf loss.
• Armyworm eggs: hairy clumps of 600 to 700 eggs on shrubs or eaves near outdoor lights. Brush them off; hosing washes them into the lawn.
• Curl grubs: C-shaped beetle larvae that eat the roots. The lawn yellows, browns and dies, and the turf slips or rolls up like a carpet. Turf Queensland counts 25 or more grubs per square metre as a problem infestation.

To look, pour a bucket of water mixed with biodegradable detergent over the area to bring larvae up, and dig where the turf lifts. Turf Queensland places curl grub damage in subtropical areas from November to January; treat that as a guide in tropical Cairns.

A circular brown patch that returns to the same low or shaded spot suggests a fungal patch disease. Mississippi State University Extension, writing about warm-season lawn grasses, says large patch recurs where dew and moisture linger, is favoured by free moisture and mild temperatures of roughly 15 to 24 degrees, and is usually not active in summer heat. In Cairns, look for it in low, damp or shaded spots during mild, wet spells rather than in open lawn in the January heat.

The check: at the patch edge, pinch a brown leaf tip and tug gently. If it comes away easily with a brown base, a fungus is likely.

Lawn Solutions Australia lists compaction, shade, overwatering and over fertilising among the conditions behind lawn disease, and suggests pruning trees to let light in and avoiding dusk watering so the lawn does not sit damp overnight. Changing those conditions comes first. Choosing a fungicide is a job for a turf specialist, working to the product label.

Dog urine does kill patches of grass. NC State Extension describes the worst damage as dead, straw-coloured spots about 8 to 15 cm across with a ring of darker green grass. That ring helps tell it from disease, which usually lacks a darker green border, and Lawn Solutions Australia adds that urine patches do not spread.

• Dry ground: damage is worse on dry soil, in hot weather and when the grass is not actively growing, so water dry spots.
• Female dogs and puppies tend to cause worse spots because they squat.
• Flushing the spot with plenty of water straight after limits the damage.
• Training the dog to use a mulched area or the yard's edge is the lasting fix. Baking soda and gypsum are not recommended.

Spreading lawn grasses fill small spots in over time, and NC State says warm-season grasses such as couch (bermudagrass) and zoysia can recover in as little as a couple of weeks while actively growing.

Mowing causes some brown patches, cures others and does nothing for the rest. Sorting them first saves money on the wrong product.

• Mowing fixes scalping: a correct height, sharp blades, cutting only when the turf is dry, and taking no more than a third of the leaf.
• Mowing helps prevent armyworm: Turf Queensland recommends regular mowing through the warmer months, October to April.
• Specialist or label: a confirmed armyworm or curl grub infestation. Turf Queensland says to read the label before you buy and check the product is registered for home lawns against that pest.
• Specialist: a suspected fungus that persists after you change the conditions behind it, or any patch you cannot identify.

For landlords, a patch noted at a routine inspection is easier to sort when the lawn is already on a round. Our rental yard maintenance calendar sets the mowing round alongside the other seasonal yard jobs.

Trisim's part in a brown patch problem is the mowing. We run a standing fortnightly round across Cairns and turn up on the same day each cycle, so the grass does not get far enough ahead for the next cut to scalp it. We mulch mow, returning fine clippings to the root zone, and edging, line trimming and a clean-up of the hard surfaces come with the cut. A yard that has already got away is cleared back to a maintainable height first, then put on the round. For lawn pests or disease, see a turf or pest specialist.

We mow across Cairns, including Mount Sheridan, Bentley Park and Mooroobool. See the lawn mowing page for how the round works, or request a free quote. Every job is quoted before it starts.