Why Does My Lawn Have Brown Patches? 8 Common Causes in Ontario

You've been watering, mowing, and doing everything right — but brown patches keep appearing on your lawn. Before you panic, know that brown spots are extremely common in Kitchener, Waterloo, Cambridge, and Guelph lawns, and most are fixable once you identify the cause.

Here are the 8 most common reasons Ontario lawns develop brown patches, how to tell them apart, and exactly what to fix.

1. White Grubs (June Bugs / European Chafer)

This is the number one cause of unexplained brown patches in the Waterloo Region. White grubs — the larvae of June bugs and European chafer beetles — live under the soil surface and feed on grass roots from July through October.

How to identify:

• Brown patches appear in late summer (August-September)
• Affected turf pulls up easily like a carpet — the roots are gone
• You'll find C-shaped white grubs in the soil beneath dead patches
• Skunks, raccoons, or crows may be digging up your lawn to eat the grubs

The fix: Nematode treatment is the most common organic approach in Ontario (chemical grub killers are restricted under Ontario's cosmetic pesticide ban). Apply beneficial nematodes in late August when grubs are small and close to the surface. Water the lawn thoroughly before and after application.

Prevention: A thick, healthy lawn resists grub damage better than a thin one. Regular mowing, proper fertilization, and aeration create denser turf that can tolerate some grub feeding without dying.

2. Snow Mould

If you see circular grey or pinkish-white matted patches when the snow melts in spring, that's snow mould. It's a fungal disease that develops under prolonged snow cover — and Kitchener-Waterloo gets plenty of that.

How to identify:

• Appears immediately after snowmelt in March-April
• Grey snow mould: circular patches 5-30 cm wide, grey-white matted grass
• Pink snow mould: similar but with a pinkish tinge at edges, can be larger

The fix: Rake the affected areas vigorously to break up the matted fungal layer and improve air circulation. Most grass underneath is still alive and will recover once exposed to sunlight and air. Our spring cleanup service includes aggressive raking of snow mould patches.

Prevention: Mow the lawn short (5 cm) for the final cut in fall. Don't pile leaves on the lawn over winter. Avoid heavy nitrogen fertilizer in late fall — it encourages the lush growth that snow mould feeds on.

3. Dog Urine Spots

The classic sign: small circular brown patches (10-20 cm) with a bright green ring around the edge. The nitrogen concentration in urine burns the centre but fertilizes the surrounding area.

How to identify:

• Small, circular, well-defined brown spots
• Bright green ring around each spot
• Location matches where your dog (or neighbourhood dogs) frequently visits

The fix: Flush the area with water immediately after your dog goes — dilution prevents the burn. For existing spots, rake out dead grass, loosen the top centimetre of soil, and overseed with perennial ryegrass (fastest germination). Keep the patch moist for 2 weeks.

Prevention: Train your dog to use one designated area. Water that area regularly. Some homeowners add a dog waste cleanup to their regular lawn service — removing waste promptly also reduces burn spots.

4. Drought Stress

During hot, dry stretches in July and August, lawns without irrigation go dormant and turn brown. This is actually a survival mechanism — the grass isn't dead, it's sleeping.

How to identify:

• Large, irregular brown areas (not circular)
• Grass blades are dry and crispy, not matted
• Footprints stay visible — the grass doesn't spring back
• Pattern follows sun exposure — south-facing slopes brown first

The fix: If the lawn is dormant, you have two options. Water deeply (2.5 cm per week) to bring it out of dormancy, or let it stay dormant until fall rains return — it will green up on its own. The worst thing you can do is water inconsistently, forcing the grass in and out of dormancy repeatedly.

Region of Waterloo note: The outdoor water use bylaw restricts lawn watering to specific days and times. Check your assigned watering day before setting up sprinklers.

5. Scalping (Mowing Too Low)

Cutting the grass too short — especially on uneven ground — removes too much of the leaf blade and exposes the brown stem beneath. This is extremely common on the first mow of the season when homeowners set their mower too low.

How to identify:

• Brown patches appear immediately after mowing
• Patches follow mowing lines, especially on high spots and bumps
• The "brown" is actually exposed stem, not dead grass

The fix: Raise your mowing height immediately. For KW cool-season grasses, never cut below 6 cm (2.5 inches). The lawn will recover in 1-2 weeks as new growth fills in. Lawn rolling in spring flattens bumps and frost-heave spots that cause scalping.

Prevention: Follow the one-third rule — never remove more than one-third of the blade height in a single cut. CutDay crews adjust height by season and never scalp.

6. Fungal Disease (Brown Patch / Dollar Spot)

Ontario's humid summers create ideal conditions for lawn fungus. The two most common in the KW region:

Brown patch (Rhizoctonia):

• Circular patches 15 cm to over 1 metre wide
• Grass blades have tan lesions with dark borders
• Appears during hot, humid weather (night temperatures above 20°C)
• Worse on lawns with excess nitrogen fertilizer

Dollar spot (Sclerotinia):

• Small circular spots about the size of a silver dollar (5-8 cm)
• Tan-coloured with reddish-brown borders
• Morning dew reveals cottony white fungal threads
• Common on under-fertilized lawns — the opposite trigger of brown patch

The fix: Improve air circulation by mowing at proper height. Water in the morning so grass dries during the day (evening watering stays wet overnight, feeding fungus). For persistent fungal issues, a balanced fertilizer program helps the lawn outgrow the damage.

7. Compacted Soil

High-traffic areas — along walkways, around play structures, near driveways — develop compacted soil over time. Compacted soil prevents water, air, and roots from penetrating, leading to thin, brown grass.

How to identify:

• Brown or thin patches in consistently high-traffic zones
• Soil feels hard when you push a screwdriver into it
• Water pools on the surface instead of soaking in
• Especially common on KW's clay-heavy soil

The fix: Core aeration is the most effective treatment. Aeration pulls small plugs of soil out of the lawn, reducing compaction and allowing water and air to reach roots. In the Waterloo Region, clay soil makes annual aeration almost essential for healthy lawns.

8. Salt Damage

A KW-specific problem: road salt and sidewalk salt drift onto lawns along driveways, walkways, and boulevards during winter. By spring, the salt concentration in those areas kills grass.

How to identify:

• Brown or dead strips along driveways, sidewalks, and roads
• Appears in early spring as snow melts and salt concentrates
• Soil may have a whitish crust on the surface

The fix: Flush affected areas with heavy watering in early spring to dilute and leach salt from the soil. Rake out dead grass, add a thin layer of topsoil or compost, and overseed with salt-tolerant perennial ryegrass. Recovery takes 4-6 weeks.

Prevention: Use calcium magnesium acetate (CMA) or sand instead of salt near lawn edges. Shovel thoroughly so less salt is needed.

When to Call a Professional

Most brown patches can be diagnosed and treated at home. But call for help if:

• Patches are spreading rapidly despite treatment
• You've tried multiple fixes with no improvement
• Animals are tearing up large sections of your lawn (likely severe grub infestation)
• You suspect the issue is underground (drainage, buried debris, contaminated soil)

A healthy, regularly maintained lawn resists most of these problems naturally. Consistent mowing, proper fertilization, and annual aeration create dense turf that shrugs off minor stress. CutDay handles the mowing, edging, and fertilization — get your instant price and let the lawn take care of itself.