When brown patches show up in summer, most people jump straight to “fungus.”
But the truth? Heat stress causes more brown lawns in NZ than disease ever will.
The challenge is they can look similar — especially when your lawn is already under pressure.
This guide breaks down the difference so you can fix the problem faster (and avoid wasting time on the wrong treatment).
What Heat Stress Looks Like
Heat stress happens when hot, dry weather pushes your lawn past its limits.
It shows up most often on exposed, full-sun areas… right when you want the lawn to look its best.
Common signs of heat stress
- Brown or straw-coloured patches across the top of the lawn
- Grass feels crispy or snaps easily
- The ground feels dry and hard
- The patch improves slightly after watering
- The pattern follows full sun or the hottest parts of the day
Why it happens
- High temperatures + dry soil
- Poor watering habits (light, frequent watering instead of deep and infrequent)
- Compacted soil
- Mowing too short
- High foot traffic during hot weather
Quick fix
Give the lawn a deep soak (20–30 minutes) and avoid mowing it short.
A light liquid feed can also help the lawn bounce back faster and improve colour.
What Fungus Looks Like
Fungal diseases love warmth + moisture, especially when the lawn stays wet overnight or is stressed.
Unlike heat stress, fungus usually has distinct patches or patterns — often smaller and more circular.
Common signs of lawn fungus
- Brown patches with a red, pink or grey tint
- Small circular spots, often spreading outward
- Fine, thread-like or powdery growth on the blades
- Grass pulling away easily at the crown
- Appears after rain, humidity or overwatering
Why it happens
- Humid weather
- Overwatering or watering at night
- Dense lawns with poor airflow
- Thatch build-up
- Recent stress (drought, renovation, scalping)
Quick fix
If it’s fungus, you’ll need a fungicide like LAWNZ Shield — watering won’t solve it.
Treat the affected area and avoid evening watering while the lawn recovers.
How to Tell the Difference Fast
If you’re not sure, use the rule of three:
1. Time of year
- Hot + dry = more likely heat stress
- Warm + humid = more likely fungus
2. Shape of the patch
- Irregular, sun-exposed = heat stress
- Circular or patterned = fungus
3. Water test
- Improves noticeably after a deep soak → heat stress
- Stays the same → fungus
If the patch is crunchy and dry: heat stress
If the patch is slimy, threadlike, or has a colour tinge: fungus
Preventing Both Problems
A healthy lawn can fight off almost anything NZ summer throws at it.
Here’s how to prevent brown patches altogether:
To avoid heat stress:
- Water deeply 1–2 times a week
- Keep your mower a little higher in summer
- Avoid mowing during peak heat
- Feed the lawn before heat arrives
To avoid fungus:
- Water in the morning, not at night
- Improve airflow by mowing regularly
- Reduce thatch
- Treat early if you see signs of disease
- Apply LAWNZ Shield as a preventative treatment during humid conditions.
When to Treat — and When to Wait
If patches are popping up during a dry spell, start with water and light feeding.
If they appear after humidity, rain or overnight watering: think fungus.
And if you’ve tried the water test and nothing changes?
That’s your sign the issue might be more than heat — and a fungicide could be the next step.
