It may be a scorcher outside, but your lawn has gotten long. Should you mow it, or wait? When is it too hot to mow and water?
It is too hot to mow the lawn when it’s above 80F (26C), or any time when you, the human, are at risk of heat-stroke. Cool-season grass (and even warm-season grass) that isn’t consistently watered will go dormant and won’t need to be mowed. The best plan of action is to just wait until the heatwave passes.
When is the Best Time of Day to Mow Lawn in Hot Weather?
While it’s best to avoid mowing cool-season grass during heatwaves, if you really need to mow, the best time of day is in the evening after the heat of the day has passed. The grass then has all night to recover and will keep its moisture without the sun beating down on it. Any dew or irrigation in the morning will give the grass a much-needed drink before facing the heat of the day again.
What’s the Best Temperature to Mow Grass?
The best temperatures to mow grass are between 40F (10C) to 80F (26C). Below that temperature, grass won’t be growing. Above that temperature, grass will be heat-stressed or dormant. Humans should also be wary about mowing the lawn and other yard work, as dehydration and heat stroke are very real consequences.
Is it Better To Leave Grass Longer in Hot Weather?
Yes, it is better to leave grass longer in hot weather. 4 inches is ideal for cool-season grasses, and 2 inches is best for warm-season grass.
Longer grass encourages grass to extend roots deeper into the soil, where roots are safe from the heat and can access more water. Watering deeply less frequently will also help.
The extra shade also keeps more moisture in the soil, rather than evaporating in the sun. It also keeps weeds from germinating and competing for water, as well as protecting the beneficial microbes in the soil, so they can support your grass.
When Should You Not Cut Your Grass?
Avoid mowing your grass when:
- The grass is not growing, like when daytime temperatures are consistently below 40F (10C), and early spring.
- The grass has gone dormant, because of the summer heat for cool-season grass or during winter for warm-season grass.
- During a heatwave, except if it really needs a cut. Then take a few shallow cuts, no more than ⅓ of the grass’ current height.
How To Avoid Heat Exhaustion
Heat exhaustion and sunstroke can have dire consequences. Heat exhaustion can not only cause physical harm and death, but because you’re exhausted, you’re more prone to making serious, potentially fatal mistakes. When possible, it’s best to avoid working outdoors during midday during heat waves or high temperatures.
If it’s not possible, then:
- Drink lots and lots of water, especially with electrolytes. Avoid sports drinks (the high sugar content is not your friend when it’s hot out), but use coconut water, electrolyte packages like Gastrolyte, or make your own.
- Wear lightweight, light-coloured, and loose-fitting clothing. Avoid polyester and jeans (polyester is like wrapping yourself up in plastic wrap), and go for natural fibers like cotton and linen.
- Definitely wear a hat and sunscreen! Continue to reapply the sunscreen as directed.
- Rest often in the shade or a cool area. Your body is overworking to keep you cool, so you’re going to feel way more tired than usual. Help your body out by taking frequent breaks. If your heart is pounding, you’re gasping for breath, and/or you feel lighthearted, faint or confused, then take a break in a cool space immediately!
- Eat light, cool meals — but don’t skip! The heat can make you feel less hungry than usual, but your body still needs calories. Just avoid hot food and large meals, as they can raise your body temperature.
- Avoid working at the absolute hottest part of the day.
The CDC has an excellent chart of what to look for and what to do for heat-related illnesses.
What Temperature Is Too Hot to Water Grass?
There is no temperature that makes it too hot to water the grass. If your grass is wilting (stems and leaves drooping or curling), then you should water it, even mid-afternoon during a heatwave without a cloud in sight. Once the grass reaches the terminal wilt stage, that grass is dead. There’s no saving it.
Watering when it’s hot will NOT cause leaf scorch [PDF]. Some people claim that if you water the grass while the sun is shining, the water drops act like magnifying glasses to scorch the grass blades. You can water your lawn during the day in the middle of a heatwave without killing it. If your grass is wilting under the heat, then you definitely should.
So yes, let your children run screaming through the sprinklers on a hot day. It’s fine.
Outside of wilting, though, it’s best to water your grass in the early morning, before temperatures go above 75F (24C). This gives the water time to soak into the soil for long-term use and for the grass to take hearty gulps before the heat of the day evaporates the water.
If your grass has gone dormant (the whole lawn has gone brown), then you don’t need to water as much or as frequently.
Want more summer watering tips? Check out How Often Should You Water Your Lawn in the Summer?
The most essential key to lawn care when it’s hot out is protecting you, the person doing all the work. Next is to keep the lawn from wilting. If you have to leave mowing the lawn a little longer than you’d like, then do it, and wait for cooler days.
Jamie is the founder of The Backyard Pros. When he was 15 years old he started working at a garden centre helping people buy plants, gardening products, and lawn care products. He has real estate experience and he is a home owner. Jamie loves backyard projects, refinishing furniture, and enjoys sharing his knowledge online.