How much water does your lawn actually need?

If you've ever stood on your lawn with a hose wondering how much water your lawn actually needs, you're not alone. Too little and it goes crispy; too much and you're basically paying to grow weeds and disease. The good news is there's a simple rule of thumb, a cheap tin-can trick to measure what you're putting down, and a bit of primary-school maths that tells you exactly how long to run your sprinkler. Let's sort it out once and for all.

The rule of thumb: about 25mm a week

For most lawns, the target is roughly 25mm (1 inch) of water per week, and that includes any rain that falls. Not per day, not per session, per week in total.

That figure isn't magic, it's just what a typical lawn drinks up and loses to evaporation over seven days in mild weather. When it's hot, windy or peak summer, your grass will want more, closer to 40mm. In cool, overcast, damp weather it might want almost nothing.

The key insight: aim for a weekly total, not a daily habit. If it rained 15mm this week, you only need to top up with 10mm. A cheap rain gauge (or a straight-sided jar left out) saves you watering when nature already did the job.

Things that push your number up or down:

• Heat and wind: hot, gusty days can double evaporation, so nudge the total up.
• Grass type: warm-season grasses are thriftier than cool-season ones (more on that below).
• Soil: sandy soil drains fast and needs more frequent top-ups; clay holds water and needs less.
• Shade: shaded areas lose less to evaporation and genuinely need less water than the sunny bits.

Why deep and infrequent beats little and often

Here's the mistake almost everyone makes: a quick daily sprinkle. It feels caring, but it's the worst thing you can do.

Light daily watering only wets the top couple of centimetres of soil. Your grass responds by keeping its roots up there where the water is, giving you a shallow-rooted lawn that faints the moment you skip a day or the weather turns.

Deep and infrequent does the opposite. A good long soak once or twice a week pushes water 15-20cm down. The roots chase it, growing deep, and a deep-rooted lawn is a tough lawn, one that shrugs off heat, stays greener between waterings and out-competes weeds.

• Little and often: shallow roots, thirsty lawn, more disease, more weeds.
• Deep and infrequent: deep roots, drought-tough lawn, fewer problems.

So take your weekly 25mm and deliver it in one or two big sessions, not seven small ones.

Measure what you're actually putting down: the tuna-can test

You can't hit a target you can't measure, and almost nobody knows how fast their sprinkler delivers water. Time to find out, and all you need is some empty tins.

The catch-cup (tuna-can) test

1. Grab 4-6 straight-sided containers, empty tuna or cat-food tins are perfect because they're low and wide.
2. Scatter them across the area your sprinkler covers, some near, some far.
3. Run the sprinkler for 15 minutes.
4. Measure the depth of water in each tin with a ruler, then work out the average.

Say your tins average 6mm in 15 minutes. That tells you your sprinkler delivers 24mm per hour. Now you can do the maths instead of guessing.

The test also reveals dry spots and heavy spots, if one tin has 12mm and another has 2mm, your coverage is uneven and you'll get patchy results no matter how long you run it.

The run-time maths

Once you know your rate, working out run time is easy:

Run time (minutes) = (target mm ÷ mm per hour) × 60

Worked example, aiming for 25mm a week in two sessions, so 12.5mm per session, with a sprinkler that does 24mm/hour:

• 12.5 ÷ 24 = 0.52 hours
• 0.52 × 60 = about 31 minutes per session, twice a week

That's it. No more guessing, no more standing there wondering.

How to tell it's had enough

Numbers are great, but your soil will tell you the truth. Two quick field tests:

• The screwdriver test: push a long screwdriver into the lawn. In well-watered soil it slides in easily to about 15cm. If it stops short, the water hasn't penetrated deep enough yet.
• The soil probe / spade check: slice a small wedge out with a spade or push in a soil probe and look at how far down the moisture goes. You're aiming for damp soil to 15-20cm, not a swamp.

If water starts pooling or running off before you've delivered your target, your soil can't absorb it fast enough. Stop, let it soak in for 20-30 minutes, then finish the rest. This "cycle and soak" trick is especially useful on clay and slopes.

Adjusting for your soil and your grass

One number doesn't fit everyone. Here's how to tune it.

Sandy vs clay soil

• Sandy soil drains quickly and won't hold a big soak, so it does better with the same weekly total split into more, smaller sessions (say three medium waterings instead of two big ones).
• Clay soil absorbs slowly but holds water for ages, so water in shorter bursts with soak-in breaks, and you can go longer between sessions.

Warm-season vs cool-season grass

Warm-season grasses (couch/Bermuda, buffalo/St Augustine, kikuyu, zoysia) are efficient and drought-hardy. Cool-season grasses (fescue, ryegrass, Kentucky bluegrass) stay greener in cool weather but drink more in the heat.

Let warm-season lawns go dormant

Here's a money-saver: in a serious drought or water restrictions, warm-season lawns can be allowed to go brown and dormant. They're not dead, they're sleeping. Give them a light drink every 2-3 weeks to keep the crowns alive and they'll green up fast once the rain returns. Fighting to keep a warm-season lawn lush through a heatwave is often a waste of water.

The best time to water: early morning

Water early in the morning, ideally before 9am. Here's why:

• Less waste: cool, still morning air means less evaporation, so more water reaches the roots.
• Fewer diseases: the grass dries through the day, so leaves aren't sitting wet overnight, which is what fungal problems love.
• Avoid evening: watering at night leaves the lawn damp for hours, rolling out the welcome mat for disease.

Midday works in a pinch but you'll lose a chunk to evaporation. Morning is the clear winner.

Frequently asked questions

How much water does a lawn need per week?

About 25mm (1 inch) per week including rainfall for most lawns, rising to around 40mm in peak summer heat. Deliver it in one or two deep sessions rather than daily sprinkles.

How long should I run my sprinkler?

It depends entirely on your sprinkler's output. Do the tuna-can test to find your mm-per-hour rate, then use: run time = (target mm ÷ mm per hour) × 60. A typical sprinkler needs 25-35 minutes to lay down 12.5mm.

Is it better to water every day or a few times a week?

A few times a week, deeply. Daily light watering grows shallow, weak roots. Deep, infrequent soaks train roots to grow down, giving you a far tougher, more drought-resistant lawn.

Can I let my lawn go brown in summer?

If it's a warm-season grass, yes. It goes dormant, not dead. A light drink every couple of weeks keeps the crown alive and it'll green back up when the rain returns.

What time of day is best to water the lawn?

Early morning, before 9am. It minimises evaporation and lets the grass dry through the day, which helps prevent fungal disease.

How Lawnova takes the guesswork out of watering

Working out your weekly target, adjusting for heat, soil and grass type, and remembering to skip a session when it rains is exactly the kind of thing Lawnova handles for you. Tell us your grass type, soil and location, and we'll build a watering schedule tuned to your actual conditions, then adjust it as the weather changes so you're never over- or under-watering again.

Get your free Lawnova plan

Water smarter, not harder, and your lawn will thank you for it.