How often to water a lawn: amount, timing and signs

How much water a lawn needs

Illinois Extension puts the need of cool-season grasses such as Kentucky bluegrass, perennial ryegrass and fine fescues at about 1 to 1.5 inches of water per week to keep green color and active growth, with soil, weather and management all playing a part. Cool-season grasses naturally slow down and may go dormant in hot weather, and Illinois says it is perfectly acceptable not to irrigate at all: most summers these lawns survive hot, dry spells by going dormant and resume growth in autumn. Our reading is that rain counts toward the weekly total.

The University of Minnesota Extension frames it by depth. It suggests watering one time a week or less, with enough water to wet the soil to a depth of six inches, and says that depending on soil type a lawn may need as little as a half inch. In Florida, UF/IFAS tells homeowners to put 1/2 to 3/4 inch in each irrigation zone when the lawn needs it, using the higher rate on sandy soil and the lower on heavy clay. Our takeaway: the right amount depends on your grass, soil and weather, so use the ranges above as a start and watch the lawn.

Why deep and infrequent works better

Illinois Extension says to water infrequently, and thoroughly enough that moisture reaches the depth of the roots. Frequent watering promotes shallower roots and weeds such as crabgrass. Minnesota says watering infrequently with enough volume encourages rooting and drought tolerance.

Illinois adds that allowing a lawn to go under mild drought stress at the start of summer increases rooting, so the first warm days are not an automatic signal to start the sprinklers. It also says to pick a plan for the season. Do not let the grass turn brown, water it green again and then let it go dormant again, because breaking dormancy drains the plant's food reserves and makes it more prone to disease.

Measure what your sprinklers deliver

Sprinklers differ, so measure. Illinois says to put coffee cans or similar straight-sided containers on the lawn to measure how much water is applied. UF/IFAS suggests five to ten cans and gives the arithmetic: run the sprinkler for 15 minutes, add up the water depth in each container and divide by the number of containers to get the average depth. Multiply by four for inches per hour.

A worked example, using that arithmetic: if the cans average a quarter inch after 15 minutes, the sprinkler delivers about 1 inch per hour, so half an hour gives half an inch. On heavy clay or slopes, Illinois says to watch for runoff, and it may be necessary to apply the water in two applications so it soaks in.

The signs that a lawn is thirsty

Illinois lists footprinting, where footprints stay visible after you walk across the lawn, and a darker color as drought stress begins. UF/IFAS lists three wilt signs: folding leaf blades, a blue-gray color and footprints that remain visible. Its advice is to water when 30 to 50 percent of the lawn shows at least one of them, instead of running a fixed schedule.

Illinois also suggests pushing a long screwdriver into the soil, since how far it goes often shows how deep the water has soaked in. Minnesota recommends running the controller in manual mode and turning it on only when the lawn shows signs of drought, instead of a set-and-forget schedule. Our suggestion for rain credit: put one can out for rain too and count what falls toward the week's total.

The best time of day

Early morning. Illinois says to water early in the day, when lawns are normally wet from dew, and to avoid midday, when water evaporates before it soaks in. UF/IFAS gives 4 a.m. to 8 a.m. as the best window and says that if you cannot manage that, night is better than 10 a.m. to 4 p.m. Illinois says watering at night has the potential to promote some turf diseases, and UF/IFAS explains why: water that stays on the blades for long periods can cause them.

New seed and new sod

A newly seeded lawn follows different rules. Illinois says to irrigate often enough to keep the seed zone moist, and once seed begins to germinate, to water deeper with longer gaps between waterings. Penn State says the soil should be moist at all times but not excessively wet, usually with light, frequent irrigation, sometimes daily, which can be reduced in rainy or cool weather. It calls the 4 to 8 weeks after seeding or sodding critical to survival.

For sod, Illinois says to water thoroughly after laying it and to lift a corner to check that the soil below is moist. For seeding in the cold months, see our dormant seeding guide; for when to cut, our guide to mowing new grass.