How Much to Water Vegetables in a Dry Northwest Summer

People who move to the Northwest are often surprised by summer. After months of rain, the tap shuts off: July and August are usually among the driest months of the year, west of the Cascades and even more so east of them. Vegetables are growing their fastest just as the rain stops, so watering well makes a big difference.

How much

A common rule of thumb is about an inch of water a week for an established vegetable garden, from rain and watering combined. Hot spells, sandy soil, windy sites and the dry east side all push that number up. Large, fruiting plants like squash and tomatoes drink more than small leafy crops.

Don't rely on the rule alone. Push a finger or a trowel two or three inches into the soil next to your plants. If it's dry at that depth, it's time to water.

Water deeply, less often

A long soak once or twice a week is better than a quick sprinkle every day. Deep watering pushes moisture down where roots can follow it, so plants handle hot days better. Light daily watering keeps the roots near the surface, where they dry out first.

Seeds and newly planted seedlings are the exception. Keep the top inch moist until they're established, which can mean watering every day in warm weather.

Water at the base, in the morning

Soaker hoses and drip lines put water right at the roots, waste very little to evaporation and keep leaves dry. Keeping leaves dry helps prevent diseases like powdery mildew and tomato blight. Add a simple timer and the garden nearly waters itself.
Morning is the best time. Less water evaporates than at midday, and any splashed leaves dry quickly.
If you hand-water, aim at the soil, not the leaves, and go slowly so the water soaks in instead of running off.

Mulch keeps the water in

Once the soil has warmed in late spring, spread 2 to 3 inches of straw, shredded leaves or compost around your plants. Mulch slows evaporation, keeps the soil cooler and more even, and smothers weeds that would compete for water. It's the single cheapest way to cut your watering.

Crop by crop

Tomatoes: keep watering even, since swings between dry and soaked can cause cracked fruit and blossom end rot. Ease off a little in late summer to encourage ripening.
Squash and cucumbers: heavy drinkers once they're fruiting. Bitter cucumbers are often a sign of drought stress.
Beans: water is most important when they're flowering and setting pods.
Lettuce and leafy greens: shallow roots need frequent water, and drought stress makes them bolt and turn bitter.

Watering is one job in the monthly garden calendar, along with everything else to do each summer month.