Watering tips for Seattle landscapes

The Seattle area had a remarkably dry spring. From March through May we received just over half of normal rainfall. Plants are already feeling the effects. And we expect less rain in summer. While June is often rainy, July and September are normally the driest months in Seattle, with less than an inch of rain each month.

If you have not already set up your irrigation system or started watering your plants, now is the time! Here are watering tips to help you keep your landscape plants alive and healthy this summer.

Watering tips: What to water

• All new plantings—trees, shrubs, perennials, lawn—need regular water for the first two to three growing seasons until their roots are established. Read more about watering newly planted trees and shrubs.
• Your lawn needs about an inch of water per week. It is better if you water once or twice a week, rather than every day, so the water will penetrate into the root zone and then partially dry out. See our blog post about summer lawn health and our video about how to measure one inch of water. If you let your lawn go dormant in summer, you are more likely to get invasions of weeds, moss and pests such as chafer beetles in the fall when the rains return.
• Shrubs and perennials need occasional water to stay healthy. Some are drought-tolerant, but they may still need water during extended drought periods. “Drought-tolerant” does not mean “zero watering.” Read more about drought-tolerant plants.
• Trees also need occasional deep watering in summer heat. In recent years, as our summers have gotten hotter and drier, we have seen signs of drought stress in many of our clients’ trees. Even if you think your trees look OK now, the damage may show in future years. Stressed trees are more susceptible to insects and diseases, so watering will help your trees stay healthy and fend off pests. Read more about drought stress in trees.
• Annuals generally need frequent watering, perhaps every day. Because of the thirsty nature of annuals, we recommend limiting the number you have in your landscape. You may want to plant them only in high-visibility areas, such as near the front door or patio.

How to water effectively

Water slowly and deeply for healthy plants.

The best practice is to water deeply and less often. Shallow, frequent watering (such as a few minutes every day) creates plants with shallow roots. Deeper watering creates plants with deeper roots. They will be healthier and will tolerate hot, dry conditions better.

You need to water your plants so that water penetrates the soil rather than evaporating. If water cannot get into the soil, it is essentially wasted.

Soil is like a sponge. If it is allowed to dry out completely, you cannot just pour water quickly onto the top of the sponge. It will run off and not penetrate the top at all. But if you slowly pour water on the sponge and allow it to soften up the surface, then it will begin to percolate through the rest of the sponge.

Check your soil with a trowel, soil probe, screwdriver or chopstick. If it is dry, you will need to water slowly to get the top to soften. Read about how to check soil moisture.

We highly recommend oscillating sprinklers as they allow the water to infiltrate slowly. The water will penetrate more deeply into the soil and root zone.
Watch our video: Watering 101: Create Strong and Healthy Plants.

Water the entire root zone area, and then let it partially dry out.

For most plants, such as trees and shrubs, the active root zone is at the outer tips of the leaves or even further out. The area under the outer edge of a tree is also known as the drip line. For many trees this can be a large area.

Most water is absorbed at the drip line and beyond. This is where tiny terminal rootlets take up water and nutrients from the soil.

Make sure you are watering the entire root zone area of the plant. It is not helpful to water a tree near the trunk; you need to water at or beyond the drip line. Then wait until the upper few inches of soil are dry before watering again. This encourages the plant to grow deeper roots.

Know your plants. Some have shallow roots near the surface. Others have deeper roots. Those with deeper roots will need more water to soak down to their feeder roots.

Let the soil partially dry out between waterings. No roots want to be waterlogged all the time, except for rice. Unless you are trying to grow rice, the roots need oxygen. Roots get air when the soil is allowed to dry out some between waterings.

Watering tips: when to water 

There are a few general rules.

Water when it is cool. This makes perfect sense. If you try to water in the heat of the day, most of the water will evaporate into the air before it has a chance to get to the soil.

Mornings are the best time to water. The day is cool, the wind is usually not blowing and water has a chance to percolate into the soil profile before it warms up.

The next best time is early evening after the day begins to cool. Be aware that watering in the evening can promote some diseases. If you water in the evening, start in a shady area for less evaporation from the sun.

If it is cloudy, any time during the day will be ok. The key is to get as much water into the soil profile as possible for plants to use with minimum waste from evaporation.

If your plants are showing water stress, water them immediately. Do not wait for the perfect time. If the plant shows signs of struggle, such as wilting or curling leaves, this moment is the perfect time.

How long to water 

There are many factors that go into watering correctly, and every landscape situation is unique and different. How long to water depends on:

• Types of plants. For example, lawns need more water than drought-tolerant ground covers.
• Maturity of plants. Anything that has been newly planted, including trees, shrubs, lawns, perennials, grasses, vines, etc., needs to be watered regularly for the first two or three growing seasons so it can establish a strong root systems. Mature plants will need less water.
• Plant canopies. Plants under trees or shrubs that offer shade will need less water.
• Property microclimates. Perhaps you have an area that is more exposed to the wind. That will dry out plants sooner.
• Types and depth of soil. Sandy soils won’t hold water as long as clay soils.
• Organic mulch layers. Mulch insulates the soil, so plants need less frequent water.
• Sun orientation and exposure. Plants in the sun will need more water than those in the shade, and those with afternoon sun exposure will need more than those with morning sun exposure.
• Irrigation efficiency. Every type of sprinkler or sprinkler system has certain advantages and disadvantages when it comes to efficiency. Our video on watering systems shows some options for wands, hoses and sprinklers.

Watering tips: resources

Watering playlist on YouTube

Our YouTube playlist on Watering Your Lawn and Garden has several videos on the basics of watering, watering systems, watering new plants and trees, and more.

• Watering 101: Create Strong and Healthy Plants
• A Good Start: How to Plant and Water a New Plant
• Tree Gators: How to Water a New Tree
• Water Timers: Make Your Watering Easier
• How Much Water? Rain Gauges, Sensors and Tuna Cans
• Watering Systems: Wands, Hoses and Sprinklers
• Choosing and Using Mulch
• Measuring One Inch of Water
• Natural Lawn Care: Smart Watering

Blog posts on watering 

Check out some of our blog posts for information about how and why to water.

• Give your plant a good start: how to plant and water it
• Tips for summer lawn health
• Tips on landscape watering
• Choose drought-tolerant plants and reduce time you spend watering
• Watering will help manage insect problems
• Invest in watering for landscape dividends
• Plants need water to survive and thrive
• Smart watering: how much, when and how long to water

Other resources

• The Saving Water Partnership has loads of good information on watering, including automatic systems, choosing and using drip irrigation and soaker hoses and more.
• Seattle Public Utilities has a guide on Smart Watering.
• Swanson’s Nursery has a good article on best watering practices.