The Pacific Northwest has an abundance of rain during the winter, but summers are usually dry when plants need the most water. Portland averages five to seven inches of rain per month in the winter, but less than one inch in July and August. Daily water usage almost doubles during the summer months. The largest part of that summer bulge goes to water lawns and landscapes.
The goal of Water Wise Gardening is to conserve water by using it efficiently. But, it is not necessary to go the extreme of planting only plants that never need watering. An attractive, low water landscape can be created by using Water Wise principles and practices.
The main components of Water Wise Gardening are: limiting the size of lawns, choosing plants which need less than average water, grouping plants according to water needs, preparing the soil properly to encourage a deep root system to gather water, and conserving water with wise watering and mulch.
Since lawns use the most water, they offer the greatest water savings. Replacing part of the lawn with ground covers and hardscapes will reduce water usage accordingly. Perennial Ryegrass and Fescue require much less water than Bentgrass or Bluegrass. A mowing height of 2.5 to 3.5 inches in the summer reduces water needs by keeping the soil cooler. The extra grass blade surface also keeps the lawn greener with less water and fertilizer. Lawns require about an inch of water each week when temperatures stay below 80 degrees, or 1.5 inches above 85 degrees. Applying a half inch of water two or three times a week is much better than applying a small amount every day. Deep watering encourages a deeper root system which can draw water from a larger soil volume. For more information on lawns, see Lawn Care.
Eco-Lawns can save even more water. Most are a mixture of grasses, clovers, and flowering plants, such as, English Daisies, Roman Chamomile, Sweet Alyssum and Yarrow. Eco-Lawn seed mixes are available locally from Pro Time Lawn Seed in Portland. Their Rough and Ready mix of grasses plus Microclover resembles a conventional lawn. They will stay green all summer with only 1.5 inches of water once a month. I mow my Eco-Lawn three to four inches high every two weeks in the spring and fall and leave it un-mowed in the summer. Eco-Lawns need much less care than a manicured lawn, but they still can get unattractive if neglected. A big advantage is that clover roots support colonies of bacteria that can pull nitrogen out of the air and convert it to a form that plants can use. I have not fertilized my Eco-Lawn since I planted it in 2018 and it is always a healthy green.
Landscape plants which need less water usually fall into two groups: plants from arid climates and plants from Mediterranean climates which have dry summers. Arid plants need less water year round, and include cactus, succulents, junipers, pines and some grasses. Dry summer plants grow and flower in the fall, winter and spring and need little water during the summer. These include fall and spring flowering bulbs, Mediterranean heather, brooms and native plants, such as Oregon grape and flowering currant.
There are a few fruits that require less water. Apricots, Figs, Grapes, Pineapple Guavas, and Pomegranates require little additional watering once they are established. The other fruits and vegetables require regular watering through the summer. Some vegetables can get by with less water because they can be planted in the spring or fall when there is natural rainfall. Broccoli, cabbage, cauliflower, onions, peas and lettuce will grow well in cool weather. They can be planted as early as March. All but peas and lettuce can be planted as late as September. The cooler weather also encourages them to grow without going to seed (bolting).
Soils in the Portland area may be sandy loam near rivers. Adding compost to sandy loam soil makes it hold more water and nutrients. However, Washington County is mostly clay which prevents roots from growing deep because of a lack of oxygen deep in the soil. Adding compost and sand will loosen clay soil and let oxygen penetrate deeper into soil. Sand should be concrete sand, not river sand, because concrete sand has sharp edges which hold the sand grains farther apart. Pumice is even better, because it has air pockets which can hold moisture and nutrients. Profile Soil Conditioner is like a manufactured pumice. Potting soil which contains sand or pumice is a good soil amendment if a small amount is needed. Large amounts can be purchased by the truckload from landscape supply yards. Amendments should be dug or tilled into the top foot of soil. For more information on soil, see Improving Soil.
Proper watering is necessary to encourage deep roots. Watering every day produces a shallow root system. This can cause tree roots to protrude above the lawn. Instead, water deeply to wet the entire root zone, then let the soil dry out before watering again. Deep watering flushes stale air from the soil. Then, fresh air and oxygen are drawn deeper into the soil as it dries to encourage deeper roots. Both clay soil and sandy loam require a half inch to wet the root zone of lawns and annuals, and an inch of water to wet the root zone of trees and shrubs.
For most plants, the top inch of soil should be somewhat dry before it is watered again. Letting the soil dry before watering also prevents root rot and crown rot, which are caused by constantly soggy soil. This may take two days or two weeks, depending on the soil, the weather and the number and size of plants. A screwdriver with a six inch long shaft is a good tool to check soil moisture on lawns, because it will indicate soil moisture deep in the root zone. If it can easily be pushed all the way in to the handle, then the soil is probably wetter than it needs to be. If it takes fair amount of pressure to push it in most of the way, that is probably the right amount of soil moisture. If it can only be pushed in two inches, then the soil needs more water. A plant water meter is an even more accurate way to tell when lawns, trees and shrubs need watering again, because it can measure soil moisture deeper.
Several simple methods can be used to conserve water. A hose shut off on the end of a hose can save quite a bit of water. A watering wand will put water on the soil, not on leaves where it might encourage diseases. A water timer will automatically shut off hose end sprinklers. Battery powered water timers can turn the water on and off automatically.
A well designed irrigation system can end up saving water if it applies the water uniformly in the right amount. Water sensors can be added to irrigation controllers to automatically adjust watering. Rainfall sensors shut off the irrigation until rainwater has evaporated from the collector. Soil moisture sensors are even more accurate. With these sensors, irrigation controllers are set to run everyday, but the irrigation is skipped until the soil actually needs water.
Run sprinklers at sunrise when the air is cool and there is less wind. Spread tuna cans around the yard to see how much water is being applied. Adjust or replace sprinkler heads so water is applied evenly. Never apply more than one inch of water at a time to trees and shrubs, or more than a half-inch on lawns and annuals.
Drip irrigation is the most efficient way to water. It places water directly onto the soil so it does not evaporate into the air. It can apply different amounts of water to different areas according to the number of emitters. Also, it does not waste water on areas where only weeds grow. Drip irrigation systems are easy to design and install. Any one that is handy at all can set up one. The drip system can be connected to a hose bib, or underground sprinkler systems can be converted to drip. Normally the half inch drip line is snaked through the landscape and emitters are inserted directly into the half inch drip line, or connected by a short length of quarter inch line. The drip lines can be buried but the emitters should be visible so they can be checked to see that they are not clogged.
Covering soil with mulch conserves water four ways. Mulch prevents the soil surface from being compacted by rainfall. Raindrops can pack the soil surface into a hard crust that does not absorb water. Mulch absorbs water and holds it until it can soak in instead of running off. Mulch keeps soil cooler so water does not evaporate as fast. Bare soil in full sun can heat up to 120 degrees, which can kill roots. Mulch reduces weeds, which can use quite a bit of water. Two or three inches of mulch is very effective, but do not cover lower branches or pile mulch more than an inch deep around stems, because diseases might develop.
Bark dust, compost, leaves, even newspaper can be used as mulch. But, decorative rock may increase water use because it transmits heat to the soil and reflects heat onto the plants. Black plastic is not good mulch because it does not let air or water soak in. Landscape fabric is better because it allows both water and air to move into the soil. Landscape fabric will also prevent perennial weeds from pushing up from the soil. However, it does not prevent weed seedlings from growing roots down through the fabric. Also, it interferes with earthworms which provide nutrients and aeration.
Mulching is also an effective way to conserve water in vegetable gardens. Newly planted seeds can be watered, then covered with damp newspaper. They will not need to be watered again until the seedlings emerge. Just be sure to remove the paper immediately when seedlings emerge so they do not overheat. Mulch also prevents a crust from forming on the surface of the soil, which hinders seedling emergence. Plants planted close together act like a mulch by shading the soil and reducing weeds.
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