Drought-tolerant landscape plants for western Oregon: site selection and preparation

Western Oregon gardens and landscapes can flourish without summer irrigation - when you choose the right plants. Drought-tolerant plants include Oregon natives from summer-dry upland sites, as well as flora from Mediterranean regions of the world. The benefits of growing plants without supplemental irrigation include saving money on an irrigation system and water, as well as the environmental benefits of conserving water. Up to 30% of yearly household water use is estimated to be used for irrigating landscapes, therefore reducing summer watering can lead to substantial savings¹.

The climate in western Oregon is temperate Mediterranean, with cool, wet winters and warm, dry summers. The summers are warm and dry enough that irrigation is required for many common landscape plants not adapted to summer-dry climates. We do occasionally have Arctic freeze events in the winter which can inflict damage on insufficiently hardy plants, so consider your USDA Hardiness Zone and microclimate when making selections for your landscapes.

This series, based on decades of on-farm research, can provide some ideas and go-to plants to help create a successful low-water use landscape. Researchers with the Oregon State University Department of Horticulture have been evaluating plants grown without summer irrigation in Aurora, Oregon since 2000. Plant groups studied include California lilac (Ceanothus), Rockrose (Cistus and Halimium), Manzanita (Arctostaphylos), Spider flower (Grevillea), and ground cover shrubs. The results of these trials will be covered in this series of research briefs.

Is your garden suitable for an unirrigated landscape?

Planting drought-tolerant landscape plants, including those that survive without irrigation, saves water and helps to protect the environment. However, it is important to do so thoughtfully and with appropriate site preparation and location. Before selecting plants, review the key characteristics for success with an unirrigated landscape.

Sun or shade

Most landscapes have both sunny and shady areas and plants should be selected that can tolerate these conditions. In some ways, sunny areas, though they are hotter, are easier to landscape as many drought tolerant plants are best grown in full sun. The plants highlighted in this series were grown in full sun, therefore, these plants will do best when grown in these conditions. For shady areas, ensure that your choices tolerate both drought and shade. Sun loving plants planted in shade will not thrive!

Site aspect

Planting areas that slope south or west will be warmer than those that face north or east. In addition, rain landing on sloped areas is less likely to be absorbed into the soil, instead moving off site and reducing water availability during the dry season.

Presence of existing trees or shrubs

Planting adjacent to existing trees and shrubs can cause problems because of competition for water. The root systems of these plants may make it harder to dig planting holes. The larger the plant and the closer it is to your new landscape, the more problematic this might be.

Nearby heat-radiating structures

If your landscape is near a south or west-facing wall, that will potentially increase the heat load, especially for those planted nearest the wall. Plants that are from Mediterranean climates, like many plant species in our trials, can appreciate the extra heat and will perform better in this environment than many of the typically used landscape plants.

Soil quality, compaction and drainage

The quality of your soil will impact how well your plants establish and grow. It’s worth taking a soil sample to be analyzed for important measures of soil quality including pH, organic matter content and major nutrients like nitrogen, phosphorus, potassium, magnesium, and calcium. If the soil is very compacted when you attempt to dig (for example, from construction activity in new homes), this problem should be addressed by digging the entire area to be planted to alleviate the compaction. Also, areas where water stands after it rains will be a problem for almost all landscape plants, so installation of drain tile or other means to improve drainage should be taken.

Soils such as these, that have been compacted or are deficient in major nutrients, or which have a texture with high amounts of clay or sand, should be also amended prior to planting. Organic matter, which is both living and dead things which are being decomposed, such as roots, leaves and soil organisms, can improve soil quality when added to soil. Organic matter accomplishes this by providing nutrients, habitat for soil organisms, holding water, and making soil more workable. Mixing plant residues, aged manure, or compost into the soil will increase soil organic matter. Ultimately, this improves soil structure and makes it a better environment for all your landscape plants.

Planting and establishment

The best time to plant an unirrigated landscape is in the autumn after fall rains begin. If the soil is not yet moist from fall rains, or if the rain is not consistent, supplemental irrigation immediately after planting will be necessary. Access to a hose bib and watering system may be necessary in this situation. The plants will be able to establish during the cool, wet season and will not need supplemental irrigation the following summer. If you plant in the spring or summer, supplemental irrigation will be needed the first growing season.

After planting: mulch

Mulch is helpful for weed control in any landscape, including un-irrigated ones. Mulch can be either an organic product, like bark products or yard debris compost. Or, you may choose to use an inorganic product, like gravel or lava rock. In the longer term, it’s easiest to plan your landscape such that the canopy of your plants covers the landscaped area, eliminating the need for re-application of mulch and reducing weeds.