Drought Tolerant Landscaping for Washington State (Home Garden Series)

Deciduous trees whose leaves have deep sinuses (indentation between the lobes) reduce water loss as they have less surface area (Mickelbart 2012). Red maple, for instance, is a very popular shade tree for Washington State as it very drought tolerant (Tirmenstein 1991). There are a number of different cultivars, ranging in width from 15 feet (cultivars Bowhall, Armstrong Gold, and Red Rocket) up to 35 feet (cultivars Red Sunset, October Glory, Northwood).

Turf grass, such as perennial ryegrass (Lolium perenne), is classified as a drought avoider as it will go into summer dormancy where the blades of grass turn brown and dry out when not irrigated. However, the plant will come back almost entirely in the fall when the rains return. Herbaceous perennials vary tremendously in their ability to withstand drought as they do not have the reserve capacity of woody plants. Once again the perennials that lack thick leaves or lignified (woody) stems are the first to grow, flower, and then go into seasonal dormancy as summer heat progresses. By filling a yard with a greater preponderance of plants with low and medium water requirements, homeowners can reduce the need to water their lawns, thus masking the appearance of light-colored grass foliage.

Back to top

Hydrozones

A proven technique for reducing overall water use in a landscape is the establishment of hydrozones. A hydrozone is defined as a portion of the landscape area where plants with similar water needs are grouped (Detwieler, Griffiths, and Olson 2005; Kesseler 2008; Kopp, Cerny, and Heflebower 2002). By using a hydrozone-based design, a landscape manager encourages intelligent grouping of compatible plants—factoring in water needs, soil characteristics, and microclimate—to provide optimal conditions for their successful establishment and long-term vigor (United States Environmental Protection Agency 2013). Also by planning for different hydrozones, homeowners can determine the type of irrigation supplies that will be needed.

When choosing a drought-tolerant landscape design, group as many of the plants as possible in a low-water-use hydrozone (Figure 3). Landscape trees, screening hedges, and evergreen groundcovers all fall into the low-water hydrozone. Low-water hydrozones are often located on the perimeter of the yard. The trees, shrubs, and groundcovers listed in Table 1 are all classified as being appropriate for the low-water hydrozone. The medium-water hydrozone consists of plants that only need watering occasionally during the dry summer months. The medium-water hydrozone is typically located closer to the home. Look for signs of leaf wilting and off-green coloration (depending upon the base color), which indicate that water should be applied. Herbaceous perennials, best known for their seasonal flowering periods, can be used in the medium-water hydrozone. Plants in the high-water hydrozone will need frequent irrigation in order to thrive during the summer. Included in this category are annuals, vegetable gardens, and lawns. Most native plants would be placed in the low-water-use hydrozone (Detwieler, Griffiths, and Olson 2005).

Plant Invasiveness

None of the plants in this guide are classified as invasive for Washington State, according to the Center for Invasive Species and Ecosystem Health (Invasive 2015), and/or the USDA Natural Resources Conservation PLANTS database (USDA PLANTS 2015). Before selecting a particular plant, check the plant’s hardiness zone rating to ensure that it will survive the minimum winter temperatures found at the planting site. Gardeners can
use zip codes to look up United States Department of Agriculture hardiness zones (USDA Hardiness Zone Map 2014) (Figure 4).

Soils and Home Construction

In the past contractors were not required to protect or improve the native soil either during the building process or following completion of the structure. All too often sites were left with too little topsoil, resulting in a landscape built on compacted layers of soil or exposed subsoils. The compromised lawns and landscapes that resulted required additional irrigation during the summer months.

The Washington State Department of Ecology (WDOE) recognized the problems of degraded landscape sites and their contributions to stormwater runoff (WDOE 2014). In western Washington, builders are now required to stockpile existing topsoil during grading and replace it prior to landscape planting (WDOE BMP T5.13 2012). If the topsoil has a minimum organic matter content of 10% dry weight for planting beds and 5% for turf areas, and is a least 8 inches deep, it does not require the addition of supplemental soil amendments prior to planting. However, if the site lacks suitable topsoil or new topsoil is not brought in, it will not meet the standards that have been developed to regulate post-construction soil quality and depth.

For planting beds, the WDOE states that builders must spread 3 inches of compost and rototill it to a depth of 5 inches (a total amended depth of 9.5 inches; settled depth of 8 inches) (Figure 5). For turf areas, builders
are required to add 1.75 inches of compost and rototill it to a depth of 6.25 inches (a total amended depth of 9.5 inches; settled depth of 8 inches). WDOE’s BMP T.5.13 policies do not pertain to eastern Washington municipalities.

Compost is the most commonly utilized soil amendment. It is made from yard debris, woody debris, animal manure, and potentially wastewater biosolids (Cogger and Stahnke 2013). When establishing a landscape site amend the entire area and not just the planting holes (Cogger 2005). Research at the WSU Puyallup Research and Extension Center found that additions of compost, followed by incorporation to a depth of 8”, improved soil health longer than 5 years (Cogger et al. 2008).

Back to top

Hardscape

Often a landscape designed to withstand periods of low precipitation will incorporate “hardscape” features, such as stone pathways, rock walls, statues, and water features (Figure 7), to create interest as spring and summer flowers fade (Love et al. 2009). Other types of hardscape include decks, arbors, shade structures, fire pits, pavers, flagstones, and stone and gravel beds. Hardscape can personalize and beautify the
landscape, while supporting a system that uses less water.

Slopes

Slopes tend to shed water quickly and thus are drier. It is often very difficult to safely mow grass on slopes, as both the lawnmower and operator can slip, especially during the spring when the lawn might be wet. Dense groundcovers (Figure 8) are excellent slope stabilizers and generally do not require supplemental water once established.

Shade

There are a host of different plants (from landscape trees to groundcovers) that can take varying amounts of shade. Plants that can tolerate dry, full shade to semi-shaded sites are listed in Table 1. Large landscape deciduous and coniferous trees often have extensive root systems that use soil moisture stored during the winter months. Low-water-use deciduous and evergreen shrubs that tolerate shade can be used underneath landscape trees. These selections are also suitable as turf replacements since the principal Northwest turf species do best under full sun conditions.

Back to top

Limiting the Use of Turf

The U.S. Environmental Protection Agency (EPA) has found that 30% of a homeowner’s daily water use goes to outdoor irrigation. If irrigation water runs off the property, it can carry lawn fertilizers and pesticides along with it, which can negatively affect nearby watersheds. For example, the use of phosphorus in lawn fertilizer is now being questioned because it has been implicated in the growth of algae blooms in nearby watersheds (Stahnke 2012).

The WDOE requires municipalities in western Washington to incorporate compost into future turf (lawn) areas (WDOE BMP T5.13, 2012) to help preserve soil moisture, thus reducing the need for supplemental irrigation. Applying heavy layers of compost to an established lawn is not appropriate however, because thicker layers may create a barrier that restricts water movement (Cogger 2013).

A lawn is best suited to sunny, relatively flat areas that are well drained. Assess winter drainage during the winter months. Soggy spots are prone to thatch buildup and the growth of moss. Landscapers can install plastic drain tile at a depth of 18 inches to help improve drainage. Lawns should be aerated in the spring with a power core aerator. By removing numerous plugs of soil, the lawn grass will extend its roots deeper into the soil profile (Figure 9).

Efficient Irrigation

A drought-resistant landscape requires a minimal amount of supplemental irrigation (Wade et al. 2007) once it has become fully established. Trees, shrubs, groundcovers, and herbaceous perennials are often planted in the fall in order to capitalize on the readily available soil moisture levels. Also air temperatures are lower, putting less stress on plants. While plants may not put on top growth, the root system will continue to grow all winter as roots do not go into winter dormancy. By the following spring, the fall transplants should have a healthy start for the spring and summer. It may take 2 to 3 years for a new tree to become fully established (Ophardt and Hummel 2011).

It is difficult to gauge a plant’s water needs by simply observing the soil surface. It is best to dig down into the soil profile to determine the wetting pattern. Apply water no faster than the rate at which it can be absorbed. A sandy loam soil can absorb up to 3 inches of water per hour, while clay loams can absorb as little as ½ inch per hour.

Check newly established plants periodically during the first year for signs of leaf wilting. When trees, shrubs, and groundcovers start to put on an extensive amount of new seasonal growth without wilting, they are well on their way to becoming fully established. All of the plants listed in Table 1 should survive well without supplemental summer irrigation as soon as they are fully established.

Irrigation during Establishment

There are different water-saving irrigation tools for use during the plant establishment period (Peters 2011), but most of them wet only a small surface area of soil near the target plant. “Micro-irrigation” is the universal term used for drip, trickle, or micro-spray irrigation systems. Micro-irrigation systems apply water at a low pressure directly to the plant roots, which is where it can best be absorbed. Less water is lost through evaporation, misplacement (watering sidewalks), runoff, and leaching. Overall weed-seed germination is reduced for the entire planting. Foliar disease problems can be reduced for plants whose foliage should stay dry.

Figure 12. Soaker hose.

Figure 13. Micro-spray stake.

Figure 14. In-line emitter tubing.

Soaker hoses (Figure 12) can be used for micro-irrigation. These hoses are made of a flexible, porous material that distributes water along its entire length. These hoses can be laid throughout the irrigation site and covered with mulch to hide its presence. Check a soaker hose periodically to ensure it has not become plugged with calcium deposits or clogged with sand. If it is laid on top of the soil surface, the wetting pattern will be wider than if it is covered.

Micro-spray stakes (Figure 13) work well for individual shrubs. They can be inserted into thin plastic tubing, which can be laid throughout the irrigation site. Finally, hard hose in-line emitter tubing (Figure 14) can be used where the trees or shrubs are planted in rows. This product is generally more durable than other types of micro-irrigation equipment.

In-line emitter tubing comes as both orifice emitters and turbulent-flow emitters (Peters 2011). The former
is simply tubing with a series of spaced holes from which water flows, although these holes can become plugged over time. The more expensive turbulent-flow tubing sends water on a tortuous path before it is released. This type of tubing is less prone to plugging. In-line emitter tubing is generally laid on top of the soil surface.

Lawn Irrigation

In western Washington, high-maintenance perennial ryegrass lawns often need 1 inch of supplemental irrigation per week in order to stay green all summer (Voderburg and Kowalewski 2014). Low-maintenance lawns that do not receive supplemental summer irrigation will become straw brown during their summer dormancy, but will regrow and turn green again with the onset of fall rains. Thin patches of lawn
will need to be overseeded before the middle of October.

A lawn filled with perennial weeds such as dandelion, false dandelion, and plantain, all of which are deep rooted, will place more stress on the ryegrass lawn. Weeds should be removed no later than May 1 to keep them from using too much of the limited soil moisture (Goss, Miltner, and Stahnke 2006). In eastern Washington, Kentucky bluegrass lawns could require up to 1 inch of supplemental irrigation per week in order to stay green all summer. While this type of lawn can be allowed to go dormant during the summer, heavy foot traffic can harm it to the extent that it may not come back the following year (Stahnke 2012).

Irrigation Systems

Irrigation systems for turf range from aboveground rotating sprinklers attached to a hose to underground systems with popup heads (Figure 15) and remote timers. No matter what system is used, the key to saving water in turf irrigation is timing water application correctly—taking into account for both the soil type as well as prevailing weather conditions.

There is no reason to treat lawns as high-water hydrozones in western Washington. The majority of soils in this region are heavier and thus hold more water than the lighter soils east of the Cascades. Other than for the two hottest summer months, the evaporative demand in areas west of the Cascades is simply not as high as drier regions.

Deep but less frequent irrigation (as opposed to shallow and frequent irrigation) is preferable because it encourages the roots of the grass plants to extend deeper into the soil profile (Charlton, Golembiewski, and Cook 2010). To measure the output from lawn sprinklers to ensure adequate irrigation, set out small,
flat-bottomed cans of equal size at approximately 5 foot intervals on lines radiating out from the sprinkler heads. On heavy clay sites with low water infiltration rates, it is preferable to apply less than ½ inch of water per hour to avoid slope runoff or ponding in low spots.

Programmable irrigation controllers (Figure 16) are an important component in permanent sprinkler systems as they allow the operator to treat the different zones in lawn appropriately. Controllers can be mounted either inside or outside the home or garage, and they can be programed much like a thermostat in the home. A “smart” controller is one that is tied to either a soil moisture sensor that measures the amount of water in the soil, or a rainfall sensor that turns off the controller when it begins to rain. Automatic lawn-watering systems should only be run in the early morning hours (4–6 a.m.) to reduce evaporation during the
day. Landscape contractors should be consulted for installation of most inground watering systems.

Back to top