Appropriate Plants for Northwest Landscapes
Appropriate Plants for Northwest Landscapes
Appropriate Plants for Northwest Landscapes
by David Johnson
Birch trees are beautiful, but anyone who owns one knows that they attract aphids like a magnet. Many varieties of roses are highly susceptible to various types of mildew and fungus. And some rhododendrons are plagued by root weevils. In trying to grow these plants which are especially prone to pest problems, gardeners often resort to extensive applications of pesticides. One way to avoid such problems is to choose plants that are more pest-resistant.
Does this mean you can’t grow rhododendrons in the Northwest without using pesticides? Not at all. Actually, many varieties of these favorite plants are quite resistant to pests. There is also a wide range of other interesting plants, some familiar and others not so well known, that can thrive in our climate zones.
When selecting a plant for the home landscape, it is necessary to consider the conditions under which it will be growing. Any plant that does not get the right growing conditions will not thrive and can be prone to pest attack. This chapter will look at the factors to consider when selecting plants with a view to reducing pest problems. It will also give some specific recommendations for plants to avoid and plants to try.
Factors in Plant Selection
There are many factors to consider when selecting plants for the home landscape. Aesthetic qualities such as flowering time, flower size and color, leaf shape and color, fall color, plant height, width, and growth form are usually considered first. It is just as important to consider the more functional factors that determine whether or not a plant will thrive in a specific location. These include soil fertility, drainage, moisture content, soil depth, and drought tolerance. Also very important is the amount of light a plant receives, as well as the length of exposure each day. Exposure to wind or salt spray and the effect of surrounding plants are other considerations. All too often plants are purchased without full knowledge of the conditions they require to thrive. The result can be a disappointing landscape with stunted or declining plants that never have the opportunity to maximize their potential. This situation encourages pest and disease problems.
A concern of growing importance is the availability of water for garden watering. Although the region west of the Cascades is noted for its rain, it also experiences a period of summer drought from mid-June to mid-September. Many landscape plants require additional water during this period. To conserve water and maintain plant health during times of drought, it is important to consider drought tolerance. Landscapes or specific portions of landscapes can be designed to minimize or completely eliminate the need for supplemental water.
To make the most of your landscape investment, be aware of the environmental factors that will affect the plants in your landscape. Know how your soil drains; look at how light patterns change both daily and seasonally. Use this knowledge when selecting plants to make sure the environmental conditions of the site match the environmental requirements of the plant.
Recommended Plants
There are literally thousands of plants that will thrive in our region. Table 1 lists a few of those that have proven themselves in the Northwest, both from an aesthetic perspective and in terms of adaptability to our environmental conditions. These plants are not bothered significantly by pests nor do they require much maintenance. Let this list serve as an introduction to what to look for when selecting plants to add to your landscape. When selecting plants from this list, look carefully at each plant’s characteristics and needs and choose those which match your own environment. Be aware that plants listed as drought tolerant can only withstand dry conditions once they have become established. For more suggestions of drought tolerant plants, see the King County Cooperative Extension Bulletin entitled "Low Water Use Plants."
Rhododendrons
Rhododendrons are a mainstay in gardens west of the Cascades. Many people say they are overused, but their lush blossoms are hard to resist. Two problems frequently affect rhododendrons. First, almost all of them require additional water during the summer. Second, most rhodies can become the feeding ground for root weevils, which chew on the roots and the leaf margins. There are a number of species and varieties which are resistant to root weevil attack. Table 2 is a fascinating comparison of the relative resistance of different types of rhodies. It is excerpted from the Cooperative Extension Bulletin “Root Weevil Control on Rhododendrons” (EB0970). Notice that a wide range of blossom colors is available even within the most resistant types.
Inappropriate Plants for Northwest Landscapes
Any plant growing under conditions to which it is not accustomed may become stressed and subject to pest problems. There are, however, some plants that seem to be plagued by pests no matter where they are growing. To maintain such plants in a healthy condition often requires repeated applications of pesticides. To avoid this situation, select plants that do not suffer from serious pest problems. The list below consists of plants that are frequently grown in the Northwest and always seem to be plagued by pests. It also includes English Ivy, which is a pest itself because it spreads rapidly and invades where where it is not wanted.
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SIDEBARS AND OTHER ITEMS FOR THIS PAGE:
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Birch trees are notorious for attracting aphids. While the aphids do not usually threaten the health of the tree itself, the honeydew they secrete forms a sticky coating on everything below. If you want birch trees, don't plant them near houses, decks, or driveways.
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Rosemary has no serious pest problems, but it is not reliably cold-hardy in many parts of western Washington.
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Other Plant Lists for the Pacific Northwest
The Plant List. A Better Way to be Beautiful. The Saving Water Partnership. (http://www.savingwater.org/docs/PlantList.pdf)
Attractive and useful list of appropriate plants for the Pacific Northwest.
Great Plant Picks (http://www.greatplantpicks.org)
A plant awards program designed to help home gardeners identify unbeatable plants for their Pacific Northwest gardens. Contains well over 300 Great Plant Picks with helpful information and pictures.
Debra Prinzing & Mary Robson. Washington & Oregon Gardener’s Guide: Proven Plants for Inspired Gardens. Cool Springs Press, 2005. 272 pages. $24.99
