Most gardens are chock-full of exotic plants, or aliens, as they are also known. Unfortunately, some alien species have the nasty habit of escaping cultivation and crowding out native plants in natural areas.

The Puget Sound area has fought an unending battle to keep English ivy from literally strangling trees and other natives in parks and natural areas. Scotch broom has apparently won the battle along highways and in many natural areas west of the Cascades. Closer to home, there has been an ongoing fight to keep purple loosestrife in check in the wetlands along parts of the Yakima River, including the Yakima Greenway.

Meet the shrub-steppe

Central Washington's ecosystem is a shrub-steppe -- contrary to the fact that most of us insist on calling it a desert. The steppe has been under siege by a highly invasive annual alien grass: cheatgrass. In addition to crowding out smaller native plants, including many beautiful wildflowers, cheatgrass greatly increases the chances of wildfires once it dries out by the time summer arrives.

Our shrub-steppe ecosystem, with its surprisingly biodiversity of native plants and wildlife, had its beginnings at the end of the last Ice Age. Over thousands of years, our native plants and native herbivorous insects evolved to rely on one another. Plants came to depend on insects to help keep their numbers in check, and the plants themselves served as the food source for herbivorous insects.

Soon after pioneers began settling the Valley, those parts of the shrub-steppe suitable for farming gave way to the plow. Fortunately, much of the shrub-steppes still cover the hills and ridges of the Yakima Valley. Unappreciated by most of us, there is more to the shrub-steppe than meets the eye.

Viewing the shrub-steppe up close

One of the best places to view the shrub-steppe up close is to take a walk along the trails on the flanks of Cowiche Mountain, located on Snow Mountain Ranch, which is a new acquisition of the Cowiche Canyon Conservancy.

This spring, take the short drive west on Summitview toward Cowiche and Tieton. Turn left onto Cowiche Mill Road and park in the Conservancy's parking lot, which is just past and across the road from Cowiche Creek Nursery.

Follow the Cowiche Mountain Loop Trail that wends its way to the top of the mountain. From late April into mid-June, it offers a riot of colorful wildflowers in a number of places along the trail.

As you walk upward along the trail, you will come upon wide expanses of lithosol -- thin, rocky soils -- covered with mounds of spreading phlox; tiny yellow bells with their bashful blooms bowed; variously colored desert buckwheat; and creamy spikes of death camas, beautiful despite its ominous name.

You will also find paintbrush, larkspur, penstemon, several species of wild parsley, lupine, sagebrush buttercups, bitterroot, several species of small daisies, sagebrush violets and many other wildflowers too numerous to mention.

A pair of binoculars will enhance your enjoyment of the panoramic views available from the mountainside. Remember to take along your camera to capture memories of your jaunt.

There is no need to hike all the way to the top; turn around anytime and return to the parking lot. But remember, the cast of characters changes almost weekly, making it worthwhile to visit Snow Mountain Ranch several times during spring.

Native plants in your landscape

After your close encounter with plants of the shrub-steppe, you may want to add some of these flowering plants to your landscape. While you might not want to plant sagebrush, bitterbrush or rabbitbrush, consider these shrubs: golden currant, mock orange, elderberry, serviceberry or other shrubs found along the south fork of Cowiche Creek. Many of the ephemeral flowers found in the lithosol would make great additions to any rock garden.

But never dig native plants from private or public lands. Odds are they wouldn't survive, plus it is illegal to collect plants without permission. You might collect a few seeds after they ripen without creating any harm, but better yet, buy your seeds and starts from nurseries that specialize in plants native to the shrub-steppe.

But why bother growing native plants when many beautiful alien plants are available wherever plants are sold? For one reason, alien plants have evolved their leaf and stem chemistry over thousands of years so that many of our native insects can't use the material as a food source.

That sounds fantastic! Plant only aliens and you will never need to spray for bad bugs. It's not that simple. Remember that without insects to keep their numbers in check, aliens may escape cultivation and wreak havoc with our native plants.

Is this to say that we shouldn't use alien plants in our gardens? Not at all. Most aliens are both beautiful and benign. Even aliens that are problematic in other parts of our state or country may be perfectly content to remain at home in your garden. However, if you have any question about the advisability of growing a particular alien ornamental, call our county's Noxious Weed Board at 509-574-2180 for information.

* Freelance gardening columnist Jim McLain can be reached at 509-697-6112 or ongardening@compwrx.com.