Return to Native Trees of Colorado

Non-native Trees Invading the Southern Rockies

by Stuart Wier

These trees are exotic foreign trees which now grow here naturally and reproduce actively. They disturb or entirely replace native trees and plant communities. Their effect on natural plant and animals ranges from somewhat disruptive to very harmful.

None of these should be planted where they can escape cultivation, for example, along streams. Since birds carry Russian olive seeds long distances, and since Tamarisk is such a prolific seed producer, so harmful, and so difficult to eradicate, there is no reason to plant either of them anywhere in the Rockies. Efforts are being made to remove Russian olive and Tamarisk.

Russian Olive

A hardy dryland shrub or small tree, growing to 25 feet tall, in floodplains, hedgerows, grasslands, and along rivers, streams, and irrigation ditches. Spreading branches near the ground form an open rounded crown, and the twigs are thorny. It has distinctive satiny leaves, pale silvery-green or gray-green, and fruit of the same silvery appearance. It is well established in the Platte River basin. In Colorado it grows below 6500 or 7000 feet elevation. A native of western Asia, the scientific name is Elaeagnus angustifolia, the narrow-leafed olive. Elaeagnus is Greek for olive.

Russian olive was planted for windbreaks and as an ornamental which could withstand the rigors of the plains climate. The Russian olive is not a relative of the common edible olive of the Mediterranean. The dry olive-shaped fruit is eaten by squirrels, grouse, and song birds. It is harmless to humans but so tasteless and mealy as to be unpalatable.

Russian olive can be an early colonizer of flooded areas and stream banks. It is drought resistant. It can grow up in the shade of other trees and then dominate that plant community, and it has as displaced cottonwood along many streams, and threatens to cover larger areas. Russian olive is now being removed as a pest.

Russian olive actively adds the nutrient nitrogen to the soil. Most native plants in Colorado have evolved to grow in a nitrogen-poor environment. Soils with enhanced nitrogen encourage weeds and non-native plants at the expense of native species.

Home-owners should consider removing Russian olive. White poplar (Populus alba) is an attractive alternative to Russian olive. The foliage is a pale whitish-green, something like the foliage of Russian olive. The White poplar has an attractive shape, unlike Russian olive's sprawling ways. It can grow to a very large tree, however.

Leaves: The pale gray-green leaf color is distinctive among Colorado trees. Leaves are 1 1/2 to 3 1/2 inches long, 1/2 inch wide, untoothed, and covered with gray fuzz making a satin or silvery cast.

Stems: twigs silvery-gray or covered with gray scaly fuss, with thorns.

Fruit: olive-shaped, silvery or pale green, 1/2 inch long. The pulp is mealy when mature, but it is eaten by birds. The seed inside, protected by a small hard casing shaped and stripped like a tiny watermelon, is very tough and can pass through a bird's digestive system unharmed. Consequently birds spread the seeds over very wide areas.

Bark: ash-gray-brown to very dark brown and thin, as long strips with shallow fissures.

Tamarisk or Salt-cedar

A shrub or tree with feathery foliage, a native of Asia, found in Colorado on river banks, streams, and irrigation ditches, below 7400 feet in elevation, commonly below 6000 feet in the drier parts of the state. It reaches 6 to 20 or more feet in height. Maximum trunk diameter is about 4 inches. Tamarisk forms pure stands, sometimes thickets of dense shrubby plants, in disturbed water-side areas. It is common in extreme western Colorado on canyon bottoms, in southeastern Colorado, and along the Rio Grande and Arkansas Rivers, but it is not yet widely established on the South Platte River. There is evidence that Tamarisk is one of the earliest plant invaders of Colorado, possibly brought by Spanish settlers in the seventeenth century and escaped from cultivation by the eighteenth century.

Its deep taproot can reach the water table (up to 25 feet deep), and a stand of Tamarisk can lower the water table, especially in dry areas where the water is most needed for native plants and human uses. Along the Colorado River an estimated 568,000 acre-feet of water each year are lost to stream-side vegetation, with Tamarisk being a major cause.

One mature Tamarisk can produce over half a million seeds a year. The seeds require saturated soil to germinate and grow, but once established this plant can survive floods, fire, and droughts. Tamarisk can sprout from roots. Some stands of Tamarisk completely replace all native vegetation; others choke natural watercourses. The constant supply of water in streams maintained by mankind aids Tamarisk in replacing native plant species.

Plant communities dominated by Tamarisk are much less valuable to native wildlife than native plant communities. Tamarisk should never be planted where they can escape cultivation - which is almost anywhere.

Tamarisk is one of the mostly widely distributed and troublesome weeds of the southwest. It is prolific and hard to control. Eradication is difficult. Cutting stems close to the ground immediately followed by application of herbicide to the stumps may succeed. The plants are resistant to burning, drought, and freezing. The scientific name is Tamarix ramosissima.

Leaves: pale green, minute scale-like leaves covering the branches, 1 millimeter or 0.02 to 0.03 inches long, pointed.

Stems: Branches are smooth, slender, flexible, and break easily.

Flowers: white to light rose.

Fruit: about 1/8 inch long.

Bark: smooth, becoming furrowed and ridged with age.

Text Copyright © 1998 Stuart K. Wier