Common buckthorn and tartarian honeysuckle: new invaders of riparian areas in Colorado

by Ray Sperger, South Suburban Park & Recreation District, Littleton

Common Buckthorn

Common buckthorn and tartarian honeysuckle cause tremendous ecological problems in wooded areas from eastern Nebraska to the eastern US, and they are beginning to get established in Colorado. Common buckthorn is a large understory shrub that invades woodland understories, prairies, and wooded edges in the Midwest although it does not do well on dry sites. In the drier climate of Colorado, it seems to prefer floodplains with a cottonwood overstory in 50 – 95% shade. It is also found on the edges of wetlands and in subirrigated areas where there is full sun, and on wetter north-facing slopes above the flood plain (personal observations). The population at South Platte Park along the South Platte River in suburban Denver has dramatically increased in the last five years, and may be a threat to other riparian areas.
The following characteristics of buckthorn make it an invader that should be eradicated before it becomes well established:

1. Buckthorn is an aggressive invader that rapidly takes over woodlands, outcompeting native shrubs and wildflowers and virtually eliminating them in many woodlands.
2. The abundant fruits are eaten by birds allowing for the long distance dispersal of these horticultural plants into native areas.
3. Buckthorn is an alternative host for oat crown rust, reducing yields and quality.
4. Additionally, buckthorn is an alternative host for the soybean aphid.
5. Nesting success of two thrushes in urban natural areas in Illinois was reduced when birds nested in either common buckthorn or exotic honeysuckle species, as compared to nests in native shrubs.

Observations in Minnesota suggest that whitetail deer avoid browsing on buckthorn, and if it takes over the understory, then deer may leave the area.
 

Tartarian Honeysuckle

The following characteristics of tartarian honeysuckle make it an invader that should be eradicated before it becomes well established:

1. The dispersal of this now widespread horticultural shrub into native areas is aided by birds, which eat the ripe fruits in the summer and fall.
2. Vigorous growth inhibits development of native shrubs and ground layer species.
3. By shading the ground and depleting soil moisture and nutrients, they out compete and replace many native understory shrubs.
4. Early leafing of this exotic shrub is detrimental to spring ephemeral wildflowers, which are designed to flower before trees and shrubs leaf out.

I am involved in a project with the goal of eradicating common buckthorn and tartarian honeysuckle in two high quality suburban natural areas, South Platte Park and Wheat Ridge Greenbelt, using Youth Corp Workers. Also, the project will map these weeds along the S. Platte River through Denver and in likely parts of the Chatfield Basin, Bear Creek, and Clear Creek. Additionally, our goal is to communicate the urgency of managing these species to affected Front Range communities. Because buckthorn and honeysuckle leaf out early and retain their leaves well after native plants drop their leaves, they are highly visible and easily mapped in early spring and late fall.