The Conservancy is excited to partner with the Mile High Youth Corps (MHYC) to perform nine weeks of Russian olive removal along the Canal corridor this spring and summer. This project will greatly advance the Conservancy’s work to protect and restore the Canal’s canopy as Russian olive is an invasive species that competes with trees and desired vegetation for scare water resources. Read more about our work with the youth corps.
Introduction to Colorado
Russian olive is an invasive species, native to southeastern Europe and western Asia. Russian olive was intentionally introduced to North America in the late 1800s, a favored ornamental in the Great Plains for its appearance and ability to function as a windbreak. State and federal agencies subsidized Russian olive distribution in the west throughout the early 1900s, touting its ability to control erosion in the wake of the Dust Bowl. Within a few decades, Russian olive began to escape cultivated lands and spread into natural areas. Russian olive is now found throughout the central and western U.S.
Spread & Establishment
Russian olive is able to spread across landscapes and displace native vegetation due to its high seed production and viability, ease of dispersal, and ability to grow in a variety of environmental conditions. Russian olives begin producing seeds when four years old, with clusters of yellow spring flowers maturing into an abundance of fruit each fall. Seedbank analyses have found an average of 415 Russian olive seeds per square meter of soil in invaded areas. These seeds remain viable for at least three years, with research indicating that seeds can remain viable for up to 28 years in certain conditions. Seeds are easily spread throughout the landscape, dispersed by the birds and mammals that consume Russian olive fruit. Seeds are also dispersed by water, with floating fruit carrying seeds down riparian corridors. Once dispersed, seeds germinate in many soil types and seedlings succeed in many climatic conditions. This adaptability and hardiness has allowed Russian olive to establish and spread aggressively in many landscapes.
Ecosystem Impacts
In Colorado, Russian olive primarily invades riparian corridors, but can survive in a variety of environmental conditions—tolerating floods and drought, extreme hot and cold temperatures, and both alkaline and saline soils. Russian olive seeds germinate in a wider variety of soil, moisture, and light conditions than cottonwoods and willows, giving it an advantage over Colorado’s native riparian trees. Though Russian olive prefer full sun, saplings are also successful in partial and full shade, allowing them to invade the understory beneath mature cottonwoods. This disrupts plant succession patterns when mature cottonwoods die: Russian olive already established in the understory grow quickly and shade out cottonwood and willow seedlings, which need full sun and moist soils to thrive.
As Russian olive invades an ecosystem, its ability to crowd out native plants can have negative impacts on wildlife. Russian olive was historically promoted for its ability to attract wildlife, as many birds and mammals eat the small fruits produced by the trees each fall. However, studies have shown that bird species richness actually decreases in Russian olive infested landscapes compared to those dominated by native plants. Cavity nesting and insectivorous birds are particularly impacted by the loss of the native vegetation with which they co-evolved, as Russian olive are not cavity forming and do not support as many insects as native trees.
Regulation & Control
The Colorado Department of Agriculture (CDA) regulates invasive species, like Russian olive, to protect the state’s agricultural and natural resources. The CDA implements and enforces the Colorado Noxious Weed Act and assists local governments in the development and implementation of programs aimed at preventing the spread of and mitigating damage from noxious weeds. The Colorado Noxious Weed Act defines a noxious weed as an alien plant that meets one or more of the following criteria: (1) aggressively invades or is detrimental to economic crops or native plant communities, (2) is poisonous to livestock, (3) is a carrier of detrimental insects, diseases, or parasites, (4) is detrimental to the environmentally sound management of natural or agricultural systems. Species meeting these criteria become state-regulated noxious weeds and are categorized into three lists: A, B, and C. List A weeds are mandated for eradication on all public and private lands, with the goal of eliminating the species from the state. List B weeds are managed to contain or suppress populations, with the goal of stopping the continued spread of the species. List C weeds are widespread species which the CDA supports local jurisdictions in managing, with the goal of creating more effective integrated management approaches. Russian olive meets the criteria of aggressively invading native plant communities and is designated as a List B noxious weed by the CDA, meaning local governments are required to manage and limit its spread.
The most effective approach to controlling Russian olive is preventing its establishment. In areas that are already infested, like the High Line, the next best approach is a combination of mechanical and chemical control, known as “cut-stump” treatments. Cut stumping involves using chainsaws or brush cutters to down trees and then immediately applying small amounts of herbicide directly to the stumps. The herbicide is drawn down into the roots and prevents the trees from resprouting from the stump or from root suckers. After cut stumping, monitoring is essential to prevent Russian olive from reestablishing from the reservoir of seeds in the soil. The Canal Conservation Corps will monitor project areas after removals and control any newly germinated seedlings. Learn more about joining the CCC!
The future of the Canal’s canopy
The proliferation of Russian olive on the Canal is both a cause and a symptom of native species decline along the corridor: Russian olive directly contribute to the decline by monopolizing resources needed for native trees to establish, but its prevalence is also symptomatic of the changing conditions along the Canal. Historically, cottonwoods and willows were able to establish and thrive on the Canal due to the abundance of irrigation water that allowed the man-made ditch to mimic a natural waterway. However, as Denver Water continues to reduce its reliance on the Canal for irrigation delivery, fewer reaches of the Canal will hold enough water to support riparian tree species, like cottonwoods, which have high water requirements. If allowed to continue to spread, both established trees and new plantings will have to compete with Russian olive for increasingly scarce water resources along the Canal. By pairing new tree plantings with efforts to remove Russian olive, the Conservancy aims to take a holistic approach to protecting and restoring the Canal’s canopy. Learn more about the Conservancy’s tree planting program.
Further Reading / References
US Forest Service: Russian Olive
USDA Natural Resource Conservation Service: Russian Olive
CO Dept of Agriculture: Noxious Weed Program
High Line Canal Conservancy: Invasive Species Removal
