Preventing Biological Invasions is a Gardener’s Responsibility

Herb Robert is a common garden invader and problem for its ability to move around with garden soil, plants, pets and clothes. Photo by R. Emanuel.

Most gardeners are unaware that they often inadvertently play roles in the spreading invasion of noxious weeds. But they also can learn to avoid, guard against and eliminate plants that try to escape the boundaries of their gardens.

“Biologists estimate that between 30 and 80 percent of invasive plant species introduced to the United States originated as garden plants,” said Linda McMahan, one of the authors of a new Oregon State University Extension publication, “Invasive Species: What Gardeners Need to Know,” EM 9035, available online at

“In Oregon alone, 21 designated noxious weeds cost approximately $125 million a year in production losses, fire damage and control,” she said. Researchers have determined that almost half of the 1,000 plants and animals protected under the U.S. Endangered Species Act have been listed because of biological invasions.

Plant invasion occurs in three distinct phases:

Introduction. Gardeners can introduce and establish invasive species as they bring a new plant into their garden and perhaps share it with neighbors through plant or seed exchange. The plant population may experience a lag time in its reproductive ability, in which it will stay in a controlled situation such as a garden, forest or pasture. The plant does not appear to be overly aggressive. In some cases, the lag period can persist for decades.

Escape. During the second phase, an escaped plant appears to jump out of the garden into the neighboring landscape. Often this means a plant has overcome barriers that have kept it in check and prevented its spread. These barriers may include temperature, moisture, pests or competition from other plants.

Invasion. Eventually, the plant population shows signs of invasion. During this phase, the plant rapidly reproduces by sexual means (seeds) or asexual means (roots/rhizomes or creeping stems) and may spread over large distances.

The OSU Extension publication points out that gardeners can take control over the introduction and escape phases of a new invasion if they evaluate their potential plant choices first. “Responsible consumers can become informed prior to making a purchase,” McMahan said. Several websites can point the way (see below).

Sometimes a plant’s invasive potential can be arrested by watching it grow in the garden. When a plant starts to show an ability to escape the designated garden or landscape boundaries, the gardener needs to deadhead, trim or otherwise prevent the plant from escaping.

If someone gives you a start or seed, ask where the plant is from and if it is native to Oregon. How successful was the plant in the garden? Does the plant produce many seeds that sprout more than a few feet away from the original plant?

What are the alternatives to invasive species? The publication strongly recommends that they be replaced with Pacific Northwest natives and advises how to get rid of invasive species already in the garden. Many nurseries carry and order more native plants than previously, McMahan said.

Websites to help identify invasive species:

•  Oregon Department of Agriculture’s Oregon State Noxious Weed List is an authoritative listing of plants officially designated as noxious weeds in the state.

•   Oregon Public Broadcasting’s Silent Invasion website provides a photographic list of invasive species of concern in Oregon.

• The United States Department of Agriculture Natural Resources Conservation Service’s PLANTS Database allows the user to track plants for their occurrence in individual counties throughout 49 states. Users can use the website to determine whether a plant is invasive by referring to the Invasive and Noxious Weeds page.

• Closer to home (for me), Clean Water Services maintains a website with a focus on identifying non-native invaders in our area.

• Also, check out the 4 County Cooperative Weed Management Area and their compendium of Portland-area weeds:

•   The University of Montana’s Invaders Database System allows users to search for invasive species by name, state or area in the five northwestern states or access lists of noxious weeds for all U.S. states and six Canadian provinces.


Watershed health is a gardener’s responsibility

Gardens can be both good and bad for watershed health. Photo: R. Emanuel.

Gardens can be both good and bad for watershed health. Photo: R. Emanuel.

Gardeners can have a huge impact on local watershed health. As hundreds, thousands and eventually millions of them influence the land use, inputs, water use and runoff patterns from their home landscapes, they shape the water quality of a watershed. Even little actions add up in a watershed.

Below are some resources I’ve drawn together for OSU Master Gardener trainings I’m delivering around the state this winter and spring. Hopefully others will find them useful too.

WaterWise Gardening Multimedia Presentation:

WaterWise ™ Gardening:

Surfrider Foundation Ocean Friendly Gardens:

Oregon Rain Garden Guide:

OSU Watershed Education Team:

Oregon Stormwater Solutions:

Protecting Water from Non-point Source Pollution:

Puget Sound Partnership (non-point source pollution):

How-to guide can help gardeners restore natural water cycle

raingardensCORVALLIS, Ore. – A new guide on building sunken-bed rain gardens to collect and filter runoff water can help Northwest homeowners learn how to redesign home landscapes to help protect rivers and streams.

Rain gardens can help restore the natural water cycle, according to Rob Emanuel and Derek Godwin of Oregon State University Extension and Oregon Sea Grant Extension.

“As our landscapes became developed, rain falling on hard surfaces was directed to pipes, ditches and storm drains that route to streams or into stormwater sewer systems,” Emanuel said. “The result is too much water arriving in a short amount of time and carrying pollutants.”

Rain gardens work like a native forest, meadow or prairie.

“They capture and redirect stormwater from hard surfaces such as roof tops, driveways, parking lots and streets,” Godwin said. “Rain gardens help keep watersheds healthy by filtering out toxins before they pollute streams and lakes, and they can actually recharge aquifers by encouraging water to soak into the ground.”

The new 44-page illustrated guide, “Oregon Rain Garden Guide: Landscaping for Clean Water and Healthy Streams,” was written by Emanuel, Godwin and Candace Stoughton, who works for the East Multnomah Soil and Water Conservation District. It can be found online at or ordered by calling Sea Grant Communications at 541-737-4849. Copies are $4.95 each, plus shipping & handling.

This how-to publication provides information specific to Oregon’s conditions. No stormwater, garden or landscape expertise is necessary to use it. The step-by-step approach teaches how to determine where water flows across a homeowner’s property and the best place to put a rain garden to manage water flow across impervious areas.

The guide points out what local regulations need to be followed and how to determine slope, drainage rates and texture of the soil. Size of the rain garden and volume of water it can hold also are discussed, as are how to excavate, grade and build berms. The guide also recommends native perennials that can withstand both frequent wet and dry cycles.

The guidebook is a joint project of the OSU Extension Service and Oregon Sea Grant, East Multnomah Soil and Water Conservation District, Jackson Soil and Water Conservation District, and the Oregon Environmental Council. Partial funding for the guide was provided by a grant from Oregon Department of Environmental Quality.

Note: Anyone who is local to the N. Coast and would like a courtesy copy can contact me at my office in Tillamook (see the contact page). Depending upon circumstances, I’ll either arrange a delivery or a pick up.

Invasive Species: Resources for Gardeners

Understanding what invasive species are and how to control them are increasingly important issues for gardeners.  Approximately 50% of the total invasive plants introduced to the U.S. have a horticultural origin.   With new introductions by the nursery industry and the hunger by gardeners to experiment with more exciting varieties in their own landscapes, horticultural-origin biological invasions are sure to continue.

Despite this grim statistic, gardeners can also be a positive part of the solution. Below are some resources to get gardeners started on solving the problem.  I developed these for the 2009 OSU Master Gardener Minicollege in Corvallis, OR:

Below are other resources that I compiled for gardeners and OSU Master Gardener volunteers:

Lastly but not least, are some published guides to Pacific Northwest and regional invaders as well as their more environmentally friendly alternatives:

Happy invader-free gardening! Don’t forget to pass on the word to others so that they too can be part of the solution when it comes to stopping the silent invasion of Oregon’s beautiful watersheds. Contact me here if you need more assistance.

Beware the Yellow Flag!

There’s a gorgeous, large yellow iris called “yellow flag” or “yellow water iris” (Iris pseudacorus) found in wetlands, along riverbanks and near ponds in Oregon. It is sold in local nurseries and garden stores as well.

Image courtesy of Oregon State University Extension ServiceBut there’s a big problem with this lovely perennial iris. It is invasive and out-competes native riparian vegetation, including cattails, sedges and rushes, and it degrades native fish habitat, as well as bird nesting and rearing sites. Native to Europe, Great Britain, North Africa and the Mediterranean region, yellow flag iris has been introduced in temperate areas nearly worldwide and occurs throughout the United States except in the Rocky Mountains. It appears to be most common near developed areas.

You wouldn’t expect widely available, familiar plants like yellow flag iris to be out-of-control noxious weeds. But yellow flag iris, native to Europe, has escaped cultivation and moved into rivers of the Pacific Northwest.

With large showy yellow blossoms, yellow flag, also known as water flag, can grow in large clumps and has been know to reduce the carrying capacity of wetlands for water storage, and block irrigation canal flow and flood control ditches. Difficult to eradicate, it is common in mid-Willamette Valley riparian areas and has been found along rivers in central Oregon.

“If you see a yellow-flowered iris growing directly in water in Oregon, it is most likely yellow flag,” said Andy Hulting, a weed specialist with the Oregon State University Extension Service.

In Oregon, yellow flag blooms in late spring or early summer. Several flowers can occur on each stem, along with one or two leafy bracts. Each flower resembles a common garden iris with three large (1.5- to 3-inch) downward facing yellow sepals and three smaller upward pointing petals. The yellow sepals are often streaked with brown to purple lines. Flower color ranges from cream to bright yellow. Some horticultural varieties have been developed with variegated leaf color. The plants may grow to almost five feet in height. The leaves are mostly basal and are folded and clasp the stem at the base in a fan-like fashion.

A perennial, yellow flag iris will remain green during the winter where the weather is mild, but leaves will die back during periods of prolonged drought or below-freezing temperatures. It spreads both by seed and by stout underground stems called rhizomes from which its roots can grow to a foot in length.

After flowering, the large seed capsules of yellow iris are up to 2.5 inches long and contain many dark to reddish-brown seeds. When not flowering, yellow flag iris may be confused with cattail (Typha latifolia) or broad-fruited bur-reed (Sparganium eurycarpum). Look for the fruits in the summer, or the fan-shaped plant-base at other times of year.

Up to several hundred flowering plants may be connected by rhizomes. Fragments of rhizome can form new plants if they break off and drift to suitable habitat. The flowers are pollinated by bumblebees and long-tongued flies. Seeds germinate and grow well after being burned in late summer. Yellow flag readily resprouts from rhizomes after burning.

As a popular ornamental plant for wet areas or well-mulched soil, yellow flag is widely sold in nurseries and on the Web. It has often been planted in wastewater or storm water treatment ponds, been used to control erosion and is known to take up metals and nutrients in wastewater treatment facilities. It is a popular garden plant for wet or well-mulched soil, and has been introduced as an ornamental throughout the world.

“Yellow flag is being widely distributed by water garden enthusiasts,” said Hulting. “This likely leads to unintentional releases in urban and suburban wetlands and eventually wetlands across the landscape, spread by seed and rhizome fragments during high water events. We’d like to help gardeners become more aware of the ecological effects invasive plants like yellow iris might have on the environment.”

The best control is prevention, said Hulting. “Learn to identify it and don’t plant it. Encourage other gardeners not to plant it.”

Yellow flag is listed on the Oregon Department of Agriculture’s Noxious B list, meaning it is locally invasive and not yet widespread. The Pacific Northwest Exotic Pest Plant Council lists it as ‘A-2 Most Invasive-Regional’ (highly to moderately invasive but still with a potential to spread).

The OSU Extension Service recommends the following wetland native plants as being more ecologically appropriate alternatives to yellow flag: monkey flower, Rocky Mountain iris, Douglas iris and skunk cabbage. Also, these non-native ornamentals are less invasive: Japanese iris, Siberian iris and blue flag.

To help home gardeners and landscape designers make sound ecological choices about what to plant in their gardens, the Oregon State University Extension Service has published a 52-page booklet called GardenSmart Oregon (EC 1620), in cooperation with City of Portland, The Nature Conservancy, Oregon Association of Nurseries, Clackamas Community College, Oregon Public Broadcasting, OSU Extension Service and OSU Sea Grant. GardenSmart Oregon is available online at:

Or, call 1-800-561-6719 to request a printed copy of GardenSmart Oregon ($3 per copy shipping and handling fee). Local county offices of the OSU Extension Service have copies available for no charge.

Pulling can control isolated plants of yellow flag iris or digging, but use care and protect your skin as resins in the leaves and rhizomes can cause irritation. Because rhizome fragments can grow to form new plants, be sure to clean up all fragments. Large-scale tillage to control yellow flag should be avoided because of the likelihood of spreading rhizome fragments, warned Hulting.

“Limiting soil disturbance can help native plants survive and make the site more resilient to reinvasion by yellow flag,” he said.

If you can do nothing else, remove and destroy the seed heads and flowers of yellow flag, he advised.

Large-scale infestations of yellow flag most likely need to be managed by chemical means to achieve complete control and limit the spread. Formulations of glyphosate labeled for aquatic uses (examples: Rodeo or Aquamaster) have been effective on yellow flag iris when applied as a spot treatment. Take care to not overspray onto desirable plants when making glyphosate applications. Follow the mixing directions and application instructions on each label.

“Cut stump” treatments, or cutting or mowing foliage and applying glyphosate directly to the cut surface of individual plants have also been effective for managing smaller infestations of yellow flag iris and can negate herbicide injury to non-target plants. Because this plant is a rhizomatous perennial multiple applications over multiple years will likely be needed for complete control, said Hulting.

No biological control organisms have been approved for yellow flag iris.