Goatsrue – May 2020 Weed of the Month

goatsrue patch
Goatsrue grows in dense patches and outcompetes native plants and healthy forage species.

The story behind this month’s noxious weed is sadly ironic. First introduced from the Middle East to Utah in 1891 as a possible forage plant, it soon became evident that it contained a poisonous alkaloid making it toxic to livestock (thus the name goat’s rue!). Fortunately for the animals grazing where it was planted, goatsrue (Galega officinalis) is also bitter and animals generally avoid it. But unfortunately for us all, it had already spread over 60 square miles before this was realized. When control began in earnest, it soon became clear that this would be no easy matter due to the massive root crowns and prolific seed production. Utah first listed goatsrue as a noxious weed in 1974 followed by federal listing by USDA/APHIS in 1981. The eradication project began in 1976 in Utah continues to this day as new plants show up every year from the seed bank and established populations stubbornly resist control efforts.

In 1999, goatsrue was first discovered in Washington here in King County growing in the Federal Way area. The State Weed Board quickly added it to the Class A Noxious Weed List, and the King County Noxious Weed Program has been working hard ever since to contain and eradicate it.

Man standing in a field of goatsrue
King County has been working to control goatsrue in Federal Way since it was first discovered in 1999.

Since goatsrue is a Class A noxious weed, state law requires eradication wherever it occurs. It is also on the state quarantine list and on the federal noxious weed list, so it isn’t being introduced through sale or import anymore (at least not legally). In the United States, in addition to Washington and Utah, goatsrue is known to be invasive in Oregon, Colorado, Nebraska, Pennsylvania, New York, Connecticut, Massachusetts, and Maine. So, all eyes are on the lookout for goatsrue these days and every effort is made to act quickly to remove it.

goatsrue plants in flower
Goatsrue plants were found in Tacoma in 2017. Photo by Jeff Walker.

However, goatsrue is very difficult to get rid of once it’s taken hold, usually taking many years of eradication efforts. It rapidly colonizes areas creating monocultures. This fast growth displaces native plants that provide food and cover for wildlife.

Goatsrue can succeed in a wide range of habitats and conditions although it prefers full sun. It tolerates acid soil and seasonally wet areas, often spreading along waterways and in wet pastures. In King County, it is found in undeveloped vacant sites that are seasonally wet, historic pasture sites, and along fence lines and road right of ways. It shares habitat with grass, blackberries, shrub roses, Scotch broom, and Canada thistle.

Goatsrue grows vigorously in well-watered, full-sun sites and forms large dense crowns of up to 20 stems that can regenerate throughout the season from massive taproots. These large taproots store energy for the plant, so it can take many years of control work to completely exhaust the roots.

goatsrue regrowing in the spring
Goatsrue continues to regenerate from deep tap roots even after being controlled for several years.

As if the roots weren’t bad enough, a single goatsrue plant can make up to 15,000 seeds that can persist in the soil for 15 years or more. The heavy seeds are moved primarily by water, erosion, animals and humans, so plants generally keep coming up in the same areas year after year unless they are moved by floods, animals or soil disturbance.

Goatsrue seedlings by show
Goatsrue seedlings emerge in the early spring, especially in disturbed, open areas.
goatsrue seedlings in the grass
In grassy areas, it is difficult to find goatsrue seedlings when they are still small.

Goatsrue has an additional feature that helps it outcompete other plants. As a member of the legume (Fabaceae) family it has a special relationship with soil bacteria. The roots form nodules that provide a home for the bacteria, in return the bacteria processes nitrogen from the air making it available to the plant. Nitrogen is a key element necessary for plant growth. Goatsrue has a unique rhizome bacteria specialized for only this particular species that helps it take in more nitrogen and grow more vigorously than less fortunate plants, especially on sites with poor nutrients.

Those first sites discovered in 1999 in Federal Way totaled only 1,000 square feet. However, in the following several years more sites were found nearby. The total area of goatsrue peaked in 2004 at 155,200 square feet (3.6 acres). This increase was due mostly to finding new infestations, but also to mowing existing sites. Mowing was done to encourage seeds to germinate in order to flush out the seed bank more quickly. The thought was that if the seeds were allowed to stay in the soil they might germinate under the existing shrubs and go unnoticed, produce more seeds and re-infest the area.

With continued twice yearly control work (for almost 20 years!!), goatsrue was down to 7,000 square feet at the end of 2019. The number of goatsrue sites peaked at 44, but through sustained and diligent work, 12 of those sites are completely clear of goatsrue and 6 more sites are considered dormant (no plants in three or more years).

bar chart showing the area of goatsue in King County from 2001 to 2019
After we knocked back most of the goatsrue in the first years of control, the last plants and long-lived seeds are stubbornly resisting our efforts to eradicate this noxious weed.

However, the plants are stubborn. After an initial large reduction, the area of goatsrue has plateaued since 2008. This is due to seedlings emerging each year from the seed bank and mature plants continuing to regenerate from the energy stored in the huge taproots.

We are hopeful though. Each year our control efforts reduce the amount of energy stored in the roots, and eventually the plants are no longer able to survive. It’s just going to take time and persistence. This month, the plants are just getting tall enough to spot easily and our first control efforts of the year will start soon.

goatsrue plant in May
Goatsrue plants in May are getting tall enough to find more easily.

Currently, this involves visiting each site at least twice a year to look for plants, treating with a combination of herbicide, digging and clipping of any seed pods that occur. Most plants are small and dispersed, and hard to spot growing in the grass. We have to know what are looking for and work slowly and carefully to make sure we don’t miss any plants.

goatsrue flowers and leaves
Goatsrue flowers and leaves resemble other plants in the Pea Family. Photo by Jeff Walker.

Goatsrue emerges in late April and can be more easily identified in May or June when it flowers. It is a perennial in the pea family and looks somewhat like a bushy vetch or wild licorice with pale purple to white flowers. Because it is somewhat difficult to identify and resembles many other plants, please get a positive identification before taking action. For photos and information on identification please see visit our goatsrue identification page. Because of the potential damage that goatsrue could do and its very limited distribution, we place a high priority on early detection and eradication of this plant. Please contact us or report it online if you spot any in King County!