Leafy spurge, a noxious weed that loves Larimer County

In the mid 1970s residents in LaPorte may have observed a new plant in their pastures and hayfields. This newcomer quickly established roots and made itself a permanent resident. Thirty-five years ago, leafy spurge was a newcomer to Larimer County, now it occupies large tracts of land here. It sunk its roots deep into the moist soils near the Poudre River; it dispersed its seed into irrigation ditches and major waterways. Today it is the worst noxious weed problem in Ft. Collins and much of Larimer County. This perennial invader dominates landscapes along the Poudre River and the North Fork of the Poudre. It is also happily at home in Bellvue, Rist Canyon, Laporte, and north of Ft. Collins. Smaller infestations found throughout the County can easily be managed, if acted on before the plant is well established.

By Ellen Nelson
Larimer County Weed District

Leafy spurge is a perennial with very deep roots that is very hardy, very prolific and very difficult to control. Plants emerge in early spring, grow 2-3 feet tall and produce yellow-green flowers in April and May. The leaves are narrow, 1-2 inches long. The entire plant contains milky latex, an identification characteristic for leafy spurge. The latex can easily be seen after tearing a leaf or breaking a stem. Leafy spurge’s root system is the key to why it is so difficult to control. The underground portions of the plants are covered with vegetative buds, each bud can produce a new plant, and some of these buds can be as much as 30 feet underground! The roots grow fast, in four months a seedling can extend its roots three feet down and three feet out. They grow deep, right to the water table, providing the plant with ready access to water and nutrients. The root buds send up new shoots, allowing the plant to spread vegetatively several feet per year. Then, there are the seeds! Leafy spurge produces lots of seeds with a very high germination rate. When mature, the seed pods burst, projecting seeds up to 15 feet from the parent plant. These seeds remain viable in the soil for up to eight to ten years.

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Leafy spurge is ideally designed to displace native vegetation in prairie grasslands, rangelands, cultivated pastures and fields. Its aggressive root system out competes native plants for available water and nutrients; dense stands of leafy spurge crowd out native vegetation, and may inhibit the establishment of light-demanding species. There is some evidence to that leafy spurge exhibits allelopathy, the ability to inhibit the growth of other plants by releasing plant toxins into the surrounding soil. For all these reasons, leafy spurge is a very aggressive plant that can dominate large areas of open land, and is extremely difficult to eradicate.

Methods of Control

Several species of flea beetle (Apthona spp.) have been found effective in significantly reducing stands of leafy spurge. These beetles develop in the spurge root system. The Apthona larvae feed on the root hairs and young roots, compromising the plant’s ability to absorb water and nutrients. The adult beetles feed on leafy spurge foliage in the summer, so other management tools, such as grazing, mowing or herbicide should be used at other times of the year. Effectiveness of the beetles is site-dependant and varies from year to year. The results are not as immediate as when herbicides are used but, some reports indicate that if pesticide use is minimal, a significant population of beetles accumulate within a few years and can provide good control of leafy spurge infestations. Sheep and goats can be trained to browse leafy spurge. Mowing and grazing can deplete root reserves. Hand pulling or digging can reduce seed production and the stress plants, but this perennial will readily grow back.

There are several systemic herbicides that are effective if applied when the flowers and seeds are developing, or in early to mid-September, when the plants are moving nutrients downward into the roots. A commercially available mixture of 2,4-D and dicamba provides suppression of leafy spurge, by causing it to die back, but this herbicide does not kill the plant, and it will usually grow back from its extensive root system. Tordon and Plateau are fairly effective but can injure cool season grasses. Recently developed herbicides, Paramount and Perspective provide effective control of leafy spurge with minimal grass injury. The Larimer County Weed District has used these herbicides in demonstration projects with favorable results. It is important to recognize that it may take several years to effectively control this prolific weed species due to its extensive underground root system. Multiple treatments may be necessary for several years, making leafy spurge control a time-consuming and expensive undertaking. If left uncontrolled for a single year, leafy spurge can reinfest very rapidly.

The Larimer County Weed District provides advice to landowners on controlling leafy spurge and other noxious weeds. The Weed District provides free site visits, plant identification, advice on controlling noxious weeds, and land and pasture management guidelines. Contact the Larimer County Weed District at (970) 498-5768 or http://www.larimer.org/weeds/

The North Fork Weed Cooperative, a community-based organization focused on weed management has a cost share program targeted specifically at helping landowners in northern Larimer County control leafy spurge on their property. For more information on this program contact the North Fork Weed Coop at NorthForkWeedCoop@yahoo.com