Ask SAM: Tamarisk added to noxious weed list

This column is provided by Larimer County Extension's Small Acreage Management (SAM) Program to assist rural residents.

Dear SAM,

We have noticed a few tamarisk plants along the shore of the reservoir behind our house. Should I remove them?

W. Young

Dear W.,

Wildfires erupt from a single lightning strike and can spread to consume thousands of acres. So it is with weed infestations. What begins as a seemingly insignificant "plant out of place" can grow into a devastating infestation that permanently alters the landscape.

Tamarisk in the South Platte watershed is a raging wildfire in the making. A riparian weed, tamarisk plants were hard to find along the Front Range five years ago. Several years and a drought later, tamarisk has a foot in our door.

Tamarisk, also known as saltcedar, is a phreatophyte. Phreatophytes are water-loving plants that send roots down to groundwater. Cottonwoods and willows are native phreatophytes, but they have less drought tolerance than tamarisk. When water tables drop, tamarisk has a competitive advantage over many of our native riparian plants.

During our recent drought, water levels in reservoirs dropped and shorelines expanded. This created ideal conditions for a tamarisk invasion. Many reservoirs in the South Platte watershed that were free of tamarisk now have two- or three-year-old infestations.

Besides destroying habitat for native plants and wildlife, tamarisk consumes large amounts of water. It is estimated that tamarisk in the Southwest consumes twice as much water each year as all the major cities in southern California combined.

In May, Larimer County added tamarisk to its list of noxious weeds that must be controlled. While listing tamarisk as a noxious weed can raise awareness, the listing by itself does nothing to control the weed. It is up to land managers to find and control tamarisk on the properties they manage.

Tamarisk is most likely to be found along shorelines of reservoirs or rivers and along irrigation ditches.

Hand-pulling can control young plants. The shoots you pull up might be growing off a larger underground root system, so don't be surprised if new plants pop up in the same spot next year. If you persistently pull young shoots as soon as you see them, you will eventually starve the larger root system.

Larger plants require a combination of mechanical and chemical control. Cut stump treatments involve cutting the tamarisk with a saw and immediately treating the stump with triclopyr (e.g. Garlon) or imazapyr (e.g. Arsenal or Habitat). The stump should be treated within a minute after cutting the plant. If you wait longer, the chemical might not make it to the root system and the plant may resprout.

Plants that are too big for pulling and too small for sawing may need to be sprayed. Late summer or early fall are the best times to spray. A 1 percent solution of imazapyr is most effective. The cost of the treatment can be reduced by using a mixture of imazapyr and glyphosate.

Several insect species are being studied for controlling tamarisk along heavily infested stretches of the Arkansas River and Colorado River. Insects are most useful for containing large, severe weed infestations. Insects are less useful when the goal is to eradicate a smaller, less concentrated weed population. Since eradication is the goal in Larimer County, insects will probably play a small role.

With tamarisk added to the county's noxious weed list, cost share funds are now available to help pay for tamarisk control. For more detailed control recommendations and cost share information, contact the Larimer County Weed District at 498-5768.

SAM

Have a question about rural living? Write to Ask SAM, Larimer County Cooperative Extension, P.O. Box 543, Fort Collins, CO 80522.