K. George Beck, Ph.D.
Dept. of Bioag Science & Pest Management
Colorado State University

Notes
ECOLOGICALLY BASED MANAGEMENT OF SALT CEDAR

Ecologically based weed management, also known as successional weed management (Sheley et al., 1996) is designed to develop strategies based upon our current understanding of succession. It recognizes that plant communities are dynamic and uses technology to enhance natural processes and mechanisms that regulate vegetation change. The goal of successional weed management is to direct weed infested plant communities on a trajectory to a more desirable plant community. The causes of succession are site availability, differential species availability, and differential species performance. Successional weed management exploits these causes. The management components of this approach are designed disturbance, which correlates to site availability, controlled colonization, which correlates to differential species availability, and controlled species performance, which correlates to differential species performance. The first step in successional weed management is to determine the composition of the infested plant community and then decide upon the composition of the desired plant community based upon land use goals. Then, by exploiting the causes of succession through management input, the infested community is placed on a trajectory over time to reach the desired state. One may not achieve exactly what they wanted but, the community will be much closer to what is necessary to achieve land use goals and objectives.

Salt cedar, like any other weed species, can be managed using successional weed management. However, one must know the control options at their disposal to properly design such a management approach.

Biological Control

Salt cedar infests very large areas in the western U.S. and is a good candidate for biological control. Several papers will be presented on biological control of salt cedar and this topic will not be covered here. Several very effective classical biological control agents have been imported into the U.S. recently and are being developed under research by the federal government. Their use should make a substantial contribution to the control choices that one may use as they develop their successional weed management strategy.

Physical or Mechanical Control

Fire is ineffective to control salt cedar (Brock, 1994). Salt cedar readily resprouts from its crown after fire and it regrows at the rate of about 3 to 4 meters per year. In Utah for example, a 20 acre fire occurred in the summer of 1975 in a stand of salt cedar. A year later, the effects of the fire were observable along with lush green salt cedar regrowth over the entire burned area. By 1978, evidence of the fire was no longer apparent and salt cedar had fully recovered. Further research in Utah examined repeat burns during spring, summer, and fall for 2 years, but proved to be ineffective because of vigorous salt cedar regrowth.

Shredding, roller chopping, and chaining are all designed to decrease the canopy of the target species and ideally to decrease plant density. However, these control mechanisms fail to decrease salt cedar density, again owing to vigorous regrowth from crowns (Brock, 1994). Grubbing individual plants by cutting them below the soil surface at least 20 cm deep, also does not work well to control salt cedar. In research to examine grubbing, regrowth was apparent 6 to 12 months following the procedure. Root plowing with a horizontal blade to at least a 20 cm depth has been used frequently and is effective, particularly when coupled to revegetation efforts. Root plowing alone controlled 40% of salt cedar in a New Mexico project. But, root plowing must be done more than once if greater control is desired. In Arizona, root plowing with 1 meter long ripper blades set at 1 meter intervals on a tool bar and pulled by a D9 crawler tractor has kept a portion of the Salt River nearly free of salt cedar for 10 years. This procedure has been carried out about every 10 months to achieve this level of success. Root plowing also is most effective when coupled with revegetation.

Flooding has the potential to control salt cedar ( Brock, 1994). Totally inundating established salt cedar for 24 to 36 months caused 99% control when flooding was done during the growing season. Inundation also will prevent salt cedar seedling establishment. However, salt cedar can withstand flooding. Root crowns survived being flooded for 98 days and entire plants flooded for 70 days also survived.

Chemical Control

The first chemicals used to control salt cedar were 2,4-D, 2,4,5-T, and silvex (Duncan and McDaniel, 1998). These growth regulator herbicides controlled salt cedar foliar growth, but plants always recovered from vigorous sprouting from crowns. Silvex and 2,4,5-T were banned in 1983, but 2,4-D still is used somewhat on an individual plant basis. Triclopyr (Garlon 3A) is another growth regulator herbicide that will control salt cedar when used on an individual plant basis. It is fairly effective. A 1.5% solution of triclopyr is recommended and a total of 300 gallons of spray solution per acre should be applied. Thorough coverage is essential and best control is achieved when treatments are done in May or August.

Imazapyr (Arsenal) has been thoroughly evaluated in New Mexico to control salt cedar (Duncan and McDaniel, 1998). When treating individual plants, researchers found that a 1% solution of imazapyr in water when sprayed to wet but not to drip, controlled 90% of salt cedar. Imazapyr solutions always include the use of a non-ionic surfactant at 0.25% v/v. Best control is achieved when spraying is done in August or September, while control is much less when sprayed in April or October. Imazapyr at 1% is an expensive treatment and is often mixed with glyphosate (Roundup, Rodeo, Aquamaster) because the latter is much less expensive. A tank-mix of imazapyr plus glyphosate at 0.5 + 0.5% v/v controlled more than 90% of salt cedar regardless of date of application during the growing season. By contrast, glyphosate alone at 2% v/v controlled only 32% of salt cedar. Based upon their experience, New Mexico State University researchers developed guidelines for using imazapyr plus glyphosate to control salt cedar:

1. Treat young salt cedar or regrowth because plants less than 4 meter tall are easier to treat and control than taller plants;
2. Treat areas previously root plowed, mowed, or cleared or areas where salt cedar is just starting to invade;
3. Treat areas with tree densities less than 160 plants per acre;
4. Spray imazapyr plus glyphosate at 0.5 + 0.5% v/v plus 0.25% non-ionic surfactant. This concentration will provide control equivalent to a 1% solution of imazapyr;
5. Spray foliage to wet, but not to drip, and make certain to treat terminal ends of branches;
6. Allow two full seasons to pass before initiating follow-up treatments.

Treating salt cedar on a broadcast basis often is desirable and certainly less labor intensive than treating on an individual plant basis. Carpet rollers have been evaluated in New Mexico (Duncan and McDaniel, 1998). Imazapyr plus glyphosate at 0.125 + 0.125% v/v or imazapyr alone at 0.125% controlled 85 and 92% of salt cedar respectively, 2 years after treatments were done. Mortality dropped to 32% when the concentration was decreased to 0.1 + 0.1% imazapyr plus glyphosate. Also, glyphosate alone at 0.5% applied through a roller carpet caused only 5% mortality, while imazapyr alone at 0.25% killed 94% of salt cedar. A major advantage of the carpet roller is that it only controls the vegetation that is contacted and the understory, thus, is protected. However, many salt cedar plants when untreated, but this problem decreased as operator experience increased. New Mexico State University researchers recommend only to use the carpet roller on salt cedar that is less than 3 meter tall.

Aerial applications to control salt cedar also have been evaluated in New Mexico. NMSU researchers compared imazapyr alone at 0.75 lb ai/A to several rates of imazapyr plus glyphosate in 1993 and repeated the experiments in 1994. Control ranged from 66% (imazapyr alone at 0.75 lb) to 87% (0.5 + 0.5 lb ai/A of imazapyr plus glyphosate) 2 years after treatments were applied. Aircraft fit with conventional raindrop nozzles delivered 7 gallons per acre were compared to microaire nozzles that delivered 3 gallons per acre. Control dropped 10 to 15% with the latter nozzles, showing that coverage and canopy penetration are important to achieve desired control. NMSU researchers also partitioned salt cedar height and stem numbers to determine their effects on control. Generally, taller trees are harder to control than short trees and trees with fewer stems also were easier to control. Researchers also compared fixed wing applications to those made by helicopter. Control from helicopter applications were highly variable and ranged from 31 to 90% with no apparent rate response.

Ecologically Based Salt Cedar Management

A very large site recovery project was initiated in New Mexico to reclaim salt cedar infested flood plains in the Bosque del Apache National Wildlife Refuge (Taylor and McDaniel, 1998). These researchers employed root plowing as a designed disturbance to make the site available for colonization then piled and burned salt cedar trash. They found it necessary to spot spray salt cedar regrowth and used imazapyr or imazapyr plus glyphosate to do so (controlled colonization). They further controlled colonization (differential species availability) by planting many different native trees and shrubs. This approach was their best system and was compared to spraying, root plowing, burning, spraying, and planting. Root plowing, burning, spraying, and planting cost about 2/3 less to invoke than spraying, root plowing, burning, spraying, and planting. They also controlled species performance (differential species performance) with drip irrigation. Ultimately, they found it necessary to mimic natural flooding with controlled water level manipulations. While this tends to stimulate salt cedar recruitment, their experience shows that salt cedar remains as a minor component of the overall desired plant community.

Summary

These are just some examples of combining treatments to create a successional weed management approach. Undoubtedly, other treatment combinations will be successful and one must tailor ecologically based weed management to particular sites. To be successful, one must know the composition of the infested community and the approximate composition of the desired plant community. Then through a series of designed disturbances followed by methods to control colonization and species performance, place the infested community on a trajectory to create the desirable plant community so that land use goals can be achieved.

References

Brock, J.H. 1994. Tamarix spp. (Salt Cedar), an invasive exotic woody plant in arid and semi-arid riparian habitats in the western USA. P. 27-44 in L.C. de Waal, L.E. Child, P.M. Wade, and J.H. Brock, eds. Ecology and management of invasive riverside plants. John Wiley and Sons, West Sussex, England.

Duncan, K.W. and K.C. McDaniel. 1998. Saltcedar (Tamarix spp.) management with imazapyr. Weed Technology 12:337-344.

Sheley, R.L., T.J. Svejcar, and B.D. Maxwell. 1996. A theoretical framework for developing successional weed management on rangeland. Weed Technology 10:766-773.

Sheley, R.L., S. Kedzie-Webb, and B.D. Maxwell. 1999. Integrated weed management on rangeland. P. 57-68 in R.L. Sheley and J.K. Petroff, eds. Biology and Management of Noxious Rangeland Weeds. Oregon State University Press, Corvallis, OR.

Taylor, J.P. and K.C. McDaniel. 1998. Restoration of saltcedar (Tamarix spp.)-infested floodplains on the Bosque del Apache National Wildlife Refuge. Weed Technology 12:345-352.