Tamarix control is a significant invasive species issue in the western USA, especially in the American southwest. The woody plant genus Tamarix, also , also known by the common names tamarisk and salt cedar has many negative influences on the natural environment once this alien species becomes established including (1) increased soil salinity, (2) increased water consumption, (3) increased wildfire frequency, (4) increased frequency and intensity of flooding and (5) altering the plant community and wildlife habitat. Tamarix is considered one of the ten most "noxious weeds" in the United States. Scientists and conservation biologists have attempted to identify effective stragegies for eradicating Tamarix from areas that it has successfully invaded. The Tamarisk Coalition identifies five main methods of Tamarix control (1) hand cutting with herbicide application, (2) mechanical removal, (3) manual herbicide application, (4) aerial herbicide application, and (5) biological control by either goats, beetles or other herbivores.
Manual cutting and herbicide application
Most commonly Tamarix is removed using chainsaws, after which the remaining stumps are treated with the systemic herbicide triclopyr. This approach requires trained operators to cut the vegetation and apply the herbicide (People using this approach are sometimes referred to as tamiwhackers.) The herbicide must be applied within approximately 15 minutes of cutting. Typically, a solution of triclopyr systemic herbicide mixed in a vegetable crop oil is applied to the cut stump. Seedlings or smaller plants with smooth bark and a stems less than one inch in diameter can be treated by spraying the bark on the bottom 12-18 inches of the stem (basal bark treatment). Applying herbicide to both cut stumps and basal bark sprays are effective as long as the temperature exceeds 85 degrees F, at which point the triclopyr herbicide volatizes.
Manual cutting and herbicide costs
Because this technique is labor intensive it can be very expensive. Costs of removal by this method range from $1,500 per acre for lightly infested areas to $5,000 per acre for heavily infested acres.
Manual cutting and herbicide effectiveness
The chainsaw method for the cut-stump approach is a relatively successful method of controlling tamarisk. Approximately 15 percent regrowth requires retreatment.
Manual cutting and herbicide applicability
This hand cutting method is appropriate for canyons, washes, irrigation ditches, and along steep river banks which are difficult to access, and which do not necessarily comprise a large land area. Hand control is not as appropriate in areas that that are easily accessible because of its high costs.
Manual cutting and herbicide pros and cons
(1) Manual cutting is very effective in mixed vegetation stands without killing other desirable plants. (2) This method is best used in rough terrain that is not accessible by mechanical equipment. (3) Cut biomass must either be stacked and burned, chipped, or left in piles for wildlife habitat. (4) Some spot herbicide re-application will be necessary.
Mechanical removal involves the use of heavy equipment to physically remove the woody trees and shrubs of the Tamarix genus.
Mechanical removal costs
The costs associate with the mechanical removal of Tamarix include the costs of removal of the root crown and the costs of mulching. The costs of root crown removal using root plow and root rakes are typically about $800 per acre and using excavators range from $150 to $600 per acre depending on density of infestation. The costs of mulching Tamarix including the contracted equipment, labor, and herbicide costs will range from approximately $220 to $700 for high capacity equipment (0.5 to 1.5 acres/hr.), and $250 to $800 for medium capacity equipment (0.25 to 0.75 acres/hr.)
Mechanical removal effectiveness
Tamarix can be effectively controlled at about 85 percent efficacy via mechanical removal.
Mechanical removal applicability
Large portions of the riparian lands and adjacent uplands throughout many watersheds could be controlled by mechanical removal. The exceptions are areas that are inaccessible, such as the canyons found along the Colorado River, Rio Grande River and Green River.
Mechanical removal pros and cons
(1) Extraction and mulching equipment can be very effective at removing Tamarix from a mixed vegetation stand without killing other desireable plants. (2) Mulching and root plowing is only useful on accessible and fairly level topography. (3) The products of mulching provide a seedbed for revegetation. (4) Root crown removal with root rakes disrupts the soil, but can enhance revegetation if irrigation water is available for intensive revegetation efforts. (5) Root crown removal using excavator does not create a severe soil disturbance and is very effective for steep embankments along rivers and canals. (6) Spot herbicide re-application will be necessary for controlling resprouts.
Manual herbicide application
The application of herbicide by hand can be effective when the density of infestations are light. Herbicide can be applied using backpack sprayers, horseback sprayers,or spay equipment mounted on motor vehicles.
Manual herbicide costs
Hand spraying costs approximately $2 to $5 per plant depending on size. Thus, it is an expedient and low cost approach for very light infestations. However, hand spraying quickly becomes expensive in denser stands with larger trees.
Manual herbicide effectiveness
Both foliate and basal bark sprays are approximately 85 percent effective, and will require some level of maintenance to kill resprouts. As alien tree density increases and access becomes more difficult, this method becomes both less effective, because of limiting abilities to spray herbicide onto all exposed basal bark or leaf surfaces.
Manual herbicide applicability
Manual herbicide application of Tamarix is appropriate whenever alien infestations are light. This technique is especially useful when these levels of infestations occur in difficult access areas including canyons ,washes, irrigation ditches and along steep embankments.
Manual herbicide pros and cons
This manual herbicide technique is effective and relatively cheap in: (1) Areas of light infestations and (2) Inaccessible and remote areas. Furthermore, there is no need to remove dead materials or revegetate because native community plant succession will most likely occur naturally in these areas.
Aerial herbicide application
Herbicide can be applied from helicopter or fixed wing aircraft. Aerial herbicide application has improved with the incorporation of precision agricultural techniques, the use of GPS coordinates and flight plans structured to deliver the herbicide to only the desired locations, and improvements to nozzle design that have enhanced the control of herbicide drift. Imazapyr is a prototypical herbicide used in aerial application.
Aerial herbicide costs
Costs of control alien Tamarisk by spraying vary from $200 to $250 per acre. Because of the high cost associated with helicopter use and mobilization, the minimum acreage to justify these cost rates is approximately 1000 acres. These costs do not include the removal of dead trees or the revegetation of the areas which could add further significant costs.
Aerial herbicide effectiveness
Aerial herbicide application effectively kills approximately 95 percent of Tamarix in most cases. Trees must be left undisturbed for a minimum of two years to allow the herbicide to work properly and effectively kill the trees.
Aerial herbicide applicability
Aerial herbicide application is an appropriate method of Tamarisk control only in those areas that exhibit a high level of infestation which accounts for a large percentage of the infestations in the southwest United States.
Aerial herbicide pros and cons
(1) Aerial herbicide spray is extremely effective in killing tamarisk, but it will also kill desireable vegetation. (2) Aerial spraying is effective in mono-specific stands that are commonly found in the Southwestern USA. (3) Aerial spraying is not recommended in areas that contain a large percentage of native species. (4) Revegetation may or not be required depending on the size of the area sprayed. (5) Some spot herbicide re-application will be necessary. (6) Killing large areas of Tamarix will adversely affect wildlife habitat. (7) Aerial herbicide use delivers large amounts of chemical substances to the environment, including soils, surface and groundwater, which residues may last for considerable times and create biodiversity risks.
Biological control uses natural enemies to control an undesirable organism. Principle methods of biological control being investigated are use of goats and a Chinese leaf beetle as possible biological control agents of Tamarix. Both utilize herbivory control tamarisk by repeatedly defoliating plants over a period several years.
Control by goats
Goats will feed on Tamarix leaves if access to other food is reduced, through processes of fencing. Typically, biological control by goats requires a guard dog, herding dog and goat herder. There are several private goat herds available for biological control throughout the region as of 2010. There is only limited information about the costs or success of biological control by goats.
Goat control costs
Preliminary work in western Kansas suggests $0.50 per (goat) head per day and three years of usage. For a moderately infested area, overall costs would be approximately $1,100 per acre.
goat control effectiveness
It is too early to provide exact information about the effectiveness, applicability, and pros and cons of biological control by goats. As of 2010, a large project is underway on the Rio Grande River in New Mexico to provide more insignt on control effectiveness.
Control by insects
Investigations into biological control of Tamarisk using insects began in the 1980s using Diorhabda elongata deserticola, a beetle from Fukang, in Xianjiang Province of northwest China. Both the adults and the larvae of this beetle feed on the Tamarisk foliage, damaging it directly or indirectly causing foliage to dry out. Research has been conducted on the insect’s life cycle, reproductive rate, and dispersal. Other research has examined the impacts of beetle on Tamarix, surrounding vegetation, and on wildlife.
Insect control costs
Using Diorhabda elongata as a control technique could reduce the costs of Tamarix control to a small fraction of any herbicide and/or mechanical approach (less than $10/acre). If Diorhabda elongata are used in a maintenance role, the costs would be similar. Once the trees are killed, skeleton trees still must be removed from moderate to heavy infested areas, and these areas must be revegetated so these costs must also be included.
Insect control effectiveness
Tamarix died after three successive years of defoliation by Diorhabda elongata in tented research cages. It remains unclear whether the insects, will be effective at killing the trees or will be more effective as a control mechanism to prevent further spread. In their fourth year since release in 2001, the beetles have caused extensive defoliation of hundreds of acres of Tamarix at the Colorado, Nevada, and Wyoming research sites. Research is still progressing to determine if the insects will cause mortality to significant numbers of Tamarix trees, as defoliated trees have resprouted each year since the insects were released into the field in 2001.
Insect control applicability
Goats will likely be most effective at controlling young stands of Tamarix that do not contain other native woody vegetation. Biological control by Diorhabda elongata is applicable to all levels of infestation, is not constrained by access conditions, and could be used in both riparian zones and upland zones.
Insect control pros and cons
(1) Killing Tamarix may harm the endangered southwestern willow flycatcher. (2) Extensive research on Diorhabda elongata suggests a lack threat to other plant species; however there is always risk with introducing an alien species). (3) Using goats may be an effective maintenance technique after an initial kill from introduced insects. (4) Dead Tamarisk trees must be removed, and the area must be revegetated.
Removal of dead Tamarix
The removal of dead Tamarix plants is important after mechanical root crown removal, bio-control, or aerial herbicide control has been successful, becasue it reduces the potential for wildfires and facilitates revegetation. Dead trees can be removed by fire or by mechanical mulching equipment that transforms dead woody biomass into mulch.
Dead Tamarisk removal costs
The costs of controlled burns are approximately $50 to $150 per acre, and the costs of mechanical mulching range from $200/acre in lightly infested areas to $200/acre in moderately infested areas.
Revegetation after Tamarisk kills
Revegetation is critical to successful long-term control of Tamarix. Costs of revegetation can include labor, seeds, plant materials, fertilizer, equipment rental, weed control and water. For narrow widths less than 50 feet, natural revegetation should to occur but may require minor to moderate costs because of soil disturbance and weed control. For broader widths (greater than 50 feet) costs will shift to the higher side because less native plant/seed will be available for reintroduction/succession. Costs range from zero when natural revegetation occurs, to $500-$1500/ acre when major revegetation is required.
Costs and benefits of Tamarix eradication
The costs of a regionwide Tamarix eradication program include the costs of evaluation, control, revegetation, and monitoring. The costs of a 20 year eradication and revegetation program was estimated at $3,006/ac ($7,428/ha). However, the estimated long-term benefits outweighed costs by $3,312 to $6,975/ac ($8,184 to $17,235/ha) or from $3.8 billion to $11.2 billion across the entire region. The resulting benefit:cost ratio for the eradication program ranges from 2.1:1 to 3.3:1.
References and further reading
- Bernard R.Baum. 1978. The Genus Tamarix, The Israel Academy of Science and Humanities
- J.L.Tracy and T.O.Robbins. 2009. Taxonomic revision and biogeography of the Tamarix-feeding Diorhabda elongata (Brullé, 1832) species group (Coleoptera: Chrysomelidae: Galerucinae: Galerucini) and analysis of their potential in biological control of Tamarisk, Zootaxa, 2101: 1-152.
Options for Non-Native Phreatophyte Control Tamarisk Coalition