Journal of the NACAA
ISSN 2158-9429
Volume 3, Issue 1 - July, 2010

Editor:

Biology and Management of Perennial Pepperweed in a Riverine Environment

Reid.C.R., Extension Agent, Utah State University Extension

ABSTRACT

Perennial pepperweed (Lepidium latifolium L.) is native to southern Europe and western Asia and has become dominant on some wetlands in the intermountain west. It has been declared a noxious weed in several western states. Original importations were thought to be in contaminated sugarbeet (Beta vulgaris L.) seed. Spread of this weed is usually by water carrying seed from upstream areas. Once established, perennial pepperweed rapidly spreads by rhizomes especially in moist soils crowding out native and other desirable vegetation. Mosquito control efforts have also been impacted by this weed. This replicated, repeated study evaluated herbicide control of perennial pepperweed in two locations in a Riverine environment. Low rates of Escort (1/2 to 3/4 oz. per acre) applied during the bud stage produced excellent control the first year of application and maintained good control the following year.

 

Biology and Management of Perennial Pepperweed in a Riverine Environment
 
 
Abstract
 
Perennial pepperweed (Lepidium latifolium L.) is native to southern Europe and western Asia and has become dominant on some wetlands in the intermountain west. It has been declared a noxious weed in several western states. Original importations were thought to be in contaminated sugarbeet (Beta vulgaris L.) seed. Spread of this weed is usually by water carrying seed from upstream areas. Once established, perennial pepperweed rapidly spreads by rhizomes especially in moist soils crowding out native and other desirable vegetation. Mosquito control efforts have also been impacted by this weed. This replicated, repeated study evaluated herbicide control of perennial pepperweed in two locations in a Riverine environment. Low rates of Escort (1/2 to 3/4 oz. per acre) applied during the bud stage produced excellent control the first year of application and maintained good control the following year.
 
Introduction
 
Perennial pepperweed (Lepidium latifolium L.) is native to southern Europe and western Asia and has become dominant on some wetlands in the intermountain west. It has been declared a noxious weed in several western states. Original importations were thought to be in contaminated sugarbeet (Beta vulgaris L.) seed. Spread of this weed is usually by water carrying seed from upstream areas. Once established, Perennial pepperweed rapidly spreads by rhizomes especially in moist soils. The plant can grow to heights greater than 1 m and readily suppresses surrounding herbaceous vegetation. Research on management of perennial pepperweed is limited, with most of the work being done on cropland.
The first report of perennial pepperweed in Uintah county known by the author was at the Ouray National Wildlife Refuge in 1972 (Neil Folks, personnel communication). It was of little more than botanical interest at that time. In 1983 extremely high flows on the Green River (50,000 cubic feet per second near Jensen, Utah) caused extensive flooding of low areas along the river. After these high flows landowners started reporting perennial pepperweed in pastures and hay fields. In 1989 the county weed supervisor initiated a spray program for perennial pepperweed. Several landowners cooperated with the county to spray approximately 600 acres with 2,4-D amine applied by airplane at the rate of 2 quarts per acre. Perennial pepperweed was in full bloom. Results were disappointing with estimates of control in the 10 to 20 % range. After this, landowners were very skeptical about control of perennial pepperweed.
Perennial pepperweed is linked to other problems in Uintah County. Land areas along the Green River include some of the most productive mosquito habitat in North America. This habitat is capable of producing up to 10 million mosquitoes per acre per brood. This also creates considerable human health concerns as surveillance sentinel flocks of chickens in Uintah County frequently seroconvert for mosquito-borne Western Equine encephalitis, Saint Louis encephalitis and more recently West Nile virus. Perennial pepperweed directly interferes with mosquito control in the area because of changes in the vegetation canopy in the invaded areas. Vegetation in these areas would normally consist of Salt grass (Distichlis spicata L.), Alkali sacaton (Sporobolus airoides Torr.) and Poverty sumpweed (Iva axillaris Purch.), all of which are low growing plants. When these areas become dominated by perennial pepperweed the canopy height and density make effective application of insecticides difficult for mosquito control. Repeated applications of organophosphate insecticides for the past 20 years to control mosquitoes have resulted in mosquito populations with a developed resistance to these insecticides. Newly developed insecticides such as Bti (Bacillus thuringiensis israelensis) require much shorter application intervals and must be applied directly to the water surface to control the aquatic larvae. The dense canopy of perennial pepperweed severely impacts these applications by intercepting the chemical before it contacts the water. (Steven V. Romney, personal communication).
The problem of perennial pepperweed along the Green River has been further intensified by the Recovery Program for Endangered Fishes of the Upper Colorado. There are currently 4 species of fish listed as endangered in the Green River. The Colorado pikeminnow (Ptychocheilus lucis), the Humpback chub (Gila cypha), the Bonytail chub (Gila elegans) and the Razorback sucker (Xyrauchen texanus). The last three species mentioned are thought to require backwaters for spawning and young fish grow-out areas. In order to increase the number and size of these areas the U.S. Fish and Wildlife Service is proposing to flood areas adjacent to the river channel on a regular basis. Flooding these areas produces large number of mosquitoes and appears to further the spread of perennial pepperweed.
These factors and poor results of past herbicide treatments lead to the initiation of a study by the author, to evaluate control methods for perennial pepperweed. On riverine areas, few control methods have been developed, though herbicides have shown the greatest potential. A review of the available literature showed the only treatment reported to be effective was 2,4-D amine (2,4 diclorophenoxy acetic acid) at a rate of 2 quarts per acre applied twice a year for 3 to 6 years and then repeated when necessary (Hackett and Post 1986). However, this strategy has not provided long term control on riverine systems in Utah. The purpose of this study was to evaluate the efficacy of herbicides on perennial pepperweed populations in wetlands in two locations in Uintah County, Utah.
 
MATERIALS AND METHODS
 
Ten treatments were evaluated including nine herbicide treatments and a control. Herbicide treatments included: Escort (metsulfuron) at three rates (1/3, 1/2, 3/4 oz. product/acre), 2,4-D amine (2 qts. product/acre), 2,4-D amine (2 qts. product/acre) + plus three rates of Escort (1/3, 1/2, 3/4 oz. product/acre) and two rates of Landmaster (2,4-D + glyphosate) (26 oz. product/acre, 54 oz. product/acre). A non-ionic surfactant (X-77) was added to treatments containing Escort. Treatments were applied to perennial pepperweed during the bud stage of development on May 25, 1994. Fall treatments using the same herbicides were applied but showed minimal effect so the results will not be discussed further. Herbicides were applied using a CO2 backpack sprayer with a hand held 6-nozzle boom (20-inch nozzle spacing), delivering 12.4 gallons per acre at 35 psi.  Herbicides, application rates and timing were determined from a pilot study. Mechanical top removal was included in the pilot study; however, since it provided no control it was not included in this study.
This study was conducted on two sites and arranged in a randomized complete block design, with three replications. Plots were 10 x 30 feet. The first site was an Utaline loam soil on an old terrace of the Green river. This is a desert loam (shadscale range site) but is now an artificial wetland because of adjacent irrigated cropland. The other site was on a wet meadow adjacent to the Green river that is flooded periodically in the spring and receives agricultural tail-water. The soil on this site is a Pogoneab clay loam and described as a wet saline stream bank (coyote willow range site). Plots were evaluated by visual inspection on September 7, 1994. Two independent observers recorded estimates of percent control, these estimates were then averaged. In addition, biomass was estimated by clipping two 2.7 ft2 frames on September 14, 1994 in each plot. Biomass was recorded using air dry weights. ANOVA was used for data analysis. Treatment means were separated using Duncan's MRT.
 
RESULTS
 
Perennial pepperweed biomass reduction and visual control ratings were significantly (P<.05) different between the Utaline loam and Pogoneab clay loam sites. Biomass values were not significantly different on the Utaline loam site and ranged from 14 to 641 pounds per acre for the herbicide treatments (Table 1). However visual control ratings for herbicide treatments during the first growing season after application were significantly (P<.05) different from each other on this site. Biomass and visual control ratings were significantly different (P<.05) on the Pogoneab clay loam site. Biomass weights ranged from 71 to 1282 pounds per acre for herbicide treatments.
Escort at 3/4 oz. per acre plus 1 qt of 2,4-D showed the greatest reduction in biomass on site one. Escort at 1/3 oz. per acre was most effective in controlling pepperweed on site two. The low rate of Landmaster was least effective on both sites. Biomass production on plots treated with the high rate of Landmaster was not different from the Escort treatments. The Utaline loam site had a lower soil water holding capacity and was subjected to drought stress associated with irrigation on the adjacent farm ground not being started until late summer. This added stress appeared to increase the efficacy of most herbicide treatments. Higher rates of Escort and Escort combined with 2,4-D did not significantly (P<.05) increase control based on first year evaluations. Data from this study and a pilot study (Table 2) indicate that low rates of Escort are the most economical way to treat perennial pepperweed. While higher rates of Landmaster gave equal control, the Glyphosphate in the product had a negative effect on desirable grasses in the plots. Also, data from the pilot study indicated that treatments with 2,4-D or Glyphosphate (Rodeo) showed less control during the second growing season.
 
SUMMARY
 
Low rates of Escort (1/2 to 3/4 oz. per acre) applied during the bud stage will give excellent control the first year of application and good control the following year. Further research is needed looking at longer term control of perennial pepperweed including competition with desirable species.
 
LITERATURE CITED
 
Folks N. 1996. Personal communication, Former Superintendent, Browns Park Waterfowl Management Area. Utah Division of Wildlife Resources.
 
Hackett, I and D. Post. 1986. Perennial pepperweed control in Nevada
University of Nevada, Reno, Cooperative Extension 86-11 4pp.
 
Romney S. 1996. Personal communication, Former Director, Uintah County Mosquito Abatement District.
 
 
 
Table 1. Percent Visual Control and Biomass of Perennial Pepperweed at Two Locations
 
Treat.
Herbicide
 
Rate
% Visual Control
Biomass
(lbs/acre)
 
 
 
Site 1
Site 2
Site 1
Site 2
1
Escort
1/3 oz. per acre
99 d
82 d
71 a
71 a
2
Escort
1/2 oz. per acre
90 bc
91 e
142a
107 a
3
Escort
3/4 oz. per acre
93 c
87 e
178 a
427 ab
4
Escort+ 2,4-D (amine)
1/3 oz. + 1qt. per acre
93 c
85 e
14 a
320 ab
5
Escort+ 2,4-D (amine)
1/2 oz. + 1qt. per acre
92 c
91 e
249 a
427 ab
6
Escort+ 2,4-D (amine)
3/4 oz. + 1qt. per acre
98 d
89 e
28 a
356 ab
7
2,4-D (amine)
1qt. per acre
95 cd
64 c
142 a
676 b
8
Landmaster
26 oz. per acre
85 b
48 b
641 a
1282 c
9
Landmaster
54 oz. per acre
92 c
73 cd
107 a
320 a
10
Check (untreated)
 
0 a
0 a
2777 b
2065 d
 
 
All rates listed as amount of product per acre
Site 1 = Utaline Loam
Site 2 = Pogoneb Clay Loam
 
 
Table 2. Results by Year of Selected Pilot Study Treatments
 
 
Treat.
Herbicide
Rate
% Visual Control
 
 
 
Year 1
Year 2
1
Escort
3/4 oz. per acre
97 c
76 c
2
Escort + 2,4-D (amine)
1/2 + 1qt. per acre
99 c
78 c
3
2,4-D (amine)
2 qts. per acre
95 c
70 c
4
Rodeo
2 qts. per acre
75 b
13 b
5
Check (untreated)
 
0 a
0 a
 
All rates listed as amount of product per acre