Brad W. Schultz, Extension Educator and Associate Professor, University of Nevada Cooperative Extension

ABSTRACT

Perennial pepperweed (Lepidium latifolium L.) is a deep-rooted, perennial herbaceous weed that spreads from both seed and creeping roots capable of sprouting new shoots. Previous work demonstrated that perennial pepperweed seedlings are much easier to control than mature plants that have been established for many years. An unanswered question is: does the efficacy of treatment decline dramatically as perennial pepperweed plants mature from seedlings (which have not yet perrenialized) to one- and two-year old plants with buds on their roots?  The question is important because a two-year window of opportunity for good control of perennial pepperweed provides flexibility for farners and ranchers who often have multiple tasks that compete for time and finanical resources during the growth stages that facilitate optimum treatment. The establishment of perennial pepperweed seedlings across hundreds of acres in 2007 following the drawdown of Chimney Dam Reservoir allowed for aerial treatment of both seedlings and two year-old plants in 2007 and 2009, respectively. Control was nearly equal on both populations, exceeding 99 percent. The results suggest that the window of opportunity for controlling new infestations of perennial pepperweed may last for two years post-emergence.  

INTRODUCTION

Previous work demonstrated that perennial pepperweed (Lepidium latifolium) seedlings are very susceptible to herbicide treatment (Schultz 2011). Seedlings are more susceptible than mature plants for at least two reasons. First they behave as an annual plant prior to reaching the eight-leaf stage of development (Renz 2000), after which they develop buds (perennialization) on their roots and root crowns. These buds can produce new shoots and/or plants if the canopy of the original plant is removed. Once perennialization occurs, the control of perennial pepperweed requires eliminating all of the buds, otherwise regrowth occurs. Second, seedlings have not developed overhead structures (flowers, dead stems from previous years) that can intercept herbicides and reduce chemical contact with living tissue. When less herbicide reaches living tissue the potential amount of herbicide uptake and translocation to sites of action (meristematic tissue) declines. When tall vegetation from other species overtopss perennial pepperweed plants, it also can intercept an applied herbicide before it reaches the target plants.

Mature stands of perennial pepperweed are difficult to control because they have large root systems with many perennial buds and the roots store large amounts of energy (Renz et al. 1997, Young et al. 1997, Wotring et al. 1997). After an herbicide treatment, any surviving buds have access to ample amounts of stored energy in the root, which they can use to initiate new growth and extend a shoot to the soil surface. These shoots can develop into an entire new plant. Plant architecture and translocation patterns, however, make it difficult to deliver an herbicide to the optimum location on the lower leaves. Perennial pepperweed plants and stands have high stem densities, an erect structure (1.5 to 6.5 ft tall) and a high initial leaf area (Renz and Blank 2004). At flowering, perennial pepperweed loses its apical dominance and develops secondary branching (Renz and Blank 2004). These branches are topped by a large dense flowering system that resides almost entirely above the leaves. The upper leaves tend to send most of their carbohydrate production to the flowers and the lower leaves to the roots (Renz and DiTomaso 2004). Maximum carbohydrate movement to the roots occurs between flowering and seed production, which is when the flowers and upper leaves are most likely to intercept an applied herbicide (Renz et al. 1997, Renz and DiTomaso 2004).

The ability to eliminate virtually all perennial pepperweed seedlings with a single herbicide treatment (Schultz 2011) suggests that early detection and rapid response should be a part of weed management for this species. Land owners and managers, however, often have multiple tasks that simultaneously compete with one-another for time and resources. This conflict can prevent them from conducting annual weed inventories. An important management question becomes, if seedlings go untreated for one or two years, what level of potential control is lost? If plants up to two-years old are as susceptible to herbicide control as seedlings, then landowners and managers have a longer period; hence, increased flexibility for implementation of their weed management programs. They may be able to effectively skip a year of weed control without suffering appreciable ecological or economic harm.  

In 2007, the rapid drawdown of Chimney Dam Reservoir in North-Central Nevada allowed a large population of perennial pepperweed seedlings to emerge on hundreds of acres of exposed lakebed. The drawdown lasted until 2011, which permitted control attempts on adjacent populations of seedlings (2007) and two year-old plants (in 2009). The environmental conditions of the area are described in Schultz (2011).

 METHODS:

In 2007, perennial pepperweed seed germinated and seedlings emerged in late April and early May, and were up to six weeks old at the time of treatment in early June. Most seedlings were 4-6 inches tall, had tap roots from about 6 inches to over 10 inches deep, and had developed 4 to 8 leaves per plant (see Schultz 2011). The two-year old pepperweed population was part of the 2007cohort, and was treated in its third growing season. Total root depth and mass were not quantified but the root systems penetrated the soil deeper and had more lateral spread than the seedlings. Excavation of one-yer old plants in 2008 found that many had developed lateral roots up to several feet long, and these often produced new photosynthetic shoots. This lateral root development added both mass (stored energy) and buds to the bud bank (see Klimešová and Klimeš (2007) for discussion of bud bank concept), with each bud capable of producing new shoots or plants. At the time of treatment, the two year-old plants were at the late vegetative to early bud stage of growth with multiple elongated stems. There were often 20 or more leaves per plant.

Both populations were treated only once: June 9, 2007 (seedlings) and May 29, 2009 (two-year old plants). Both populations were treated with an aerial application of Cimarron^®Max (metsulfuron methyl, dicamba and 2,4-D) at the labeled rate for perennial pepperweed. This equates to an active ingredient rate per acre of 0.60 oz metsulfuron methyl; 6.6 oz of dimethylamine salt of dicamba; and 18.9 oz of 2,4-D. The mix included a non-ionic surfactant at the rate of 0.25% v/v (1 quart per 100 gallons of spray mix). The herbicide application to the seedling population occurred when the winds were calm, the skies clear and temperature was about 73°F. No rain occurred for at least two weeks. High temperatures ranged from 83°F on the day of application to 97°F on June 17^th, with low temperatures typically in the upper 30’s to mid 40’s. Tthe frost-free period lasted until September 11, 2007. For the two year-old population, the high temperature on the day of treatment was near 90°F, but much cooler weather prevailed the next two weeks. A frost did not occur until mid-September 2009. Winds were light at the time of herbicide application and no rainfall occurred for several days.

Plant counts occurred in 3 x 100 ft belt transects, at six locations in each population. For the seedling population, data collection occurred in 2008 and 2009. For the two-year old population, post-treatment data collection occurred only in 2010, because the reservoir nearly filled in 2011. Means were calculated for each year of data collection and were compared with either the Kruskal-Wallis nonparametric analysis of variance (seedling), or the Wilcoxon Rank Sum Test (two-year old: Statistix 9 Analytical Software 2008).

RESULTS

Seedlings were visually affected by the herbicide application within three days of treatment (see Schultz 2011 for photos of their appearance). The one- and two-year post-treatment response of the seedling population was nearly identical, with only one additional plant counted the second year after treatment (Table 1). Control across both years was nearly 100 percent (Figures 1a and 1b). The Kruskal-Wallis AOV test found a high probability that the differences among the means were due to a treatment effect (p≤0.02). The carpet of annual forbs present the first year after treatment was largely replaced with foxtail barley (Hordeum jubatum L.).

There was almost complete control of the two-year old perennial pepperweed plants with the single treatment of Cimarron^®Max (Table 1, Figures 2a and 2b). Four of the six transects had no plants present one-year post-treatment. Most of the plants counted were in a single transect but control on that transect exceeded 98 percent. The Wilcoxon Rank Sum Test indicated the differences in the pre- and post-treatment means was due to the treatment (p≤0.002). As in 2007, the perennial pepperweed plants were largely replaced with foxtail barley.  

Table 1. Perennial pepperweed density in seedling and two year-old populations at the time of treatment and one or two years post-treatment. Mean values within a population, followed by different letters indicate the values were different due to the treatment applied (p≤0.05).

	Seedling population				Two year-old population
	Total density in belt transect					Total density in belt transect
Transect	20071	2008	2009		Transect	20091	2010
1	127	0	0		1	121	0
2	108	0	0		2	173	0
3	162	0	1		3	104	0
4	275	1	1		4	310	5
5	187	0	0		5	194	1
6	243	1	1		6	231	0
Mean	185a	0.33b	0.50b		Mean	188a	1b

^1.Density at the time of herbicide application 

 

   1a    1b 

Figures 1a and 1b. Seedling perennial pepperweed plants (brownish leaves) 13 days after treatment in 2007 (1a) and the same site three years later in 2010 (1b). The predominant plant is foxtail barley, a short-lived perennial grass.

                                                                                                                                  

          2a   2b 

Figures 2a and 2b.  The two-year old perennial pepperweed population 10 days after treatment (2a) and 13 months (2b) after treatment on July 7, 2010. 

 

DISCUSSION

Schultz (2011) clearly showed that perennial pepperweed seedlings are much easier to control than is a mature stand. This work shows that a single application of Cimarron^®Max herbicide, under the bio-environmental conditions for which it was applied, is as effective on two-year old perennial pepperweed plants, as for seedlings. The outcome of any herbicide treatment, however, reflects the site-specific conditions at the time of the application. An understanding of this context is necessary for successful transfer of the results from one treatment site to other infestations, where the environmental conditions may be different.

At this study site, neither the seedling population nor the two-year old population had an overstory of non-target vegetation (live or dead) above the target plants. The two-year old population had only a few erect dead stems from the previous growing season; thus, little if any of the aerially applied herbicide was intercepted by non-target vegetation. This condition may have occurred because the area was used for winter livestock grazing and most of the previous year’s flowering stems were trampled into the soil. The general lack of any vegetative material above the target plants probably increased treatment success because it facilitated good contact between the herbicide and the photosynthetically active leaves of the target plants.

This work suggests that a landowner may have a two-year (three growing season) window of opportunity for treating new infestations of perennial pepperweed. The ability to obtain nearly equal control of perennial pepperweed plants at both the two-year old and seedling age-classes has important management implications for landowners and managers. A longer potential control period adds temporal flexibility to their farming or ranching operation. As long as the operator can commit to treating the weeds within two years of their emergence the probability exists for excellent control of the weed. This has important implications for prioritizing management actions during the establishment year, when the number of activities that should occur may exceed the time and resources available for their implementation, and one or more activities must be postponed. Any treatment that occurs one or two years after emergence, however, must apply the majority of the herbicide to actively photosynthetic leaves, and preferably the leaves located lowest on the stems (Renz and DiTomaso 2004).

Delayed herbicide treatment of perennial pepperweed seedlings for up to two years may be successful, but also carries risks One-year old pepperweed plants will produce seed; thus, contribute toward a seedbank, which can last for several years (Renz 2005). A more important risk is that the growing conditions for other perennial pepperweed infestations (in other areas) may differ substantially from those in this study. Two-year old plants on other sites may respond quite differently than the two-year old plants in this study. Many perennial pepperweed infestations inhabit soils that are much less alkaline than in this study area, and/or have growing seasons that are much longer and warmer. Both conditions could result in two-year old plants that are much larger than those in this study. Larger plants would have longer and deeper roots. Longer root systems would result in more total buds on the root system, and many buds that are located further from the point of herbicide uptake. The need to eliminate more buds, and buds further from the point of chemical uptake, could have a negative feedback on herbicide efficacy, particularly if uptake and/or phloem transport are less than optimum. The same amount of herbicide applied to two populations, which differ in plant size and leaf area (but not age), may have differential rates of effectiveness. This results, in part, due to the potential dilution of the number of molecules of the active ingredient across more biomass that occurs in larger plants. It is possible that larger plants with many buds may not have sufficient uptake and transport of enough molecules of the active ingredient to reach all of the potential sites of action. The buds on large root systems which survive an herbicide treatment probably would have more stored energy (soluble carbohydrates) available for regrowth, compared to the buds on small root systems. This has important implications for potential regrowth from buds on the deep roots associated with larger/older plants. To produce a new plant (leaf) capable of photosynthesis, regrowth from a bud on a deep root will need more stored energy to produce a shoot that can reach the surface and produce a new leaf. Two-year old plants that occupy sites under conditions that promote the rapid development of large root systems may be harder to control than the plants treated in this study. All weed control efforts must consider the bio-physical context of their specific situation and not rely completely upon the results related to time or dose parameters in other studies. 

Literature Cited

Analytical Software 2008. Statistix 9.Tallahassee, FL. 454 p.

Klimešová, Jitka, and Leoš Klimeš. 2007. Bud banks and their role in vegetative regeneration – A literature review and proposal for simple classification and assessment. Perspectives in Plant Ecology, Evolution and Systematics. 8:115-129.

Renz, M.J. 2000. Element stewardship abstract for Lepidium latifolium L. The Nature Conservancy.  http://www.imapinvasives.org/GIST/ESA/esapages/documnts/lepilat.pdf. Accessed March 7, 2012.

Renz, M.J. 2005. Perennial pepperweed. Lepidium latifolium. Pages 91-98. In: Invasive plants of range and wildlands and their environmental, economic and societal impacts. C.L. Duncan and J.K. Clark (eds.). Weed Science Society of America. Lawrence, KS. 222 p.

Renz, M.J. and R.R. Blank. 2004. Influence of perennial pepperweed (Lepidium latifolium) biology and plant-soil relationships on management and restoration. Weed Technology. 18:1359-1363.

Renz, M.J. and J.M. DiTomaso. 2004. Mechanism for the enhance effect of mowing followed by glyphosate application to resprouts of perennial pepperweed (Lepidium latifolium). Weed Science. 52:14-23.

Renz, M.J., J.M. DiTomaso, and J. Schmierer. 1997. Above and belowground distribution of perennial pepperweed biomass and utilization of mowing to maximize herbicide effectiveness. Proceedings California Weed Science Society 49:175.

Schultz. B. W. 2011. Differential herbicide effectiveness on adjacent populations of young (seedling) and mature perennial pepperweed (Lepidium latifolium). Journal of the NACAA. 4:2. Available at: http://www.nacaa.com/journal/index.php?jid=103

Wotring, S O., D. Palmquist, and J. Young. 1997. Perennial pepperweed (Lepidium latifolium) rooting characteristics. Pages 14-14. In: T. Svejcar (ed). Management of perennial pepperweed (tall whitetop). Special Report 972, USDA Agricultural Research Service and Agricultural Experiment Station. Corvalis, OR: Oregon State University.

Young, J.A., D.E. Palmquist, and S.O. Wotring. 1997. The invasive nature of Lepidiumlatifolium: a review. Pages 59-68 in J.H. Brock, M. Wade, P. Pysek and D. Green, eds. Plant invasions: studies from North America and Europe. Leiden, The Netherlands: Backhuys.

Young, J. A., C.D. Clements, and R.R. Blank. 1998. The ecology and control of perennial pepperweed. Weed Technology 12:402-405.  

 

 

 

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