Perennial pepperweed (Lepidium latifolium), also referred to as tall whitetop, is native to eastern Europe and Asia and was accidentally introduced into North America early in the 20th century. A member of the mustard family, this weed has spread to portions of all western states and adjacent Canada.
In the Intermountain Area, perennial pepperweed first became established in riparian and wetland areas. It spread to native hay meadows and alfalfa (Medicago saliva) fields. Native hay meadows located on flood plains are particularly susceptible to perennial pepperweed invasion (Figure 1). The weed is highly adapted, but not restricted to growth on salt affected soils and soils with sodium hazards.
Perennial pepperweed plants can produce large quantities of viable seeds that can reach 16 billion seeds/acre. The plants have an extensive creeping root system with the ability of very small fragments of these roots producing new plants. This makes mechanical control by tillage virtually impossible as following mechanical tillage can result in a significant increase in perennial pepperweed density and cover.
The invasion of native hay meadows by perennial pepperweed is a threat to the range livestock industry of the Intermountain Area because these meadows are a critical portion of the forage base for both hay production and grazing of crop aftermath. Perennial pepperweed can be partially suppressed by grazing, but if grazing is deferred for hay production the plants become quite woody and the resulting hay is of very poor quality. If herbicidal weed control is practiced on these meadows with near mono-cultures of perennial pepperweed, spontaneous regeneration of the meadow is very slow and re- invasion of perennialweed will occur. This makes seeding of a perennial forage species necessary.
The choice of revegetation for these seasonally dry meadows with salt affected soils is limited, with tall wheatgrass (Elytrigia elongata) being the most widely used species. The standard herbicide for control of perennial pepperweed has traditionally been 2,4-D [(2,4-dichorophenoxy) acetic acid]. More recently, herbicides of the sulfonylurea family, especially chlorsulfuron {2-chloro-N-[(4- methoxy-6-methyl-1, 3, 5-triazin, 2-yl) – aminocarbony] benzenenesulfonamide } have been found to be very effective for the control of perennial pepperweed. Both 2,4-D and chlorsulfuron can be applied at appropriate rates and kill perennial pepperweed in established stands of meadow species and not injure the perennial grasses and grass-like species.
To give the perennial grass plants a chance to biologically suppress perennial pepperweed, repeated herbicide applications are necessary over the seedling stands. This obviously requires the use of a selective herbicide.
We tested mechanical (discing), biological (goats) and chemical (herbicides), as well as a combination of these treatments to control perennial pepperweed infestations (Figure 2). In all we had a combination of 52 treatments. Here we report on what we consider the more important portions.
Discing, conducted in late May and early June, initially reduced perennial pepperweed cover from 95% cover down to 5% cover, but by the end of summer (October) perennial pepperweed cover was up to 30% and reached 100% the following July.
Discing in early May followed by the application of herbicides [2-4D 2.2 kg/ha (2 lbs/ac), or Chlorsulfuron (Telar®XP) 0.11 kg/ha (0.10 lbs/ac)] initially appeared to be very effective as perennial pepperweed leaves looked very necrotic. Even though perennial pepperweed failed to seed due to the combination of discing and spraying, perennial pepperweed became productive and vigorous by the following July and cover increased to more than 20%. There were no significant differences in cover between the combination of discing and herbicide application and that of herbicide application by itself.
We also investigated the control of perennial pepperweed by grazing Spanish goats (Figure 3 above). Eight 0.1 hectare (0.25 acre) enclosures were constructed in which four were grazed and combined with herbicide and seeding treatments while the remaining 4 enclosures were grazed and seeded. Heavy grazing of perennial pepperweed decreased forage yield by 78%, yet did not decrease the number of perennial pepperweed plants in the plots. By the time the goats completed the grazing on all 4 enclosures, enclosure 1 had regrown enough to place the 25 goats back in it with plenty of available forage.
Grazing perennial pepperweed as a control method was not successful as the sprouting perennial grass seedlings could not compete with the dense creeping rooted perennial pepperweed. The control of perennial pepperweed using goats and herbicide did not significantly reduce perennial pepperweed when compared to herbicide treatments by themselves.
Realizing that the increase in perennial pepperweed cover in such a short period of time was a major problem, we tested a variety of plant species in an attempt to suppress perennial pepperweed. After testing a variety of plant species such as purple needlegrass (Stipa pulchra), crested wheatgrass (Agropyron cristatum), Great Basin wildrye (Leymus cinereus), and intermediate wheatgrass (Thinopyrum intermedium) we chose tall wheatgrass (Elytrigia elongata) which performed better in these salt-affected soils.
We followed up the May/June (one-half bloom stage) herbicide application (same herbicides and rates) with the seeding of tall wheatgrass at [10.3 kg/ha (9 lb/ac) rate]. The objective was to reduce perennial pepperweed with the herbicide application and then suppress perennial pepperweed with a long-lived perennial grass.
The following June, well after tall wheatgrass seedlings had emerged and developed 3+ leaves, we applied 2,4-D at 1.1 kg/ha (1 lb/ac) rate as to negatively affect perennial pepperweed, but not injure the tall wheatgrass seedlings with this broadleaved selective herbicide.
Tall wheatgrass seedlings in the Telar plots did not fair very well as the herbicide residue from Telar was still strong enough to kill the tall wheatgrass seedlings, therefore resulting in less than 0.5 tall wheatgrass plant/m² (< 0.04/ft²). The 2,4-D treated plots resulted in more than 4 tall wheatgrass plants/m² (> 0.38/ft²) (Figure 4).
Following a couple of years of spot treating perennial pepperweed plants, tall wheatgrass had vigorously grown and taken over the site which resulted in excellent suppression of perennial pepperweed which allowed for the site to be converted back into production agriculture (Figure 5).
The cost of weed control can often be very expensive and more than what the land provides in value for specific needs such as grazing pastures. In this rehabilitation practice the cost averaged $40.30/ha ($99.50/ac) with the less accessible pastures costing roughly 20% more. These large, replicated experimental treatments were repeated for 3 consecutive years with highly consistent results.
Diehard defenders of chlorsulfuron point out that we have done no chemical analysis of the soil to prove that residues of the herbicide are responsible for the failure to establish tall wheatgrass seedlings. The consistency of seedling failure in plots treated with chlorsulfuron while side by side plots treated with 2 applications of 2,4-D have excellent stands of the perennial grass is very compelling evidence for persistent residues.
The failure of annual weeds to invade areas treated with chlorsulfuron for 3 years is additional evidence for the existence of residues. The few perennial pepperweed plants that escape initial applications of chlorsulfuron are a cause of concern. They increased in area by extending creeping roots and putting up additional stems even though very few perennial grass seedlings could be established on the plots. The plants are not necessarily resistant to chlorsulfuron. They may be accidents of faulty herbicidal distribution in the initial application and their depth of rooting allows them to spread below soil horizons with herbicide residues.
The existence, persistence, and spread of these plants creates an additional hazard with the use of chlorsulfuron to control perennial pepperweed. The apparent persistence of chlorsulfuron residues that limit perennial grass establishment is probably heightened by the high pH of the salt affected soils on the study site. Most native hay meadows where perennial pepperweed is invading have similar soils.
By Charlie D. Clements and Dan N. Harmon