Research Update: Control of Perennial Invasive Weeds Requires Repeated Herbicide Applications

Field tour of purple loosestrife management study
Figure 1. Nebraska research showed the need for multiple years of treatment to achieve complete control of purple loosestrife. Choosing the right herbicide for the job and practicing patience and persistance were key to success. (Photos by Stevan Knezevic)

Research Update: Control of Perennial Invasive Weeds Requires Repeated Herbicide Applications June 4, 2018

It is well known that herbicides can manage perennial invasive weed species; however, a single application of most herbicides provides only short-term efficacy (one- or two-year suppression). These perennials are not easily managed with one application because they propagate by multiple means, including seeds and perennial roots (rhizomes, stolones and crowns). The secondary buds of these plants can sprout new shoots, even after initial herbicide damage.

To examine what's needed to achieve complete control, we conducted a 10-year herbicide efficacy study with a pilot species, purple loosestrife (Lythrum salicaria L.). Purple loosestrife is an invasive weed introduced from Eurasia to North America in the early 1880s. It has now become widespread in the United States, reducing the biodiversity of riparian areas across Nebraska and elsewhere in the US. In 2001 purple loosestrife was declared a noxious species in Nebraska as it poses a serious threat to the economic, social, and aesthetic well-being of the state’s landscape.

We have evaluated the effectiveness of 14 herbicide treatments for purple loosestrife control over a 10-year period. Studies were conducted at four sites:

  • Along the Missouri River near Newcastle in northeast Nebraska in a 10-year-old (according to the landowner);
  • Buffalo site, established along the Platte River near Kearney in three-year-old stands;
  • Brown site, established along the Niobrara River north of Johnstown in five-year-old stands; and
  • Holt site, along a local private lake north of Atkinson in three-year-old stands.
Purple loosestrife
Figure 2. Purple loosestrife (Lythrum salicaria L.)
Stevan Knezevic in PPE preparing to treat purple loosestrife
Figure 3. Nebraska Extension Weeds Specialist Stevan Knezevic applying one of 14 treatments studied in this plot of purple loosestrife, a perennial weed declared a noxious weed in Nebraska.

Fourteen herbicide treatments were evaluated:

  • Rodeo (glyphosate, 4 and 6 pt/acre);
  • 2,4-D Amine (2,4-D amine, 2.5 and 5 pt/acre);
  • Garlon 3a (triclopyr, 3 and 5 pt/acre);
  • Habitat (imazapyr, 4 and 6 pt/acre);
  • Escort (metsulfuron, 2 and 4 oz/acre);
  • Krenite (fosamine, 3 and 5 gal/acre);
  • Garlon+2,4-D (triclopyr, 3 pt/acre + 2,4-D amine, 2.5 pt/acre); and
  • Escort+2,4-D (metsulfuron, 1 oz/acre + 2,4-D amine, 2.5 pt/acre).

Herbicides were applied when purple loosestrife was blooming, which was approximately the last two weeks of June each year. Herbicide efficacy rating was conducted at 60 and 360 days after treatment (DAT) using a scale of 0-100% (where 0 = no control and 100% = plant death). It is important to note that not all treatments were applied every year. For example, during the initial years of the experiments all herbicide treatments were applied every year, but the decision whether to respray a particular treatment was based on the rating at 360 DAT, hence all treatments with ratings lower than 100% were resprayed.

Purple Loosestrife Control

Procrastination Penalty

Our research showed that age of the purple loosestrife stand was critical to effective control. The younger the stand, the faster the control was achieved. Delaying the start of treatment could extend the total time needed to achieve control from three to as many as nine years.

Our data showed that the age of purple loosestrife stand was critical for effective control. The younger the stand, the faster the control was achieved. For example, the three-year-old stands from Buffalo and Holt required two to three years of consecutive spraying to provide complete control of purple loosestrife (Table 1). At the Holt site, the three-year-old stands were completely controlled by glyphosate, imazapyr, and metsulfuron after two consecutive years of spraying, while 2,4-D, triclopyr and fosamine required three years.

The five-year-old stands at the Brown site required two to five years of consecutive spraying to achieve 100% control, depending on the herbicide (Table 1). For example, the earliest complete control was achieved with metsulfuron sprayed for two consecutive years. When spraying glyphosate, imazapyr, and a mix of 2,4-D dimethylamine plus triclopyr, three consecutive years of spraying were needed to provide complete control. When spraying 2,4-D dimethylamine, triclopyr, and fosamine, it took five consecutive annual applications to achieve complete control.

The 10-year-old stand at the Dixon site required three to nine years of spraying to achieve complete control, depending on the herbicides used (Table 2). For example, imazapyr required three years of consecutive spraying while a mixture of 2,4-D, triclopyr and fosamine required nine years of annual sprayings (Table 2).

As these results indicate, selecting the best herbicide for the job is critical for faster control.  

All treatments at all fours sites were also rated and monitored for an extra three years after last spraying, and all ratings showed 100% control (data not shown in this article).

Practical Implications

Our study showed that repeated applications of the tested herbicides could provide effective control of purple loosestrife over time, but it required persistence and a well-chosen herbicide. The most effective herbicides for purple loosestrife control were glyphosate, imazapyr and metsulfuron, as they provided the fastest control (e.g., within the first two to three years of spraying).

In addition, our assessment of the negative impacts on beneficial vegetation suggested that metsulfuron appears to be the most desirable choice, as it has no detrimental effects on grassy vegetation. Presence of grasses along the edges of waterways is promoted by land managers because grasses provide habitat and food for various bird species (including migratory birds) and feed for grazing animals (deer, livestock).

Table 1. Number of annual herbicide treatments (or years) until complete control was achieved for each age group of purple loosestrife (Lythrum salicaria) stands
TreatmentRateAge of Stand & Site
3 years
(Buffalo)
3 years
(Holt)
5 years
(Brown)
10 years
(Dixon)
Glyphosate 4 pt/ac 2 (2)a 2 (2) 3 (3) 4 (6)
Glyphosate 6 pt/ac 2 (2) 2 (2) 3 (3) 4 (6)
2,4-D dimethylamine 2.5 pt/ac 2 (2) 3 (3) 5 (5) 7 (8)
2,4-D dimethylamine 5 pt/ac 2 (2) 3 (3) 5 (5) 7 (8)
Triclopyr 3 pt/ac 2 (2) 3 (3) 5 (5) 7 (8)
Triclopyr 5 pt/ac 2 (2) 3 (3) 5 (5) 7 (8)
Imazapyr 4 pt/ac 2 (2) 2 (2) 3 (3) 3 (3)
Imazapyr 6 pt/ac 2 (2) 2 (2) 3 (3) 3 (3)
Metsulfuron 2 oz/ac 2 (2) 2 (2) 2 (2) 4 (5)
Metsulfuron 4 oz/ac 2 (2) 2 (2) 2 (2) 4 (5)
Fosamine 3 gal/ac 2 (2) 3 (3) 5 (5) 7 (8)
Fosamine 5 gal/ac 2 (2) 3 (3) 5 (5) 7 (8)
Triclopyr + 2,4-D 3 + 2.5 pt/ac 2 (2) 3 (3) 3 (4) 6 (7)
Metsulfuron + 2,4-D 1 + 2.5 pt/ac 2 (2) 2 (2) 3 (4) 6 (7)
Mean 2 (2) 2 (2) 4 (4) 6 (6)

Table 2. Percent control over time of a 10-year-old purple loosestrife (Lythrum salicaria) stand from selected herbicide treatments at Dixon.
Year 1a Year 2 Year 3 Year 4 Year 5 Year 6 Year 7 Year 8 Year 9
Treatment Rate 60b 360 60 360 60 360 60 360 60 360 60 360 60 360 60 360 60 360
---------- % control -----------
Glyphosate 4 pt/ac 90 84 97 94 95 93 100 100 - - 100 100 - - - - - -
Glyphosate 6 pt/ac 98 97 97 96 97 95 100 100 - - 100 100 - - - - - -
2,4-D amine 2.5 pt/ac 61 32 94 50 76 32 57 54 57 62 65 78 81 76 91 97 100 100
2,4-D amine 5 pt/ac 96 28 97 79 95 72 82 78 71 72 81 80 78 82 90 94 100 100
Triclopyr 3 pt/ac 64 25 86 41 79 32 64 63 47 72 72 74 55 76 83 77 100 100
Triclopyr 5 pt/ac 85 29 97 50 92 70 90 87 54 90 - - 78 76 84 83 100 100
Imazapyr 4 pt/ac 84 83 97 98 100 100 - - - - - - - - - - - -
Imazapyr 6 pt/ac 87 89 95 98 100 100 - - - - - - - - - - - -
Metsulfuron 2 oz/ac 79 30 94 89 100 100 - - 94 97 - - - - - - - -
Metsulfuron 4 oz/ac 85 30 90 92 100 100 - - 96 95 - - - - - - - -
Fosamine 3 gal/ac 50 43 94 67 80 89 76 71 66 73 77 85 87 85 87 88 100 100
Fosamine 5 gal/ac 76 66 99 75 83 87 69 76 59 90 90 82 83 83 87 87 100 100
Triclopyr + 2,4-D 3 + 2.5 pt/ac 85 30 88 52 78 100 - - 70 85 77 98 100 100 - - - -
Metsulfuron + 2,4-D 1 + 2.5 pt/ac 91 30 86 74 95 97 - - 86 97 99 97 100 100 - - - -
LSD (0.05) 5.3 10.4 6 17 10 15.2 13.4 17 15.1 14.9 13.5 10.2 21.1 12.8 16.7 0 0
a Herbicides were sprayed once in each year.
b Days after treatment (DAT)
- Not sprayed because there was no regrowth