Managing Perennial Weeds in Organic Fields

by Mark Dempsey, CFSA Farm Services Coordinator

Mugwort through cover crop

Mugwort emerging through a rolled cover crop in late summer. Suppression from a rolled cover crop lasts several months, but a bad perennial weed infestation will eventually push through, requiring further treatment. Photo by Mark Dempsey.

Staying ahead of weeds this time of year is one of the organic farmer’s biggest challenges and a big drain on labor. In the Grower’s Toolbox last June, we addressed the management of annual weeds by focusing on preventing weeds from setting seed and on drawing down the weed seedbank. While annual weeds are generally the most common weed problem, if you have a perennial weed problem, you’ll probably dread it even more. In fact, since last year, we’ve gotten feedback from many farmers expressing the same sentiment: “I’ve got my annual weeds under control, but my perennial weeds just won’t die!”

 

Perennial weeds are problematic for organic farmers because they regrow vigorously after attempts to control them, can be spread around the farm by tillage equipment, and they are simply hard to kill. The take home message for perennial weed management is simple: be aggressive and persistent. Whatever treatment methods you use, be sure to use them often, or time them strategically to minimize that weed’s ability to regrow. Doing this well, however, requires diligence and knowing enough about perennial weed biology to hit them where and when it hurts most.

 

The key to the success of perennial weeds is their ability to store energy below ground in their roots (eg. pokeweed or bindweed) and underground stems such as rhizomes (eg. bermudagrass) or tubers (eg. nutsedge). This allows them to bounce back time-and-again without actually having leaves to capture sunshine for energy – something annual weeds can’t do so well. Further, knowing your particular perennial weed is important because some spread by seed in addition to roots and stems (eg. bindweed & marestail), requiring careful attention to both seeds and underground storage structures.

Rhizomes, such as those in this Canada thistle, store energy and help weeds to spread. Photo by Mark Dempsey.

Rhizomes, such as those in this Canada thistle, store energy and help weeds to spread. Photo by Mark Dempsey.

 

Nutsedge

In addition to rhizomes, nutsedge also has small tubers (aka. nutlets) at their root tips, which also store energy and help the weed to spread. Photo by Mark Dempsey.

The ultimate management goal is to draw down those energy reserves so the plant can no longer regrow after treatment (more on that below), eventually leading to its death. The longer the weed’s leaves are out capturing sunshine, the more likely it is that it’s replacing energy reserves underground. Yet, it’s important to know that the plant invests its below ground energy in two basic ways: for vegetative growth (early-season and post-control regrowth), and for flowering/fruiting, which tends to use more energy. Thus, there are two approaches to drawing down energy reserves: pull a field or area out of production and hit it hard, limiting vegetative growth (usually eradicating the perennial weed in one season), or managing the weed within a crop, targeting the weed’s flowering period to maximize returns on your effort (usually requiring several seasons). Choosing to go one way or the other depends on how bad the infestation is, and whether you can afford to pull a field out of production for a season.

 

 

 

The most common management techniques are explained below and are preferably used in combination (also see Figure 1):

  • Tillage: the most straightforward and effective way to beat perennial weeds. This cuts down the weed’s aboveground (energy-making) parts, and chops up the belowground (energy-storing) parts, limiting the plant’s ability to make energy, and requiring it to draw down its energy reserves incrementally as it regrows with each event. Tillage also opens up wounds in the roots/stems, making them more vulnerable to disease. Tillage is most effective when done repeatedly (every week or two) to limit the weed’s ability to recharge its energy storage, and also most effective without a crop in the field (ie. to uniformly treat the infested area instead of only between crop rows). One must also be careful to not spread the infestation into clean fields with tillage equipment; exercise caution and good equipment sanitation when treating infested areas. If you observe spread, be sure to follow up immediately with more treatment and don’t let the weeds get a foothold.

 

  • Mowing: generally to be used in combination with other crops, such as in a competitive forage or cover crop, and can be strategically timed for weed flowering to maximize energy draw-down. When done frequently (mow at ≤2” height) the plant becomes less vigorous over time, limiting its ability to regrow, eventually leading to death. While not as effective as tillage, mowing may be desirable for maintaining soil health compared to tillage. Also, be careful about equipment selection: mowing nearly to the ground is preferred (eg. with a flail mower or string trimmer); a close cut with a sickle bar might do, but achieving energy draw-down with a rotary mower will likely be difficult.

 

  • Suppression: a method that should be considered at any scale because it’s easy to do and should be used in combination with other methods. Suppression can be as simple as a live cover crop that competes for light and other resources, it can be a roll-killed cover crop that forms a thick light-blocking mat (and can have allelopathic properties), it can be straw mulch, or it can be a tarp or landscape fabric. Among suppression methods, using a tarp or landscape fabric is the most effective for quickly eradicating perennials, and may be as effective as tillage – it just takes a little longer than tillage. Do note that a physical barrier might not be practical at scales larger than 1 acre. Additionally, suppression from cover crops and organic mulches will contribute more to building soil than a tarp/landscape fabric.

 

  • Herbicide:  there are several organic herbicides available, but none are systemic, meaning that none will kill the entire plant in one or a few shots. Instead, all organic herbicides currently available kill only what they contact. Thus, they have little efficacy on perennial weeds, which regrow after treatment. Additionally, they are generally more expensive than the other control measures, and their effectiveness varies by weed species. However, if you wish to diversify your weed management program, organic herbicides may fit well – just plan to use them strategically and be prepared for their price tag.

 

Mugwort suppression by management type

Figure 1. Biomass of the perennial weed mugwort after 2 years of different weed control methods. All treatments (except Landscape Fabric) had back-to-back cover crops (no cash crop), with tilling, mowing or herbicide application occurring in the spring and fall between cover crops. Cover Crop Only had no additional weed control measures. Landscape Fabric was planted to tomatoes. Unpublished data.

A final note on the use of tillage: reducing tillage is the perennial weed’s best friend, and perennial weeds are not as problematic on full-till farms compared to low/no-till farms, except after fallow periods when they get a foothold. While reducing tillage is encouraged from a soil health perspective, those goals should be balanced by your ability to manage perennial weeds and achieve yield goals. It’s possible to manage perennial weeds without tillage, but anticipate a much longer timescale compared to tillage.

 

Deciding how to deal with your perennial weed problem is ultimately up to you, and it’s worth considering the trade-offs in time, yields (and therefore income), and conservation goals (such as crop diversity and soil health), to determine which way to go. You may want to try a little of all of them to judge which work best for your particular weed problem and crop rotation. For more information on perennial weed management email Mark Dempsey.

 

Are you considering using conservation practices such as weed-suppressive cover crops, low-till, or mulches, and want financial assistance to do so from the USDA? If you’re also transitioning to certified organic production, you should consider a Conservation Activity Plan Supporting Organic Transition (CAP 138). CFSA can help write these conservation plans after approval by the NRCS. Visit our webpage for more information on how we can help you!
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