A Guide to Weed Control in Hemp

Columns - From The Field

Use these tips to identify, prevent and control weeds in hemp.

May 12, 2021

Weeds are a major cause of yield loss for all types of crops, even with the advent of different herbicides. These plant pests can be problematic for several reasons. For one, they directly compete with crops, and potentially steal critical resources like nutrients, water and sunlight, which can reduce yields. In addition, weeds can be a reservoir for pathogens and other insect pests that also cause yield loss, and harboring these pests can be problematic for plant health.

For example, last year a grower in Indiana saw an outbreak of common stalk borer (Papaipema nebris), a pest that had not been previously observed in the state attacking hemp. This pest tends to stay in weedy margins around the field, but if those weeds are destroyed or the caterpillar stalk borers outgrow the plant, they migrate. This grower managed the weeds in the margin by mowing later in the season and subsequently saw an outbreak of caterpillars in his hemp. This could have been prevented by managing weeds much earlier in the season before planting. (However, researchers are still identifying pests in hemp, so determining the extent at which weeds will be a pest reservoir is a continual process as we learn more about the crop.)

Hemp is often touted as a plant that outcompetes all other plants, providing great weed suppression. This a half-truth. Once hemp forms a canopy that shades the soil surface, especially in a densely seeded crop grown for fiber, the plants protect the soil surface from invasion. But to achieve canopy closure, growers must avoid serious weed competition in the hemp field from the start.

Managing weeds in hemp demands so much more than just sowing seeds and expecting weed suppression. This article explains the basics of integrated weed management to help hemp growers battle intruders.

Identify Your Weeds

Any plant growing where it’s not wanted could be considered a weed. We often only think of weeds as wild plants that invade our fields, but sometimes they’re plants that were grown intentionally a previous year. This could be volunteer corn in a soybean planting, or in the case of many hemp grain and fiber growers, volunteer hemp coming up the following season. This presents one of the first challenges in weed management: identifying the weed you need to manage.

Weed identification helps determine the best management practices to reduce loss. That doesn’t mean you have to learn every single type of weed that could possibly be present, but it isn’t a bad idea to use weed identification guides to determine what’s afflicting the field. At the very least, growers should be able to distinguish a broadleaf weed from a grass.

Monitoring weeds throughout the year can also provide a view into their seasonality. Some species are more persistent early in the season before planting when there’s adequate rainfall. Others become a problem shortly after planting or mid-season. Since there can be differences between fields from year to year, regular observations are crucial to understanding trends in weed pressure.

In addition to understanding what weeds are in the field, growers should also determine if any weeds are coming in the hempseed they order. Start by checking the package, since seed labels include the percentage of weed seed in a bag. Growers can also send independent samples to their state seed lab for testing. This is more common among grain and fiber hemp growers because several hundred seeds are needed for these tests, and feminized seed (commonly used by cannabinoid producers) is not cheap. The nature of harvesting grain and fiber seed in large fields can also mean more weed seeds getting into the bag.

Many farmers growing all sorts of crops struggle with weed pressure. Researchers and manufacturers know this and spend considerable time and money developing new herbicides. However, herbicide use in hemp is limited. The few products available for use in hemp are not selective, meaning they will target many different plants—not just weeds. These products must be applied between widely spaced rows where hemp is not growing to prevent burning the crop. Certain herbicide residues left over from previous seasons may also cause hemp plants to grow stunted, malformed or less vigorously, so consider previous herbicide applications when choosing field sites.

Use Integrated Pest Management

Despite limited herbicide options, not all hope is lost. Many farmers try to use an integrated pest management (IPM) approach, which incorporates various methods to reduce weed, disease and insect pest pressure in a crop. You wouldn’t build a house using only a drill; the same concept applies to growing and managing crops. IPM provides growers an entire toolbox of cultural, biological, mechanical and chemical techniques to control pests.

Each category offers growers different tools to reduce pest pressure. No matter which set of techniques you use (and yes, more than one is ideal), these tools can help achieve the overarching principles of successful IPM, which include: identifying the pest, monitoring the pest, determining an economic threshold, implementing management techniques, and evaluating the success of your program. IPM is a marathon, not a sprint, and weed management is a long-term approach to reduce pest pressure—not a silver bullet.

Here are some weed management techniques growers can use in hemp production:

Cultural weed control includes planting cover crops for suppression, selecting crop rotation models to reduce pressure, following sanitary practices to make sure weed seeds do not make it into the field, selecting competitive cultivars, choosing an optimal planting window and plant density, mulching around the crop, and maintaining overall plant health. Cultural control can be thought of as weed prevention, which means most of these techniques require some forethought and planning.

Biological weed control

uses living organisms to reduce weedy plants in the field. Some biological control agents include livestock that graze on weed species or even beneficial insects that eat weeds or weed seeds. This type of control is a curative method used when weeds are already present.

The canopy of densely planted hemp suppresses weeds.

Mechanical weed control

uses equipment to manage weeds. Some examples include tractor-mounted cultivators, rototillers and tine weeders. Manual weed control, such as pulling or hoeing weeds by hand, can also be considered a form of mechanical control.

Chemical weed control typically involves the use of herbicides. They work by targeting specific functions in unwanted plants to result in their mortality.

The U.S. Environmental Protection Agency (EPA) list of the few pesticides registered for hemp can be found here. This list is subject to frequent updates as more pesticidal products are developed and registered for use in hemp production. The table above includes the active ingredients in the registered herbicides. It is imperative that the product is also registered in the state in which it will be used and the applicant understands the label.

Flame weeding is another technique to wipe out small weeds in the field. While this isn’t a chemical control, it has the effect of literally burning plants to destroy them.

Flame weeding is more common in the organic growing community. Growers, especially those who till, often wait for the flush of weeds that comes before planting, and then use a flame weeder to kill them before planting their primary crop—creating what’s referred to as a stale seedbed. Many growers achieve a stale seedbed by using an herbicide to do a spring burndown before planting. Flame weeding may be an alternative option for growers in states where applications of herbicides before planting are not permitted.

A morning glory (Ipomoea purpurea) weed

Design a Hybrid Plan

Understanding the basics of integrated weed management can give hemp growers a foundation to build a customized toolbox of effective techniques for reducing pest pressure. Not every management technique will work for every hemp production model. For example, laying mulch to suppress weeds within the rows of transplanted hemp will not work for grain and fiber production because the planting density of these cultivars is high. Plus, the canopy will suppress weeds after closing.

Certain pieces of cultivation equipment may already be in the barn or shed, so using what’s available is a great way to be resourceful and design a tailored management plan that works for your farm. Some growers work with each other or cooperatives to share equipment—just make sure it arrives free of soil and other debris to prevent the movement of weeds, insects and pathogens.

There are far more preventative tools for reducing weeds than curative tools for eradicating them. Plus, it’s easier to avoid pests altogether than to battle a whole infestation. Building a plan that has a heavy focus on cultural control, and then keeping some curative options on standby, is a good hybrid method to stay out of the weeds.

Marguerite Bolt is the hemp extension specialist at Purdue University’s Department of Agronomy. She received her M.S. in entomology from Purdue University and her B.S. in entomology from Michigan State University. Bolt’s research has focused on hemp-insect interactions and plant chemistry.