The high summertime temperatures experienced in southern Oregon can result in prolific weed growth in hemp production systems. Growers close to the coast region can experience even more weed pressure due to substantial rainfall. Weeds can outcompete hemp for sunlight, water, and nutrients if appropriate management practices are not implemented. Furthermore, some weeds produce chemicals (natural herbicides) that stunt neighboring crop growth. Research conducted in other regions of the U.S. shows that weed competition reduces hemp growth and bud production (Singh et al. 2024; Cutulle et al. 2020). In particular, keeping the field weed-free for a certain period while plants are establishing is important for bud yield. There is no general recommendation regarding the specific length of time that a hemp field should be kept weed free to eliminate yield drag. Factors, such as weed species present, environmental conditions, and hemp varietal selection, influence the period a crop needs to be weed free to avoid a yield reduction. Therefore, a general guideline for growers would be to keep their hemp fields weed-free for at least two weeks to ensure profitable bud production. However, the presence of aggressive weeds, such as puncturevine (Tribulus terrestris) (Figure 1) and yellow nutsedge (Cyperus esculentus) may require a longer period, particularly if a slow-growing, smaller cultivar is planted. Though not typical in flora hemp, if hemp is direct seeded, the critical weed free period would be substantially longer than transplanted hemp. In addition, growers in Maine observed increased THC levels after stress induced competition in floral hemp, which could put them over the allowable legal limit for CBD marketed products (Jamison 2024).
Cultivar Selection
Cultivars that are more competitive against weeds are likely show greater early season vigor, rapid germination and emergence, rapid biomass accumulation, high leaf area index, leaf angle, and reduce effective light penetration (Table 1). Taller hemp cultivars may be better suited for higher planting densities due to their dense canopies, aiding in weed suppression. On the other hand, shorter floral and dual-purpose varieties may require lower planting densities but might contribute to weed control through canopy density.
| High Biomass Cultivars | Lower Light Penetration | Leaf Area index |
|---|---|---|
| Arrowhead Abacus | Arrowhead Select | Bubbatonic |
| Bubbatonic | Hurricane Hemp Florence | Arrowhead Abacus |
| Hurricane Hemp | Hurricane Hemp | White CBG |
Management
Due to the lack of selective herbicides (zero in organic production), it is important to implement pre-season management practices, including crop rotation, the stale seed bed technique, sanitation and successful use of plastic mulch.
Crop Rotation and Cover Crops
Continuously growing of the same crop will favor weed species that thrive under those specific monoculture conditions. The same nutrient management and growth habit niche related to canopy and root architecture will select for several dominant weeds. These weeds are often problematic because they are difficult to control in the specific monoculture cropping system (i.e., yellow nutsedge in plasticulture). Over time, populations of these weed species can build up, sometimes to problematic levels. Diversifying crop rotations leads to diverse weed management tactics and growth habit niches, allowing for greater weed diversity (less dominant weeds) and helping to keep weed populations under control. As the diversity of the rotation increases, benefits for weed management also increase. If a grower it is not able to grow a cash crop in the rotation, a winter cover crop such as cereal rye is good for rotation. Strictly from a weed science perspective, it may be better to grow a monoculture of a cover crop, such as cereal rye that will provide suppression of weed seeds in the soil, as opposed to mixing it with another cover crop such as a clover species. A cover crop monoculture will typically generate more biomass per cover crop seed when compared to polycultures of cover crops as competition between two different cover crops for resources can reduce overall biomass. Overall biomass is one of the most important factors when trying to suppress weeds with cover crops. This recommendation is narrowly focused on the weed science benefits.
Stale Seed Bed Technique
An important pre-season management technique is the stale seed bed method. Weeds have dormancy mechanisms, thus facilitating germination of seedlings and then terminating them before a crop is planted is ideal in hemp production, given the limited in-season weed control options. The stale seedbed approach involves preparing the soil as if for planting a crop, without actually planting the crop. After the soil is prepared, weed seeds in the upper 1-2" of the soil are encouraged to germinate through adequate soil moisture and temperature. This can be achieved either initiating stale seed bed technique during rainy season or through irrigation. A pivot irrigation system would be the optimum equipment maximizing coverage; however, any kind of overhead irrigation strategy should work. Weeds should emerge within a week and then can be terminated with a non-selective herbicide, flame or cultivation. Killing emerged weeds with non-selective herbicides (i.e., Glyphosate, Glufosinate or Paraquat) or flaming does not disturb the soil and prevent new weed seeds for being brought close to the soil surface. If possible, conduct the stale seed bed action multiple times to encourage a maximum flush and subsequent termination of weed seedlings. After implementing the stale seedbed technique, it is important to not to disturb the soil any more than is necessary during the transplanting process to minimize the amount of weed seeds that are brought up to the surface to germinate.
Sanitation
The source of new weed infestations can originate from wind-blown seed, animal traffic, contaminated equipment, and non-certified crop seed sources. Growers have limited control over wind-dispersed seeds, but they can practice good sanitation techniques, such as preventing weeds from going to seed, to avoid the other infestation scenarios. It is important to avoid cross contamination on the farm. If entering a field with substantial weed pressure, it is important to clean the equipment (and your shoes) thoroughly before going into a field that does not have those troublesome weeds. When finished cultivating, spraying, or planting for the day, it is important to clean the equipment to avoid spreading weed seeds across multiple fields or farms. Animal tracks can also introduce weed seeds, potentially leading to new weed infestations. Hemp is a high value crop which justifies using barriers, such as an electric fence (could be temporary), around the hemp crop to keep terrestrial animals out of the crop area and reduce new weed infestations. It is also important to check cover crop, hemp, or rotational crop seed tags for the presence of weed seed in the seed bag. Maintain good records regarding historical infestation of new weed species. If there is a pattern of the infestations occurring when seed from a certain vendor, consider contacting the vendor and local Extension personnel to address issue.
Herbicides
The only registered herbicide that can be used in season in hemp production is Ethafluralin and registered for use in Oregon. Ethalfluralin is the active ingredient (ai) in Sonalan® HFP Herbicide. A supplemental product label (Gowan) allows for its use in hemp production. It is effective for controlling small seeded annual grasses (such as crabgrasses, foxtails, and barnyardgrass), and broadleaf weeds (such as pigweed species, horseweed and lambsquarters). Sonalan® HFP can be applied to the soil surface as a pre-emergent (PRE) herbicide before or after planting (but before hemp has emerged) at application rates ranging from 1.5 to 3 pt/A (0.5625 to 1.125 lbs ai/A). These product rates are dependent on soil type; use the lower labeled rate on coarse textured soils. Medium and fine textured soils and soils with high organic matter will require a higher rate to effectively control some of the labeled weed species. Sonalan® HFP is limited to one application per year. The product must be incorporated mechanically or with irrigation immediately after application. Poor incorporation will result in erratic weed control. The product can be used in both directed seeded and transplanted crops (For transplants apply before transplanting the crop). The transplanted hemp crop should at a sufficient depth that the crop roots grow beyond the treated zone. For optimal soil coverage, a flat fan broadcast nozzle (i.e., XR) should be used with an output of 15 to 20 gallons per acre.
References
- Cutulle M, Ward B, Campbell HT, Horry M. Varieties and Weed-Free Duration is Important for Hemp Bud Yield in Coastal South Carolina. Clemson (SC): Clemson Cooperative Extension, Land-Grant Press by Clemson Extension; 2020 Jun. LGP 1074.
- Jemison, J. M., Jr. (2024). Soil and weed management effects on floral hemp(Cannabis sativa L.) production and cannabinoidcontent. Agronomy Journal, 116, 1758–1774.
- Singh, G.; Slonecki, T.; Wadl, P.; Flessner, M.; Sosnoskie, L.; Hatterman-Valenti, H.; Gage, K.; Cutulle, M. Implementing Digital Multispectral 3D Scanning Technology for Rapid Assessment of Hemp (Cannabis sativa L.) Weed Competitive Traits. Remote Sens. 2024, 16, 2375.
Use pesticides safely!
- Wear protective clothing and safety devices as recommended on the label. Bathe or shower after each use.
- Read the pesticide label—even if you’ve used the pesticide before. Follow closely the instructions on the label (and any other directions you have).
- Be cautious when you apply pesticides. Know your legal responsibility as a pesticide applicator. You may be liable for injury or damage resulting from pesticide use.
