By Robert W. Pal and Nathan Carpenter
Department of Biological Sciences/
Department of Chemistry and Geochemistry, Montana Technological University

Figure 6. Field survey on a fallow hemp field in 2019 where numerous volunteer hemp seedlings emerged.

Industrial hemp (Cannabis sativa L.), a multi-purpose agricultural crop and an ideal alternative for organic farming, is becoming more popular among farmers in several states in the US, including Montana. However, in many respects, little is known about how this plant really functions in its growing environment. For instance, we do not know what the most common weeds in Montana hemp fields are. As an organic crop alternative, how competitive is hemp against problem weeds? Can the plant itself become weedy in its new growing ranges? Such questions are important to address before mass production.

Within the framework of a small research project funded by the Montana Farmers Union, we surveyed active (25 random plots) and abandoned (25 random plots) hemp farmlands, mainly in the Helena area of west-central Montana. The field surveys were conducted in early August of 2019. The hemp was seeded, in most cases, in early June. In addition, a greenhouse competition experiment tested the competitive ability of hemp with five weeds and five native plants.

The mean coverage of hemp, in the active fields, was 60.8% and there was approximately 20.5% bare ground, which was prone to weed infestation. The average height of hemp was 36.3 inches (the smallest measured plant was 2 inches and the highest was 77.16 inches). The mean stem number was 56.76/m2 (5.1 plants/ft2), which is slightly lower than the recommended target plant density. We found a very strong relationship between hemp stem numbers and weed cover. As the stem number grew the weed cover decreased, and by around 80-90 stem/m2 (7.2-8.1 plants/ft2) the weed cover dropped to nearly zero.

In these active fields, we recorded a total of 23 weed species and an average of 4.4 species per plot. The mean weed cover in the studied plots was 24.17% (the lowest cover was 1.5% while the highest reached 80%). The frequency and mean cover of the ten most dominant and frequent weeds can be found in Figure 1 and 2. All listed weeds in the active hemp fields were dicots except for quackgrass; in fact, this species was the only identified grass in the entire survey. The overall abundance of dicots is not surprising as hemp is also a dicot and that makes chemical weed control against this group extremely difficult. Lambsquarters (Chenopodium album) was the most frequent weed; it was present in 72% of the surveyed plots. Interestingly, after lambsquarters, alfalfa was the most dominant weed of hemp fields with an average 5.8% mean cover of all plots. Alfalfa is a popular crop in the area and the soil seed bank may have a significant amount of propagules which makes it a dominant and relatively frequent weed.

Figure 1 and 2. The frequency and the mean cover values of the ten most dominant and frequent weed species in hemp.

The mean coverage of hemp in the abandoned fields was 28.4% and there was approximately 40.2% bare ground. The average height of hemp was 3.83 inches (the smallest measured plant was 0.78 inches and the highest was 18.3 inches). The mean stem number was 28.4/m2 (2.55 plants/ft2). Although the investigated fields were abandoned only for one year, our results suggest that hemp could become a weedy species after its cultivation. Hemp is not a native plant species to North America and there are reports about its weedy behavior from other parts of the world; therefore, extreme caution needs to be taken if it is to be cultivated.

In the greenhouse competition experiment, we found that hemp grew 34% larger in competition against native plants compared to weeds. Hemp decreased the mean biomass of weeds by 22% and by 5.2% that of natives.(See Figure 3)

Figure 3. The competitive ability of hemp against weeds and native plants. Bars show the dry biomass of five weed and native species either alone or in competition with hemp.

Summarizing the results of our study, we can conclude that there are numerous weed species that can interfere with hemp production in west-central Montana. As chemical weed control is extremely difficult, if not impossible in hemp fields.

It is important to focus on site selection and to perform an effective pre-seed treatment. Our research also found out that optimizing seeding density could help to improve the competitive ability of hemp against weeds.Additionally, growers need to be careful with hemp production as the plant can return as a volunteer and become a weedy species on subsequent crops in fallow, abandoned fields. And finally, hemp has a potential to enter natural or semi-natural habitats, especially disturbed sites; therefore, extreme caution needs to be applied during harvesting and transporting the crop and it seeds. In addition, the growing area and its surroundings should be frequently monitored for volunteer hemp plants, so as not to increase the soil seed bank.