What You Need to Know Before You Plant Hemp

Columns - From The Field

Is your site ideal for growing hemp?

April 2, 2020

Whether transplanted or planted directly in the ground, farmers must consider soil and growing conditions, among other factors, to make sure hemp plants take root.
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So, you’ve decided to grow hemp this season. Now what?

One of the challenges of introducing a new crop to your farm is selecting a suitable field site for production. Because farmers hadn’t grown hemp for decades in the U.S., data on field production was lacking prior to the Agricultural Act of 2014 (the 2014 Farm Bill). Fortunately, we can draw some helpful information from recent research, Canada, Europe and old U.S. records.

Soil Conditions

One common myth is that you can grow hemp anywhere and expect high yields. I think part of this misconception stems from feral hemp populations—often referred to as ditch weed—that grow in marginal sites. (For more on feral hemp, read Taming Feral Hemp.) These populations have been naturalized to the area where they are growing, and they are not being planted at a field scale with yield in mind.

We knew even back in 1943 that hemp should be planted on the best fields. An old U.S. Department of Agriculture bulletin stated, “Hemp should be planted on the most productive land on the farm” (Farmer’s Bulletin No. 1935, USDA). This means growers may choose to plant hemp in a field where they would have otherwise planted a different crop.

Planting hemp on the most productive fields could affect yields of the other crops if those crops are planted on a less productive site. Growers should conduct a risk assessment of how they are going to allocate field space to hemp and the potential consequences of doing so, such as losing money on hemp when something less risky could have been planted there.

When selecting a field, look for loose, well-aerated loam soil with a pH of 6-7.5. Well-drained or tiled clay soils can be used, but poorly drained clay or poorly structured soils often result in plant establishment failures, as seedling and young plants are prone to damping-off. Hemp can grow in sandy soils; however, these sites may need irrigation at some point during the growing season.

With such a large lapse in hemp production, fertility programs have not been well developed in the United States. These programs outline plans to apply the right fertilizer products, at the right rate, in the right place, at the right time. I advise working with a crop consultant or county extension educators for site-specific recommendations using baseline recommendations from the Canadian Hemp Trade Alliance (CHTA) eGuide on the amount of fertilizer to apply and when. The type of production also factors into fertility programs. Fiber, grain or cannabinoid producers will likely follow different fertility programs, especially since many cannabinoid hemp producers are feeding through drip irrigation throughout the season.

We know that hemp grows best in fertile fields that have abundant organic matter greater than 3.5%. General baseline recommendations from the CHTA eGuide state that grain hemp production requires inputs of up to 80 to 120 pounds of nitrogen per acre, 40 pounds of phosphorus per acre, 58 pounds of potassium per acre and 13 pounds of sulfur per acre. For perspective, hemp requires about the same fertility inputs as a high-yielding row crop like wheat or corn. Soil test results should inform your decisions on inputs for each field. Recommendations are going to vary based on these results. This is where universities that have been conducting hemp research can help.

Hemp plants 24 days after direct seeding in 2018.
Photos Courtesy Marguerite Bolt

Heavy Metals

Farmers have more to consider than just soil type and fertility when choosing a field site, however. Studies are showing an ever-increasing concern regarding heavy metals (e.g., lead, arsenic and mercury) in soils that move into hemp plants and show up in post-harvest testing of hemp extracts.

Five publications currently report that hemp can pull heavy metals out of contaminated soils. One recent study demonstrated that most metals are stored in the plant’s leaves, not in the flowers (Rabab Husain, et al., PLOS ONE 2019). Leaf material is often included during processing, so this could be a concern.

Most soil testing services can screen for heavy metals when they receive samples for fertility measurements. However, soils naturally can contain some amounts of heavy metals, so we do not yet have a clear understanding of what amount of heavy metals shown in a soil test would be cause for growers to pick new field sites.

Pesticides and Pathogens

Always consider previous crops grown and any pesticides used on those crops when selecting a field site for hemp. This is especially important for herbicides that have been applied in previous years. Some products have lengthy planting restrictions for specialty crops and could reduce or prevent germination or stunt hemp growth.

Records of any former crop issues in a particular field are critical because only a few pesticides have been approved for use on hemp crops. Do not plant hemp in fields that have a history of pathogens such as white mold or southern blight, as soil inoculum can build up.

Based upon reports from Ontario, Canada, it has been recommended that hemp not immediately be planted on land that has grown canola, edible beans, soybeans or sunflowers because of the risk of white mold. Knowing the disease history of your fields should help dictate rotational order of your crops. Hemp can be used to diversify current rotations of row or forage crops. Different rotational models are currently being studied at Rodale Institute and Purdue University.

Hemp can successfully grow in continuous rotation for several years on the same land, but it comes with risks. A general rule of thumb with annual crops is to rotate them out every year, because if farmers plant the same crop in the same spot year after year, they can run into issues with pest and pathogen populations.

With very few pesticide options, hemp is especially susceptible to these issues. Growing hemp in the same spot two years in a row would probably be OK, but hemp is a heavy nutrient user. Since farmers accustomed to using herbicides have few available for use on hemp, weed control after growing hemp two years in a row is going to be challenging.

It is also a good idea to avoid planting in weedy fields if possible. Early season weed pressure paired with wet weather can lead to weeds outcompeting hemp.

Seedlings with their first set of true leaves, 15 days after direct seeding in 2019

Seedbed Preparation

Seedbed preparation will vary depending on what kind of production system you use. I see both till and no-till hemp production from farmers for direct seeding but a fine, level and firm seedbed results in a more consistent establishment.

Preparation for transplanting depends on the production model. Some growers opt to plant transplants in bed shapers, while others use plastic on top of the raised beds. This is entirely dependent on what kind of production fits into a current operation.

Seed is best planted at ½ inch to 1 inch beneath the soil surface. Although deeper plantings may be tolerated, they are more susceptible to damping off. I have also seen growers with successful crops when they planted closer to 1/4 inch below the soil surface. This may be a better option in extremely wet springs, when deeper plantings in saturated soils could lead to greater seedling mortality.

Although the seedlings will germinate and survive at temperatures just above freezing, soil temperatures of 46 to 50 degrees Fahrenheit are preferable. Just because the seeds can germinate at 35 degrees Fahrenheit doesn’t mean it is a good idea. Planting in soil that is too cool could result in damping off if it is also very wet, and lots of soil pathogens like cool, damp soils. It also means that hemp is going to grow even slower, making way for weeds to outcompete the plant if the weather is too cool and hemp can’t get a quick canopy closure. Generally, hemp should be planted after danger of hard freezes, typically around mid-April to May. Growers transplanting seedlings or clones have a larger window for planting.

Good soil moisture is necessary for seed germination, and planting after a rain is best. Heavy rainfall after seeding can stall hemp germination and slow growth, so while rainfall is needed for good growth, especially during the first six weeks, large amounts of rain is not ideal.

Crop failures due to choosing the wrong field site are not unheard of. Proper field site selection is important to get off on the right foot and set up your hemp farm for success in the 2020 season and beyond.

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.