Hemp production takes many forms, and with those various forms comes the need for different irrigation treatments.
The best irrigation systems will differ depending on production practices, where the hemp is grown and what it’s grown for. However, some irrigation principles apply to all hemp growing methods.
The goal of irrigation management should be to provide the plant with an adequate amount of water using the minimum number of applications and without overfilling the root zone. It is important to never wet below the root zone, as this wastes water and can even potentially waste nutrients. Plants need enough water to saturate the top half of the root zone at least once per week, as the nutrients in the soil need water to move into the plant.
For newly planted hemp, this zone may be only a few inches deep. But as plants reach maturity, rooting depth may be 36 inches or more. Growers should also monitor weather forecasts for anticipated precipitation to make the best use of rainfall and adjust irrigation plans as needed.
Choosing a System
While many growing operations have existing irrigation systems, farmers looking to add hemp to their crop rotations often look to integrate watering equipment that is ideal for hemp production.
The most common form of irrigation across the country is a sprinkler system (see Table 1), which includes center-pivot and traveler equipment. These systems are designed for deep-rooted crops (24 to 36 inches) where the roots also extend laterally throughout the soil between plants. Overhead sprinkler systems allow producers to germinate seed or incorporate surface-applied fertilizer.
Hemp production in 36-inch or narrower rows—usually used for fiber and grain production—will have root systems that fully recover overhead-irrigated water, making it an efficient method of irrigation in these situations. The high fixed cost of overhead irrigation, however, results in the need to utilize a crop rotation with other high-value crops, such as vegetables and crops grown for seed production, that can justify expenses in non-hemp production years.
Although hemp can be grown in the same field continuously—assuming the soil is adequate for hemp production in the first place—it would be best to grow it in a rotation to reduce the risk of increasing insect pest pressure and fungal inoculum.
Some hemp production methods present challenges for overhead irrigation. Plant spacing greater than 36 inches apart, which is common in cannabidiol (CBD) production, can increase weed pressure and create areas where overhead irrigation water will pass through the soil without reaching the crop. Plastic mulch can exacerbate the problem by directing overhead irrigated water away from the hemp plant root mass, making irrigation inefficient. In addition, frequent leaf-wetting can make cannabis susceptible to foliar diseases, such as white mold or gray mold, making frequent overhead irrigation applications even more undesirable.
Drip irrigation excels where row and plant spacing are wider than rooting areas. This method allows for water application directly to the root zone. Growers have the flexibility of using thin-walled drip tape with a single-year life span or longer-life drip tubing that can be rolled up and reused for two or more years. Either would allow for a minimal long-term investment. Some of the water supply components, such as the main distribution line and manifold, can be relocated each year to a new field if the hemp is grown in a rotation with other crops. Drip irrigation systems also work well with plastic mulch, providing efficient water application and weed control. Adding fertigation (fertilizing through drip irrigation) allows producers to spoon-feed the plant for efficient water and nutrient delivery.
Foliar diseases are a major factor in irrigation selection for hemp production. With few fungicides registered for hemp disease control (see Table 2), growers need to minimize disease potential by limiting the amount of times they wet the leaves. Drip irrigation eliminates wetting the leaves altogether. When drip irrigation and plastic mulch are used together, the ground surface remains dry, lowering the humidity at the leaf level and further reducing foliar disease potential.
How Much Should I Water?
Knowing how much water to apply is as important as knowing which irrigation system to use. Crops that close to a solid canopy will need more water than crops without closed canopies. Water requirements should be calculated based on canopy cover and sunlight interception rather than on total acreage.
With overhead irrigation, calculate the volume of water applications by adding inches of rainfall plus inches of irrigation that fall into a rain gauge.
If using drip irrigation, growers will need to convert to gallons of application. Drip irrigation tubing and tapes are rated by output per emitter, or per foot of row, in gallons per hour. As an example, a grower with plant rows 220 feet long and a crop canopy 2 feet wide has 440 square feet of canopy cover (220 x 2). This equals roughly 1/100th of an acre of sunlight interception (440 square feet of canopy cover divided by 43,560 square feet in an acre). To calculate what a 1-inch application of water would be on that area, use the formula shown below.
So, using this formula with the example given, a 1-inch application over one row would be equal to 274 gallons.
A single drip tape the length of the row is installed, and the drip tape delivers a half-gallon of water per foot per hour. Almost all crops in the Midwest require approximately one-quarter inch of water per day at the time of peak water need, which usually correlates with the reproductive stage or, for crops that do not reach flowering, peak biomass production. This water requirement is general and is based on water use coefficients that have been developed with numerous crops through research. However, more research needs to be conducted to determine these coefficients for different hemp types.
Instead of providing one-quarter inch of water per day, as is ideal in the Midwest, many farmers prefer to apply 1 inch of water every four days to cut back on frequency of running the system. In this example, the farmer will be applying 110 gallons of water per hour of run time (220-foot-long rows multiplied by the half-gallon of water per foot per hour from the drip tape). So, he will need to run the system for 2.4 hours every fourth day to provide 274 gallons (274 gallons needed divided by 110 gallons per hour), or the equivalent of a 1-inch rainfall in the given area.
Some farmers use checkbook scheduling methods to calculate how much irrigation is needed to make up the deficit between crop water use and rainfall for the previous week. A checkbook method of irrigation scheduling follows the concept that the soil in a field is like a bank checking account. As a starting point, crops will have the rainfall from the week available plus the water stored in the soil. Rainfall and irrigation applications are deposits into the checking account. Daily water removal from evaporation and transpiration from the field and crops are withdrawals from the account. Soil has a maximum amount of water it can hold (called field capacity), so any water added beyond the soil’s water holding capacity would be a loss.
The University of Minnesota has published a checkbook scheduling spreadsheet. While the spreadsheet is meant for growers in Minnesota and North Dakota, it serves as a launching point for anyone who wants to use the checkbook method. The method requires monitoring a crop’s growth, knowing the soil texture in the rooting zone, observing and logging the maximum air temperature each day, and measuring and logging the rainfall or irrigation applied to the field. (To access the spreadsheet and a full manual on how to use it, visit https://extension.umn.edu/irrigation/irrigation-scheduling-checkbook-method.)
No matter what growth stage they’re in, crops need more water when rainfall frequency and volume decrease. Matching hemp’s water needs with the appropriate type of irrigation system and amount of water applied is essential to producing a successful crop.