Understanding Hemp's Water Needs

Columns - From The Field

The frequency and amount of irrigation growers need depend on factors such as soil type, weather and plant growth stage.

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March 8, 2021

© Marguerite Bolt
As plants grow, their ability to tolerate water stress changes. Oversaturated soils damaged this seedling.

 

Changes in precipitation patterns and higher summer temperatures are becoming the norm for the U.S., with 2020 yielding one of the hottest summers for many areas, according to the National Oceanic and Atmospheric Administration. With hot summers and unpredictable or inconsistent rainfall, some agricultural producers may contemplate adding irrigation equipment to their operations.

Irrigation is one of the largest uses for water in the country, according to the U.S. Geological Survey. California, Nebraska and Arkansas have the most irrigated acres of land, while California, Idaho and Texas apply the most water on an acre-feet basis, according to the USDA’s National Agricultural Statistics Service. (An acre-foot is the amount of water required to cover one acre of land with one foot of water. This is equal to 325,851 gallons.) In Indiana, almost 60% of hemp producers used some form of irrigation in 2020.

The frequency and amount of irrigation growers need will depend on a combination of factors. These can include weather, soil type and plant growth stage. Understanding each facet can help reduce water use, which saves both money and natural resources.

Water Needs by Growth Stage

As plants grow, their ability to tolerate water stress changes. The critical period for water needs indicates the time or times in the crop's lifecycle where water is crucial to prevent yield loss due to water stress. While we lack studies on the critical growth period for hemp in relation to drought stress, it is likely that the beginning of the lifecycle is a critical period based on what we know about other crops. As hemp matures, it can develop a large root system that can access water deeper in the soil, so plants may be more drought-tolerant later in the season. Because hemp is also grown for different purposes, where different plant structures are capitalized on, water needs may also vary for each type. This means hemp grown for grain, fiber and cannabinoids could have different critical periods for water needs.

The water needs of the crop also vary depending on environmental interactions, which change throughout the growing season. Higher temperatures increase the rate of plant transpiration (water evaporating from the plant’s leaves), which in turn means greater water demands. High humidity and high temperatures cause a decrease in transpiration because it is difficult for water from plants to evaporate into saturated air.

When relative humidity decreases, transpiration increases because it is easier for water to evaporate into drier air. However, dry air and dry soil can lead to plants closing their stomata, decreasing transpiration. The same is true for high wind, which also leads to an increase in transpiration. Hot, dry and windy weather means plants are going to need more soil moisture to reduce plant stress.

Plants can be specialized for certain climactic conditions. Some plants are able to tolerate drought for weeks or even months, but it is not without consequence if drought is sustained past their tolerance period. Think about a home garden or house plant that is neglected. It may be revived when watered after a dry period, but if that period is far too long, the plant cannot be saved. Drought stress can cause a reduction in yield and an increase in plant mortality depending on the plant species.

Within species, there can be varying degrees of drought tolerance as well. Some of these genes are related to the transpiration rate of the plant. In a recent paper published in the Journal of Crop Improvement, researchers found different hemp cultivars closed their stomata earlier, reducing transpiration (and therefore conserving water) during the soil drying period. Growers that rely on rainfall would benefit from selecting drought-tolerant hemp cultivars.

While hemp, like many crops, can withstand periods of low rainfall, producing target yields will require consistent watering throughout the growing season. The ideal situation is regular rainfall across the entire growing season, which rarely happens. Researchers across the U.S. are trying to understand the water needs of hemp and how they vary across different regions.

Timing of Irrigation

The timing of irrigation (as well as the duration and amount of water used) will depend on the weather. This can vary between fields and across regions. For example, it would not make sense to irrigate right before a rain event, so keeping an eye on local weather can help with irrigation schedules. (Airport weather data and local weather stations are reliable sources.)

It’s also important to note that even though the industry uses the word “schedule,” it does not mean a regular, calendar-based prescription of irrigating. Following a strict irrigation schedule could lead to overwatering, which could cause plant mortality due to consistent saturation around the roots. However, increasing periods of drought may mean more consistent schedules in mid to late summer. In the case of western regions, the timing of irrigation may resemble a regular schedule. Regardless, it is still important to look at weather forecasts and plan when to irrigate accordingly.

[Editor's note: See here for more on how to calculate the amount of water you’ll need based on rainfall and your type of irrigation.]

Some growers may opt to use soil moisture sensors to help make irrigation decisions. Multiple options are available, but understanding the output of these sensors is necessary to maximize scheduling efforts. The University of Minnesota has detailed resources on installing and interpreting results from different soil moisture sensors.

A hemp field in Indiana
© Marguerite Bolt

The Importance of Soil Type

Soil type is an important factor in determining how much water is needed, whether it be irrigation or rainfall. Different soils have varying amounts of water-holding capacity. Sandier soil will not hold water as well as soil composed of higher clay content. That is because of the soils’ pore space, which is the space between the particles and where the majority of water is held. Sand particles in soil can be thought of like a container full of golf balls—if someone were to pour water over the top, the liquid would reach the bottom quickly. On the other hand, soil with more clay particles is like a container full of marbles, and water poured on top would take much longer to reach the bottom.

Both sand- and clay-dominant soils have advantages and disadvantages in terms of their ability to hold water. During periods of heavy rainfall, drainage is important to reduce the chance of standing water in the hemp field; this also presents challenges during dry periods, though. Fields with sand-dominant soil will need more water during these dry spells. This does not mean that growers with clay-heavy soils never need irrigation; it just means the frequency may be reduced.

The opposite advantages and disadvantages exist when we look at soils with greater clay content. Having some clay particles in the soil can help hold more water, and they increase the soil’s ability to hold on to nutrients. However, these soils can be problematic during periods of heavy rain, increasing the chance for standing water in the field. Clay-heavy soils with low organic matter are also prone to forming a hard crust, which can reduce water penetration and prevent seedling emergence after sowing. If the clay content is too high and heavy rainfall events do occur, growers could expect to see some issues related to root health and disease.

Long periods of soil saturation can result in two main issues. Firstly, prolonged saturation reduces gas exchange in the soil. This can affect root growth and reduce both water and mineral absorption by the plants. Other factors besides soil type can also increase this risk, including compaction and low organic matter. In Indiana soils, this reduction of gas exchange causes problems in any given season. Long periods of soil saturation can also increase the risk of plant disease, particularly root rot. While wild hemp is often referred to as ditchweed, it does not tolerate conditions often found in ditches (e.g., excessively flooded or compacted areas).

It is important for growers to understand the soil type and the amount of organic matter it contains, and the field capacity of particular sites. This can reduce the chance of over- or underwatering.

The decision to irrigate should be based on multiple factors. Taking the needs of the plant, the weather in different regions, and the soil into consideration when deciding to irrigate can prevent drought-related stress. It can also help conserve resources by appropriately timing the application of water.

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.