Climate change

Does it really matter when growers irrigate their crops?

Yes it does! Various crops are sensitive to water stress at different growth stages. The rule of thumb is that reproduction is sensitive to low moisture. The amount of moisture available at the different growth stages can affect how well the plants grow, their resistance to disease, and the yield of the crop. So farmers need to manage when they irrigate just as much as when they harvest!

The reproductive stage for crops refers to when they flower, but also includes the seed- or grain-filling stage. It is within these stages where the crop demands water at the highest. Water demands can even vary depending on how dry the day is – as influenced by sunlight, wind speed, and temperature.

water hoses between young wheat plants in a field trial
Irrigating spring wheat at tillering stage during the 2015 extreme drought year in Kalispell, MT. Credit: J.A. Torrion.

Using wheat as an example: Research has found that roots in wheat are seen to three feet deep near flowering and are able to use available soil moisture in that depth. But wheat roots are still short early in the season. Wheat plants are very sensitive to water stress when the plants are producing tillers. If the soil is too dry for a long period during the tiller-producing stage, yield is affected. The number of tillers per unit area is reduced when rainfall is lacking and, especially during a drought year. In this situation, irrigation during the tiller-producing stage of growth is needed to achieve an optimal yield. This is something wheat farmers have to watch for in their fields to make sure they have a successful yield – especially when it is too dry!

Another factor in the success of wheat crops is having water available when the seeds start growing. Wheat seeds start off with a milky texture inside, and eventually become more starchy, protein-rich seed. Farmers are encouraged to watch their fields after the flowering stage, and to make sure their plants have the moisture they need to produce good seed grains. Another strategy is to irrigate just before flowering and allow wheat heads to stay a bit drier for a period while flowering – this strategy can suppress diseases!

Water sprinkler spraying water above a crop.
Growers turn to irrigation at various times in the growing process. Knowing when to water, and how much, is important to farmers. Credit: Sayantan Sarkar

The amount of water needed is dependent on the farm’s location. Humid locations with high rainfall amounts may get by adding at least an inch or two more after flowering spread out into multiple events. In arid to semi-arid areas, a relatively higher irrigation amount is expected than in humid environments.

Of course, in many locations, water is limited. So, that’s why scientists study the timing of irrigation and yield results. Farmers then apply this information to make best decisions for their fields. They “scout” their fields, testing the grains to evaluate if they are more milky, or starchy. Wheat is even sensitive to breaking down the starches produced (into sugars) if growers aren’t careful about their water management plans. Once wheat seed grains feels soft and springy, the potential yield loss from water stresses is reduced.

Photo of a soybean plant with small flowers as well as tiny soybean pods.
Shown, the R3 stage, a begin-pod stage in soybean. Knowing the stage of pod development helps growers know when to irrigate. Credit: J.A. Torrion

Another crop example is soybean. Soybeans are deep-rooted, with a taproot that extends at least one-half inch per day! When they reach the begin-pod stage (R3, see photo), roots have already grown at least 3-feet deep. When farming at a dry and low-rainfall environment on a sandy soil, irrigation may be needed before R3 stage. But, in sub-humid areas with rainfall events from crop establishment before R3 and planted on fine-textured soil, waiting until the R3 stage to irrigate is proven not to lose any yield. Using this strategy, we observed that soybean has a much more controlled leaf growth. Too much leaf growth in soybean can be counterproductive.  Also, constant wetting of petals at the flowering stage – in any crop – can increase the incidence of diseases.

Waiting until R3 to irrigate in sub-humid to humid locations, we use a caveat: irrigation ‘catch up’ is required to fill the soil. At this stage, it is common that a crop uses an estimate of 0.20-0.40 inch per day depending on your geographic location. Irrigation catch-up, of course, is minimized with the occurrence of rainfall.

The takeaway for soybean is this: too much early-watering produces large leaves, which are hard for the plants to maintain later on. Plants end up using large amounts of energy to maintain the accumulated leaves than for making seeds.  In wheat, too much watering later in the season does not increase yield or quality.

Have you grown a potted tomato in your backyard? In my experience, I love watering them because I want my tomatoes to be lush and as big as possible. However, too much water can be a problem. My tomato plants can get so big, with large leaves, I can’t catch up with watering especially when the day is so hot and dry. They wilt when I get home late in the afternoon. It might be a better idea for me to allow my tomatoes to have less water at the initial growth stage – and then the smaller plants will have a more steady use of water while I’m at work! Applying my research knowledge of water management in crops to my home garden will result in fewer, but tastier and healthier tomatoes!

Answered by Jessica A. Torrion, Montana State University

Watch this video Managing Water in Gardens and Farms – Agronomy Feeds the World  to learn more.

About us: This blog is sponsored and written by members of the American Society of Agronomy and Crop Science Society of America. Our members are researchers and trained, certified, professionals in the areas of growing our world’s food supply while protecting our environment. We work at universities, government research facilities, and private businesses across the United States and the world.

4 replies »

  1. That’s good to know that if you water your plants too much too early they can grow to many leaves. I would have thought that was a good thing, but I can see why you would want the plant to focus on growing the thing you want it to grow, instead of leaves. I could also see why if there are too many leaves that could cause there to be not enough resources for the plant to sustain them.

  2. yes. good. very nice paper. It i not only when to irrigate but also how much to irrigate and which crop
    Principal Scientist
    Water-Climate-soil-plant relations

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