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Irrigation strategies for vegetables

Key points

  • Consistent soil moisture is critical to ensuring high yields and preventing problems in fruit and vegetable crops.
  • Taking the time to monitor soil moisture and develop an irrigation plan is important for farm resilience.
  • Maintaining irrigation and constant moisture in the upper soil profile can help to prevent soil crusting as seeds are germinating. 
  • Consistent moisture during periods of heat stress can help to mitigate the impacts of heat.

The importance of consistent moisture for horticultural crops

Consistent soil moisture is critical to ensuring high yields and preventing physiological problems in fruit and vegetable crops. Many crops are severely impacted by fluctuating water levels, both in terms of yield and disorders that make crops unmarketable.

Common vegetable problems related to moisture stress

Blossom end rot in tomatoes and peppers

Tomatoes on a wooden table with corky, black blossom ends.
The corky, black blossom ends on these tomatoes are symptoms of blossom end rot.
  • Blossom end rot (BER) is a calcium deficiency but is not necessarily related to the amount of calcium in the soil.
  • BER is exacerbated by soil moisture fluctuations.
  • The first fruit tends to be most severely impacted.
  • More information on blossom end rot.

Splitting in tomatoes

  • Splitting is caused by uneven cell division rates in the inner and outer tissues of fruit and is driven by uneven moisture.
  • Splitting allows for secondary pathogens and opportunistic insects to colonize fruit.

Hollow heart in potatoes

  • Potatoes with hollow heart have a hollow center, often surrounded by brown tissue. The cavity is often star-shaped, or an abstract shape with straight edges rather than circular.
  • Hollow heart occurs when there are abrupt changes in growing conditions
  • Most commonly, hollow heart is attributed to fluctuating water levels, especially droughts followed by heavy rain.
  • Water stress followed by a significant increase in moisture causes tubers to develop too quickly, causing the center tissues to pull apart. 
  • Hollow heart does not pose a food safety issue, it is simply a cosmetic problem. Depending on your markets, it may be a small nuisance, or it could mean discarding your crop.

Hollow stem in Brassica crops

  • Hollow stem in Brassica crops is complex and caused by many interacting factors.
  • Inconsistent moisture may exacerbate hollow stem, but it is rarely the only cause. Improving moisture consistency can help to reduce the severity of hollow stem, but should be combined with other management strategies.
  • Other risk factors for hollow stem include fast-growing cultivars, too much nitrogen and potassium, wide plant spacing and, in some cases, boron deficiency.
  • Hollow stem symptoms look very similar to the symptoms of soft rot in broccoli and collards; check with your local Extension educator or plant disease clinic if you are seeing symptoms.

Poor germination

  • All seeds need consistent moisture for good germination.
  • Small seeded crops like beets and carrots are especially sensitive, especially in clay soils that are prone to crusting.
  • Maintaining irrigation and constant moisture in the upper soil profile can help to prevent soil crusting as seeds are germinating. 
A pepper plant against a blue sky showing dried up flowers.
The blossoms on this pepper plant are dried out and ready to drop off.

Woody, flavorless carrots

  • Carrots become woody when they are water-stressed.
  • Heat stress also causes woodiness; consistent irrigation can help to cool the soil. 
  • Carrots are susceptible to discoloration and rot with excessive moisture. 

Multicentered bulbs in onions

  • Yield is reduced in onions if they experience water stress at any stage.
  • Onions have shallow root systems and require more frequent irrigation.
  • Water stress earlier in the growing season results in more multicenter bulbs. 

Blossom drop and fruit abortion

  • Heat stress is the primary cause of other common problems such as blossom drop or fruit abortion in tomato, cucumber and other cucurbits.
  • Consistent moisture during periods of heat stress can help to mitigate the impacts of heat. 
Hand holding a green tomatillo that has split down the center with a picnic bug in the split area.
Splitting on a tomatillo
A potato that has been cut in half to show the white inner flesh; in each half, the center of the potato is hollow.
Hollow heart in potato
Broccoli head cut in half the long way on a metal surface; the stem is hollow and slightly brown in the hollow areas.
Hollow stem in broccoli

Weather and climate trends in Minnesota

Based on trends over the past 100 years, Minnesota climatologists predict the following trends in our weather: 

  • Highest likelihood: More rainfall overall, and more intense rain events.
  • High likelihood: Longer, more intense heat waves, more days between rain.

These predictions have important implications for vegetable growers. 

  • More intense rainfall events can cause flooding or waterlogging on farms and can make it difficult to get into the field on time in the spring, or harvest crops on time in the fall.
  • Heatwaves can cause plant stress with impacts to crop yield and marketability.
  • The combination of these factors makes maintaining consistent soil moisture a significant challenge.
  • Taking the time to monitor soil moisture and develop an irrigation plan is important for farm resilience. 

More information about Minnesota climate predictions.

The importance of timing

Supplemental irrigation in vegetable crops is beneficial because rainfall is erratic and rarely uniformly distributed. Even in a year when we get above average precipitation, which is very common in Minnesota, sometimes it does not happen in the critical growth period of the crop. Supplemental irrigation is required to mitigate the yield loss risk.

While consistent moisture is important throughout the lifespan of all vegetables, each crop has critical periods during which moisture is especially important.

In almost all cases, the critical period is during flowering and fruit or pod formation for flowering crops, during head formation for head forming crops, and during bulb or root enlargement for bulb and root-forming crops.

In many cases, these critical periods correspond with the hottest, driest part of the summer.

Critical growth period for irrigation requirements in vegetable crops

Crop Most critical period
Broccoli, cabbage, cauliflower, lettuce head development
Carrot, radish, beet, turnip root enlargement
Sweet corn silking, tasseling, and ear development
Cucumber, eggplant, pepper, melon, tomato flowering, fruit set, and maturation
Bean, pea flowering, fruit set, and development
Onion bulb development
Potato tuber set and enlargement

Table source: Midwest vegetable production guide for commercial growers

In addition to the critical periods, it has been observed that moisture deficiencies occurring early in the crop cycle may delay maturity and reduce yields. Shortages later in the season often lower quality as well as yields.

In addition to under-watering, too much water in vegetable crops, especially late in the season, can also reduce the quality and postharvest life of the crop. Applying excessive water wastes water and increases the risk of nutrient leaching.

In-season irrigation management strategies

Deciding when to irrigate to optimize production is a daily judgment call that requires you to consider several factors. Many of these factors change as the crop develops.

Below are some general guidelines to consider when developing a water management plan and setting allowable soil water deficit limits.

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Soil water monitoring

Soil water monitoring is an excellent method for vegetable growers. It is more precise than scheduling irrigation on a calendar basis or by feel and works well for growers with multiple crops. It is also simpler and less time consuming than calculating crop water demand or water budgeting.

Soil moisture sensors are divided into two categories depending on the technology they use:

  • Sensors that measure volumetric water content, or
  • Sensors that measure soil tension when placed in the soil profile.
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Authors: Natalie Hoidal, Extension local foods and vegetable production educator, and Vasudha Sharma, Extension irrigation specialist

Reviewed in 2020

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