Drought monitoring and irrigation for apples and grapes

The climate of the upper Midwest is variable and at times, extreme. This can include excess rainfall during one season and extended dry periods later in the same season or the next. 

Irrigation use is less common in apple and grape production than in berry production in the upper Midwest. Most Minnesota grape growers lack irrigation systems because they have historically been rarely needed.

Most, but not all, modern apple orchards in the upper Midwest have drip irrigation systems. Irrigation is crucial for apples every year to maintain tree health and can be beneficial for cold-climate grapes in extremely dry years, as well as for newly planted vines.

Grapes

Water availability impacts grapevine vegetative growth, fruit composition, and potentially fruit set and winter hardiness. Grapevine drought-stress symptoms are slow shoot growth, dried tendrils, drooping leaves, and yellowed or dried leaves.

Recommendations for monitoring and irrigating

During extended dry periods, consider irrigating when the top 2 to 3 inches of soil are dry. This can be done using a finger test for certain soil types, a shallow-probed moisture meter, or by observing visual cues. If you notice drooping leaves, this is another indication that the plants are severely water-deficient. 

The age of the grapevines will determine how much and how often they require irrigation. For new vines, a simple guideline is to irrigate 1 inch every 7 to 10 days. Mature vines require less frequent irrigation due to their more extensive root systems; irrigate 1 inch every 2 to 3 weeks. The recommendation is the same whether you are using an irrigation system or a watering tank.

Apples

Water stress decreases fruit size, limits root growth, and may restrict calcium uptake, leading to the development of bitter pit. Apples grown on dwarf rootstocks and trellised are more prone to drought symptoms due to their smaller root systems.

Some modern irrigation models, such as the Cornell Network for Environment and Weather Applications (NEWA ), use data from weather stations to estimate the daily water usage of apple trees. The model takes into account the age of the tree and its phenological stage (the extent of canopy development) to calculate the daily water usage. It also takes into account the amount of water that has entered the system through rainfall to calculate the total amount required.

Available water by soil type

In addition to rainfall, the volume of water stored in the soil must be taken into account when calculating irrigation needs. Different soil types have varied available water capacities.

• Sandy soils that drain well will have ~30,000 gal/acre of available water
• Loam will have ~80,000 gal/acre
• Clay soils will have ~110,000 gal/acre

If a mature tall spindle orchard consumes 4,000-5,000 gallons a day, it means that a sandy, loam, and clay soil will have 7, 20, and 27 days of water supply, respectively.

However, as water availability decreases, it becomes increasingly difficult for trees to use the water remaining in the soil, and the general recommendation is to start irrigating when 5,000 to 10,000 gallons per acre have been used. NEWA and other irrigation models help to calculate this.

Avoid allowing mature trees to reach the 10,000 gallons per acre water-use threshold. 

Young trees (1 to 3 years) have a smaller root system than mature trees and should be irrigated more frequently. Young trees should be irrigated when the cumulative water balance reaches 5,000 gallons per acre to avoid water stress. 

Resources for managing fruit crops during drought

The University of Nebraska National Drought Mitigation Center has a variety of resources for specialty crop growers to deal with drought. They include a set of production calendars designed to help fruit growers decide when and how to irrigate during dry years. One helpful feature of the calendars is that they associate each fruit phenological stage with the corresponding drought concerns during that time.