Jun 17, 2021

Irrigation recommendations for fruit crops during drought

The climate of the upper Midwest is variable and at times, extreme. Fruit growers find themselves managing the effects of excess rainfall one season and extended dry periods the next.

While berry growers use irrigation throughout every growing season, irrigation use is less universal in apples and grapes in the upper Midwest. For example, most Minnesota grape growers lack irrigation systems because they are so rarely needed, while others with drip tape report having never turned it on. Irrigation is very important for apples and can be helpful for cold climate grapes in extremely dry years and for newly planted vines.

Grapes

Water availability impacts grapevine vegetative growth, fruit composition, and potentially fruit set and winter hardiness. Symptoms include slow shoot growth, dried tendrils, drooping leaves, and yellowed or dried leaves.

During extended dry periods, the irrigation recommendations depend on the age of the vines. For new vines, a simple guideline is to irrigate 1 inch every 7-10 days. Mature vines require less frequent irrigation due to their more extensive root systems – irrigation 1 inch every 2-3 weeks. The recommendation is the same whether the grower is using an irrigation system or watering tank.

Apples 

These past two months have been unusually dry and have resulted in pressing needs for irrigation in high density apple orchards. Lucky for us we have a great tool to estimate water needs with the Cornell apple irrigation model available at the NEWA website.

The Cornell apple irrigation model uses weather data to estimate the amount of water apple trees use daily. The model takes in consideration the age of the tree and the phenological stage (how much of the canopy has developed) to calculate the daily amount of water used. The model also incorporates how much water has entered the system through rainfall to calculate the total amount needed. In addition to rainfall, the volume of water stored in the soil must be considered when calculating irrigation needed. Different soil types have different available water capacities. Sandy soils will have ~30,000 gal/acre of available water; while loam and clay soil will have ~80,000 gal/acre and 110,000 gal/acre, respectively. If we consider that a mature tall spindle orchard will consume 4,000-5,000 gal/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 is harder and harder for trees to use the water left in the soil, and the general recommendation is to start irrigating when 5,000 to 10,000 gal/acre has been used.

In 2021, rainfall has been insufficient in most of Wisconsin (between 2.5 to 4.5 inches of rain since May 1^st). The data from Madison (Table 1), shows that the cumulative water balance fell below 10,000 gal/acre in mid-May for a couple of days until we got some rainfall, but has been under 10,000 gal/acre since June 1^st and no rain is forecasted in the next 5 days (the model only allows 5 days of forecasting). Based on the recommendation to irrigate when cumulative water balance reaches 10,000 gal/acre, growers should have began irrigating by May 1^st.

Table 1. Daily and cumulative mature tall Spindle apple orchard water use at Verona, WI for the last 30 days.

This drought period could negatively impact fruit size, and if the drought continues through the month of July fruit size will be greatly affected. In addition, the low moisture content in soil will limit root growth and thus calcium uptake, which could result in higher incidence of bitter pit. Growers should use the Cornell apple irrigation model to irrigate trees before the cumulative water balance reaches 10,000 gal/acre to avoid water stress. Young trees (1-3 years) have a smaller root system than mature trees and should be irrigated more frequently. Young trees should be irrigated when cumulative water balance reaches 5,000 gal/acre to avoid water stress.

Resources for Managing Fruit Crops During Drought

The National Drought Mitigation Center has released a variety of resources for specialty crop growers to deal with drought. Find a full list of these resources here.

This includes 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 relate each fruit phenological stage to the drought concerns during that time.

Quick links:

Apple Decision Calendar

Grape Decision Calendar

Cranberry Decision Calendar

Perennial fruits drought monitoring resources fact sheet

The fact sheets also discuss how to use tools like soil moisture maps, soil temperature maps, the US Drought Monitor, and the evaporative stress index (ESI).

This is a project of the National Drought Mitigation Center and the University of Wisconsin, with the U.S. Department of Agriculture Midwest Climate Hub and the National Integrated Drought Information System.

– Annie Klodd and Amaya Atucha, University of Wisconsin