Are You Over-Watering or Under-Watering Your Drip Irrigated Vegetables with Plastic Mulch?

Gordon Johnson, Extension Ag Agent, Kent Co.; gcjohn@udel.edu

One of the most common problems in drip irrigated vegetables with plastic mulch is over-irrigating or under-irrigating. The following are guidelines from the Delaware Commercial Vegetable Production Recommendations guide:

“Calculating the length of time required to apply a specific depth of water with a trickle irrigation system is more difficult than with sprinkler systems. Unlike sprinkler systems, trickle systems apply water to only a small portion of the total crop acreage. Usually, a fair assumption to make is that the mulched width approximates the extent of the plant root zone and should be used to calculate system run-times. Table C-4 has been prepared to calculate the length of time required to apply one inch of water with a trickle irrigation system, based on the trickle tube flow rate and the mulched width. The use of this table requires that the trickle system be operating at the pressure listed in the manufacturers specifications.”

Hours Required to Apply 1 Inch Water to Mulched Area (Table C-4 from the Delaware Commercial Vegetable Recommendations)

“On coarse-textured soils, applying an inch of water to the mulched width may not be effective. Because water is not held in large pore spaces, it moves below the plant root zone, carrying nutrients and pesticides beyond the reach of the plant roots. Table C-5 has been prepared to calculate the maximum recommended irrigation period for trickle irrigation systems. The irrigation periods listed are based on the assumption that 50 percent of the available water in the plant root zone is depleted. Soil texture directly influences the water-holding capacity of soils and, therefore, the depth reached by irrigation water.”

Maximum Irrigation Periods (Hours) for Trickle Irrigation Systems to Result in a Water Infiltration Depth of 12-18 Inches (Table C-5 from the Delaware Commercial Vegetable Recommendations)

Choosing the right emitter spacing for the crop that you are growing is very important. Melons grown on wide spacings are quite different than crops, such as peppers, grown on double rows with tight spacings. The emitter needs to be close enough to planting holes so that young transplants can survive without having to wet the bed so much that you cause leaching. In very sandy soils, this is even more important because the zone of wetting is much narrower.

As to how much to irrigate, during the initial growth phase after seedling establishment and up to the time that plants cover the beds, one inch or less of water per week is generally sufficient based on a mulched acre. As plants reach their full size, this will need to be increased based on the crop. Some crops will need over two inches per week at peak in mid-summer. Irrigation will need to be run multiple days per week to achieve this and on very sandy soils, this will require more management attention so as not to water out of the root zone. Irrigation can be twice daily, daily, every 2 days, or every 3 days depending on the soil type during peak growth.

Estimating crop needs based on evapotranspiration data will be necessary to determine the amount of time to run the drip system under plastic mulch, especially at peak usage. This is complicated by trying to assess the impact of rainfall and water infiltrating under the plastic from row middles. To better manage irrigation under plastic, it is recommended to install or use portable moisture sensors to determine moisture status of the plant root zone under mulch. The standard has been the tensiometer but there are several other moisture sensors that will work equally well or better. Irrigate when available soil moisture is depleted 30-50% based on sensor readings.