Should Farmers Adjust Irrigation After Heavy Rain?

By Blake Jackson

Determining whether irrigation schedules should be adjusted after significant rainfall remains a common challenge for vegetable growers using plastic mulch systems.

According to Penn State Extension specialists Elsa Sánchez and Tom Butzler, rainfall movement beneath plastic mulch is unpredictable, making it difficult to estimate how much water actually reaches crop roots.

Soil moisture sensors can help growers make more informed decisions by providing real-time information about moisture levels in the field.

In many vegetable production systems across Pennsylvania and the Mid-Atlantic region, crops are grown on plastic-covered beds with drip irrigation.

Traditionally, rainfall is not included in irrigation calculations because plastic mulch prevents most rain from directly entering the root zone. This approach generally works well, especially during light to moderate rainfall events.

However, changing weather patterns have brought longer dry periods followed by intense storms, prompting growers to reconsider whether heavy rainfall can contribute meaningful moisture beneath plastic mulch.

Water naturally moves downward through gravity and sideways through capillary action. During heavy rain events, water entering uncovered areas such as row middles, bed edges, and planting holes may travel laterally beneath the plastic and reach plant roots.

The degree of water movement depends on factors such as soil texture, rainfall intensity, field conditions, crop type, and plant growth stage. Clay soils generally allow more sideways water movement than sandy soils.

As crops mature, expanding root systems may also access moisture that has moved under the mulch from surrounding areas.

Despite these possibilities, rainfall contributions remain highly inconsistent and difficult to predict. For this reason, drip irrigation continues to be the most dependable method of supplying water to crops.

Proper irrigation management is essential because overwatering can lead to fruit cracking, disease development, and nutrient losses, while underwatering can reduce yields.

Soil moisture sensors offer a practical solution by helping growers monitor moisture within the active root zone.

For best results, sensors should be installed at multiple depths and used to track trends over time, ensuring irrigation decisions are based on actual field conditions rather than assumptions.

Photo Credit: gettyimages-dmytro-diedov