Making Drip Irrigation Decisions After Rain in Plasticulture

One ongoing challenge is deciding how, or even if, irrigation scheduling should be adjusted after heavy rainfall events. Rain moving under plastic mulch is inconsistent and difficult to predict. Soil moisture sensors provide a practical, field-based way for assessing whether rainfall has contributed to crop water needs.

Why Rainfall is Usually Ignored in Irrigation Scheduling, and Why that Still Makes Sense

In Pennsylvania and across the region, many growers plant vegetables in plastic-covered beds with drip irrigation. In these systems, irrigation scheduling typically does not account for rainfall when deciding when to turn the system on (see the Mid-Atlantic Commercial Vegetable Production Recommendations guide ). That approach is sound: plastic mulch blocks most rainfall from reaching the crop root zone, especially during light and moderate rain events.

However, weather patterns are shifting. Many growing seasons now include prolonged dry spells during the early and mid-season, followed by episodes of intense rainfall in the late season. This raises the question: Does intense rainfall meaningfully contribute water to crops grown under plastic mulch?

How Water Can Move Beneath Plastic Mulch

When water enters the soil, it moves in two directions:

• Downward by the pull of gravity
• Laterally by capillary action

Capillary action allows water to move through soil pores, even against gravity, similar to how water travels up a paper towel partially dipped in liquid. Once soil pores near the soil surface are filled, excess water can move sideways, and if conditions are right, underneath plastic-covered beds.

The extent of this lateral movement depends largely on soil texture. Soils with higher clay contents have smaller pores and can support greater lateral water movement than sandier soils. The Natural Resources Conservation Service has soil maps that can help understand soil texture at the individual farm level. They are available at usda.gov.

This Purdue Extension article includes visual examples of wetting patterns across different soil types.

What Research Tells Us About Rainfall Movement Under Plastic Mulches

Plastic mulches shed most rain from the bed surface, so water does not infiltrate evenly into the soil beneath the plastic. However, research indicates that rainfall can still indirectly contribute some moisture to the root zone, primarily through uncovered areas such as row middles, bed edges, or planting holes. Water entering the soil in these areas may move laterally underneath the plastic mulch, particularly during intense rainfall events, although the extent of this movement is highly variable. Modeling studies show that plastic mulch alters the direction and distribution of soil water movement compared to bare soil, helping explain why rainfall effects under plasticulture systems are often inconsistent from field to field (Kader et al., 2021). Overall, while rainfall can supplement soil moisture under plastic mulch, its contribution is difficult to predict and is generally less reliable than drip irrigation, which remains the most consistent method for delivering water to the crop root zone.

Why Timing and Crop Stage Matter

Later in the growing season, the effect of rain moving under plastic-covered beds can become more relevant. As crops grow, root systems expand outward and downward, often reaching the edges of the plastic mulch or beyond it. At this stage, roots are well positioned to access water entering the soil from areas outside the bed, further increasing the chance that rainfall contributes to plant-available water.

That said, the amount of rainfall reaching the root zone remains highly variable. It depends on many factors, including soil texture, rainfall amount and duration, topography, plant type, plant growth stage, and more. Even on a single farm, rainfall effects under plastic mulch can differ substantially from field to field. As a result, there is no universal threshold or guideline for reliably counting rainfall when making irrigation scheduling decisions.

Why Accounting for Soil Moisture Still Matters

Over- and underwatering can affect crop performance and quality. Excess water can contribute to problems such as:

• Reduced fruit quality (for example, fruit cracking)
• Increased risk of some diseases during wet periods (for example, diseases caused by Phytophthora capsica)
• Nutrient leaching below the root zone (here are some related articles: What is the Potential for Nitrogen Losses from Extreme Summer Rainfall? ;  Nutrient Management to Improve Nitrogen Efficiency and Reduce Environmental Loss).

At the same time, underwatering can limit the potential yield of crops.

Using Soil Moisture Sensors to Support Irrigation Decisions

Using moisture sensors provides a way to ground irrigation decisions in what is happening in the soil after intense rainfall, rather than relying on estimates. Michigan State University provides practical guidance in Utilizing Soil Moisture Sensors for Efficient Irrigation Management.

Key recommendations for vegetable production include:

• Placing sensors within the active root zone
• Placing sensors at multiple depths (shallow, mid, and deep in the active root zone) to track moisture dynamics
• Interpreting trends over time rather than relying on a single reading

Sensors placed only at the edge of plastic-covered beds should be considered supplemental, not standalone. The edge of the bed is more sensitive to rainfall. Measurements at the edge are not representative of the soil moisture throughout the root zone. Relying solely on sensors at the edge of the bed can result in under- or overwatering plants.

Bottom Line

Intense rainfall can contribute to plant-available water in plasticulture systems. However, the extent of that contribution is unpredictable. Soil moisture sensors can be used to assess when rainfall actually changes soil moisture in the crop root zone, allowing for more confidence in irrigation decisions.