Lettuce, spinach, and the other leafy vegetables require frequent irrigations for optimal quality. As many as 8-10 irrigations and 10-12 inches of water per acre may be necessary depending on seasonal variation, variety, and planting date. Soil type does not affect the amount of total water needed, but does dictate frequency of water application. Lighter soils need more frequent water applications, but less water applied per application.
If available, use drip irrigation to improve marketable yield and performance and to reduce irrigation inefficiencies associated with other methods. Since most of the leafy green vegetables have shallow root systems, drip is often a more efficient way to irrigate. Most of the leafy green vegetables are extremely sensitive to water stress. Critical times when irrigation can aid productivity is when establishing the crop (seeding or transplanting), during rapid leaf sizing, and right up until harvest. Different irrigation methods are commonly used to irrigate leafy green vegetables, each with different management considerations. Historically, furrow irrigation was the common method of choice. Furrow irrigation results in large fluctuations in soil moisture, nutrient leaching, and low water use efficiency. Sprinkler irrigation is slightly better than furrow, but not as good as drip.
Plastic mulches conserve water and reduce weeds. White-on-black plastic mulch (white one side; black on the other) can be used for growing transplanted lettuce and other leafy green vegetables. White-up helps keep the soil temperature several degrees cooler which can reduce bolting in high temperatures. Blackup may have a place for very early or late season crops and for use in high tunnels in winter when extra heat can help with growth.