All of the brassicas require regular, uniform watering during the growing season. Water shortages during establishment can limit seed germination, transplant establishment, and early growth. Inconsistent watering around heading set can cause misshapen, rough broccoli or cauliflower heads, induce splitting or tip burn in cabbage, and affect flavor. Over-watering wastes water and encourages root rots and foliar disease. Sprinklers are regularly used to germinate seed or establish transplants. Then after the first cultivation, you can switch to furrow irrigation. For small production areas it is common to use drip irrigation.
Soil water status should be monitored regularly to maintain consistent, uniform water supply. Use soil moisture monitoring sensors and weather based irrigation scheduling to monitor plant needs. Place sensors at various locations in the field and at several depths in the soil profile to get an accurate measure of soil water content. Start irrigating at 20-25% depletion when irrigating by drip and at 35-45% depletion for furrow or sprinkler systems. Water extraction estimates, using reference evapotranspiration adjusted with a crop coefficient (kc), are closely related to row canopy cover in the brassicas. The kc for a crop with 25% row cover is about 0.3, for 50% cover 0.6, and 85% cover the kc is 1. Note that irrigation also depends on your soil type (Table 4.4). Other low cost tools and methods to monitor soil water can be found at attra.ncat.org/ attra-pub/soil_moisture.html.
Soil Tension Values for Different Soil Textures
|Soil Tension Values (centibars)|
|Sand, loamy sand||5-10||17-22||25-30|
|Loam, silt loam||15-25||25-30||40-50|
|Clay loam, clay||20-40||35-45||55-65|
Use in scheduling drip irrigation, based on various percentages of depletion of available water holding capacity (Field capacity).