Cool season annual vegetables such as brassicas (broccoli, cabbage, etc.) and leafy greens typically root at shallow depths and need to be irrigated frequently for shorter periods of time – just long enough to moisten the root zone. Most warm-season annuals such as tomatoes and vine crops (squash, cucumber, melon, etc.), as well as perennials, have deeper, more extensive root systems. Irrigate deeper-rooted crops more heavily, but less frequently to encourage their roots to grow deeper.
Many crops, especially in humid climates, prefer to be watered at the base of the plant to discourage bacterial and fungal leaf diseases. These include beans, cucumbers, eggplant, gourds, squash, melons, peppers, tomatoes, okra, onions, and potatoes. Most flowers, herbs, and vegetables (including carrots, beets, chard, radishes, turnips, leeks, rutabaga, and parsnips) can be overhead or bottom watered regardless of climatic conditions.