If your vegetable patch is constantly moist, growing plants that thrive in wet conditions improves your chances of success. Excess water causes rotting and encourages root diseases in most vegetables, but varieties that were originally aquatic or marginal plants thrive in very moist soil. Slugs and snails also love moist soil, however, so go on patrol after dark or before dawn with a flashlight to find and dispose of these annoying pests.
Vegetables that thrive in waterlogged soil include watercress (Nasturtium officinale) and taro (Colocasia esculenta). Hardy in U.S. Department of Agriculture plant hardiness zones 3 through 11, watercress grows best in slowly moving water about 2 inches deep, but can thrive in waterlogged soil in a shady position. Growing 20 inches tall and 39 inches wide, this perennial plant sprouts new plants where its leaf nodes touch the soil and can be harvested year round. Taro is a perennial vegetable in USDA zones 8 through 10, and grows 3 to 6 feet tall and wide. Thriving in water up to 6 inches deep, taro provides edible tubers, shoots and young leaves.
Constantly wet roots don't affect American groundnut (Apios americana) or arugula (Eruca versicaria subsp. sativa). A perennial climbing plant, groundnut is hardy in USDA zones 4B through 8B and grows up to 4 feet tall. Edible tubers appear along its long thin roots, which take two years to grow to a usable size. American groundnut bears brown, bronze or maroon spring flowers and can be invasive in its favored moist conditions. Arugula is an annual vegetable that provides salad leaves and edible flowers. Grown from seed sown in winter or early spring, arugula grows 24 to 36 inches tall and 12 to 18 inches wide and can self-seed around the garden.
Some vegetables only grow steadily and healthily in permanently moist soil. Celery (Apium graveolens var. dulce) grows best in full-sun sites and rich, constantly moist soil. Usually grown as an annual plant, celery is a perennial that's hardy in USDA zones 3 through 6 and is invasive in some areas. Another plant that thrives in constant moisture is garden pea (Pisum sativum var. sativum). An annual vegetable, garden pea provides edible pea shoots as well as peas in their pods. Although garden pea grows well in moist soil, pea seeds rot in cold soil, so wait until late spring before sowing or sow smooth-seeded varieties, which resist rotting.
Vegetables in the brassica family grow best in soil that never dries out. To form large, firm heads, cauliflower (Brassica oleracea Botrytis group) requires constantly moist soil, and related plants such as cabbage (Brassica oleracea Capitata group) also thrive in damp conditions. Brassicas are grown as annual, cool-weather plants. They can't tolerate heat or drought and are usually sown during early summer and ready for planting out during late summer or fall. Because brassicas suffer from a range of soilborne diseases, they should be planted in a new site each year.