Watercress

Sow seed about a quarter inch deep in a wet, organically rich mix roughly three weeks before your frost-free date, and never let it dry out. Germination wants cool, wet conditions around 50 to 60 F. You can skip seed entirely and start plants from stem cuttings instead. Watercress germinates in the cold and does its best growing in the cool seasons, so seed started at the front edge of spring gets rooted and bulked up before summer heat pushes it toward flowering.

After the last frost, set plants 8 inches apart in a sunny spot in wet, organically rich soil at a pH of 6.5 to 7.5. Without a stream, stand potted plants in a bucket or saucer holding 2 to 3 inches of water so the mix stays saturated from below. Watercress is a semi-aquatic plant with no drought tolerance at all - its roots need standing water or permanently saturated soil, and a pot sitting in water reproduces a streambed closely enough for a home garden.

Hold 2 to 3 inches of water in the tray, bucket or bed at all times and replace it once or twice a week rather than only topping it up. Never let the soil dry out, even briefly. The plant drinks constantly through summer and a saucer can run dry in a day, and in water that is never circulated the dissolved salts left behind as the crop feeds concentrate over time and check growth. Replacing the water resets both.

Look at the surface of the tray, tub or tank each week for wriggling mosquito larvae. Treat with a Bacillus thuringiensis subspecies israelensis (Bti) product, or keep small fish in a larger tank. Cool air and dry, windy conditions suppress mosquitoes on their own. Nutrient water that is never circulated or aerated is prime mosquito habitat, and a watercress bed is standing water by design - CTAHR calls the resulting mosquito populations both a health menace and a nuisance to workers.

Cut the plant back to about 4 inches tall with scissors rather than pulling it, and let it re-grow for the next cut. Harvest before the flowers open, and expect the best quality in the cool stretches of spring and fall. Cut stems keep in the refrigerator about a week. Watercress regrows from the stubble you leave behind, so cutting turns a single planting into a whole season of harvests, and leaves taken before bloom are the mild ones - once the plant commits to flowering it is putting its growth into seed rather than leaf.

Watercress has low nutrient needs, so feed only after a cut, using a complete soluble fertilizer at the label rate. In a tub or tank, refill the water first and dose based on the gallons you added. A second ratoon crop follows roughly three to four weeks later. The cutting carries off the leaf mass the plant built out of the water's nutrients, and in standing water both the solution level and its nutrient load fall steadily across the growing period, so regrowth stalls unless you put them back.