Greens are mostly a cool-weather crop. With consistent moisture, they thrive during short, cool days and cold nights. When these conditions occur in your garden depends on your planting zone. (The USDA Plant Hardiness Zones will help you learn about how plants may react to conditions in your area.
Kale, Collard Greens & Swiss Chard
Planting: Put seeds and transplants out at least six weeks before frost. In mild winter climates, transplants and seeds can go out eight to 10 weeks before the first frost. Space kale and collards 24 inches apart; chard 8 to 10 inches apart, either by seed or transplant.
Maintenance: Kale and collards are fairly pest-free, enjoy well-nourished soil and can withstand a frost. A heavy freeze will kill kale and chard, but in mild winter climates they will keep on producing through winter. Collards are the hardiest of the greens and will continue to grow beyond the frost, through winter and on into springtime. Mulch plants well for best overwintering results.
Harvesting: Harvest larger outer leaves for cooking and young, tender leaves for salads.
Turnip & Mustard Greens
Planting: The seeds of turnip greens (the leafy part of a turnip plant) and mustard greens can be broadcast into a prepared bed and lightly raked in about two months before the first frost. Water well to ensure good germination and continue to water as plants emerge. They make a great winter cover crop. (Read about cover cropping in All About Cover Crops.)
Harvesting: Harvest mustard greens by the single leaf. Turnips can be grown for both their greens and their roots. Many people harvest the greens while they wait for the roots to develop. To protect the health of the root, don’t harvest all the leaves at once. Instead, harvest single leaves or snip high enough above the soil to leave tiny new leaves intact.
Lettuce & Spinach
Planting: Lettuce and spinach can be sown by seed directly into the garden or transplanted. Both plants have shallow root systems and need consistent moisture. Seeds sown directly into the soil bed should be thinned to 4 to 6 inches apart after their first true leaves come in. True leaves have the shape of the mature plant’s leaves and come in after the first set of rabbit ear-shaped leaves. To grow a full lettuce head, plants should be 12 inches apart.
Maintenance: Both plants are easy to care for and require only light weeding. Pests may be a problem; consider light row covers to keep animals away. Spinach is cold-hardier than most lettuces and can survive chilly weather, especially with help from row covers.
Harvesting: Harvest (and plant) salad greens depending on your desired end product. Plants can be harvested by the outermost leaf. This reduces their likelihood of bolting and encourages them to put more energy into new growth. Leaves can be harvested in aggregate, taking all the leaves on the plant at once but leaving the stem and root intact (this will result in the plant growing another flush of leaves). Alternatively, let lettuce develop until it forms a full head, then harvest it in one piece using a sharp knife to cut below the lowest leaves, or pull the plant out by the roots.