Green garlic is young garlic which is harvested before the cloves have begun to mature. The resulting vegetable resembles a scallion, with a deep green stalk and a pale white bulb. It can often be found for sale at a farmers' market in the spring, and can also be grown at home relatively easily.
When garlic is grown normally, it is harvested when the lower part of the stalk visible above the ground starts to turn brown. The bulb of the garlic has differentiated into garlic cloves, and it is cured so that it will last in storage. Typically, garlic is harvested in the middle of summer. Green garlic, on the other hand, is harvested before the plant is mature. The stalks are still totally green, usually around one foot (30 cm) long, and the bulb resembles that of a green onion, rather than a segmented head of garlic.
The flavor of green garlic is still garlicky, but is much more mild with less of a bitter bite. When cooked, the garlic sweetens, lending a new layer of depth to a dish. The whole plant, including the leaves, can be used. Some cooks use it instead of mature garlic or scallions for a different flavor in a favorite dish, and others invent entirely new dishes to showcase its mild flavor. It can be used raw or cooked in a broad assortment of cuisines.
Other names are sometimes used to market green garlic, including spring garlic and baby garlic. In all cases, you want to pick out sturdy crisp stalks which do not appear wilted, and you should check for mold and mildew on the garlic. The garlic can be stored under refrigeration for three to five days before being used, and it will not cure like regular garlic, so make sure to use it up.
To grow green garlic, set aside a small patch in the garden to plant with seed garlic, cloves of garlic picked from especially high quality plants. You can also grow garlic from cloves purchased at the grocery store, but many gardeners prefer the assurance of better quality offered by seed garlic. Plant the cloves in fall or spring, depending on whether or not the winter is cold, and mulch the garlic well to protect it from freezing. Harvest in the late spring, and enjoy.