Cultivation of Costmary
Site: Costmary grows best in full sun and a light, dry but fertile soil. It's a perennial that can grow up to almost 3 foot and it spreads similar to mint. If you can manage to give it a sunny corner in your garden it will be happy, and one plant will be plenty. Many gardeners complain it's invasive, which it can be. If you deadhead the blooms before they go to seed and pull up any roots that spread, it can be kept under control. I do this with my mint too, and as long as I stick to the maintenance it's fine.
Propagation: Costmary is hardy to Zone 4 and is propagated by root division. The seed is not readily available, so it's best to buy a plant.
Growing: You can grow it in shade, but it will become leggy and won't bloom. I've seen quite a few resources labeling it a shade herb, but it really won't do its best in a shady location. It can be cut back to keep it from getting to full or tall.
Harvesting: The young tender leaves can be gathered and used fresh at any time during the season, but for winter use they should be harvested when the plants begin to flower, and should be dried rapidly in a well-ventilated, darkened room. If the leaves are at all dusty or gritty, they should be washed in cold water and thoroughly drained before drying.
Culinary Uses: Usually used for medicinal purposes, but can be used as a flavoring in soups (especially carrot soup), stews, and salads. Use only in small amounts as it has a sharp tang. Use it to clear, preserve and flavor beer.