Alyssum (Lobularia maritima) is classified as a short-lived perennial that is usually grown as an annual due to its poor second-year performance. Hardy in U.S. Department of Agriculture plant hardiness zones 4 through 9, alyssum can be started indoors to get a jump on the growing season or planted directly in the garden after the soil warms in the spring. Noted for the clusters of small fragrant flowers, varieties grow 4 to 16 inches tall, but they can spread about a foot, making them useful as bedding plants or when used to soften the hard edges of a rock garden. Alyssum blooms year-round in frost-free locations.
You can grow alyssum by planting the seeds outside three to four weeks before the last frost date for your area. Space the seeds about 6 inches apart to allow for spread and just press them lightly into the soil. Keep the seedbed moist until germination occurs. You can also start them indoors about 6 weeks before the last frost date. Press the seeds into the top of the growing medium but do not bury them, wet the soil and keep it moist but not saturated until growth appears. Keep the flats or pots in a bright spot at a temperature of 65 to 70 degrees Fahrenheit until the seeds germinate in about 14 days.
Transfer alyssum seedlings to the garden when soil temperature is at least 60 degrees Fahrenheit. Before placing them in their permanent location, harden off the seedlings gradually by leaving them outside in a protected area receiving partial sun for several hours each day for several days. This minimizes the shock of going from a cozy indoor environment to a harsher one where weather fluctuations are unpredictable. Established seedlings are readily available at most nurseries and garden centers. Harden them off the same way as you would any that you start yourself before setting them into the garden.
To plant seedlings, dig a hole in prepared soil that is as deep as the growing container. Remove the seedling from the pot or flat and place it in the bottom of the hole. If you started the seeds in peat pots, place the entire pot in the hole. Bring the soil gently to the base of the leaves and tamp it down lightly. Allow about 6 inches between each plant on all sides and water each at ground level.
Alyssum does best in average soil as long as it is well drained, and it grows in either full direct sunlight or in part shade. Before planting, enrich the soil with organic matter such as aged manure or mature compost, working it into the top 6 inches. In warm climates, plant alyssum in an area that receives some afternoon shade. Alyssum prefers moist soil but tolerates dry spells and blooms all summer long in cooler areas. After plants are established, apply 5-10-5, 5-10-10 or 10-10-10 fertilizer at a rate of 1 pound per 100 square feet of space. Work it in as a side dressing near and around the base of the plants and then water to distribute the fertilizer and prevent root burn.