Sweet alyssum is a cool-season annual that is easy from seed. Sow it outdoors several weeks before your last spring frost, or start seed indoors 5 to 6 weeks before the last frost (NC State Extension). Hardened-off transplants can go out a week or two before the last frost (Wisconsin Horticulture). Plants usually flower within 6 to 8 weeks of sowing, slow down in summer heat and pick up again when it cools. For fall flowers, NC State suggests sowing in August, and in Florida UF/IFAS says you can plant from October until late spring.
Sweet alyssum (Lobularia maritima) is a low, mat-forming plant in the mustard family, about 3 to 9 inches tall (Wisconsin Horticulture), covered in tiny honey-scented flowers. NC State describes it as an annual or short-lived perennial. White is the classic color, and cultivars come in pink, rose, lavender, purple and apricot. White-flowered plants tend to be the most fragrant (NC State, Wisconsin Horticulture). For vegetable gardeners its main draw is what it feeds: Wisconsin Horticulture says the flowers are very attractive to bees, flower flies and tiny wasps, and that California organic lettuce and broccoli growers plant it to attract natural enemies of aphids.
Spring planting dates by last frost
NC State says to sow seed in the ground several weeks before the last frost, or start it indoors 5 to 6 weeks before and set seedlings out just before the frost date. Wisconsin Horticulture says hardened-off transplants can go out a week or two before the average last frost. "Several weeks" isn't a number, so the outdoor column is GardenPal's estimate:
| Average last spring frost | Sow outdoors (GardenPal estimate) | Start indoors (NC State: 5 to 6 weeks before) | Set out hardened plants (Wisconsin: 1 to 2 weeks before) |
|---|---|---|---|
| Mar 1 | Feb 1 – Feb 15 | Jan 18 – Jan 25 | Feb 15 – Feb 22 |
| Mar 15 | Feb 15 – Mar 1 | Feb 1 – Feb 8 | Mar 1 – Mar 8 |
| Apr 1 | Mar 4 – Mar 18 | Feb 18 – Feb 25 | Mar 18 – Mar 25 |
| Apr 15 | Mar 18 – Apr 1 | Mar 4 – Mar 11 | Apr 1 – Apr 8 |
| May 1 | Apr 3 – Apr 17 | Mar 20 – Mar 27 | Apr 17 – Apr 24 |
| May 15 | Apr 17 – May 1 | Apr 3 – Apr 10 | May 1 – May 8 |
| Jun 1 | May 4 – May 18 | Apr 20 – Apr 27 | May 18 – May 25 |
Summer slump and fall flowers
Like many brassicas, alyssum slows down in hot weather. Wisconsin Horticulture says plants look ragged in midsummer but revive as temperatures cool and last well into fall; a light shearing, watering and a little fertilizer help them recover. NC State says blooms decline in hot summers and suggests cutting plants back to keep them flowering. Where summers are cool, NC State says flowers can keep coming from spring to the first frost.
For a fresh fall display, NC State suggests sowing seed in August. In hot-summer areas, Wisconsin Horticulture says partial shade helps, and NC State notes alyssum appreciates some afternoon shade.
Florida and mild-winter areas
UF/IFAS treats sweet alyssum as a fall-to-winter bloomer in Florida, flowering from winter through spring, and says you can plant it from October until late spring. It adds that the old seed-grown types perform poorly there compared with newer heat-tolerant types grown from cuttings, such as the Easy Breezy series and Snow Princess. NC State and Wisconsin Horticulture also list Snow Princess as heat tolerant.
Site, soil and care
| Step | What extension sources say |
|---|---|
| Sun | Full sun in cool climates, part shade in hot ones (Wisconsin Horticulture); full sun to part shade (NC State); full sun, takes some shade (UF/IFAS) |
| Soil | Well-drained; NC State calls very good drainage a must |
| Water | Does best with consistent moisture, tolerates short dry spells (Wisconsin Horticulture); water regularly (UF/IFAS) |
| Feeding | Don't fertilize heavily, or you get leaves instead of flowers (Wisconsin Horticulture) |
| Shearing | Deadhead, or shear leggy plants and plants with more seedpods than flowers (Wisconsin Horticulture) |
| Self-sowing | May reseed, but not aggressively (Wisconsin Horticulture); seedlings from named varieties tend to be white and taller |
Problems to watch for
Most alyssum problems come from wet soil. NC State lists downy mildew, stem and crown rots, and decline in poorly drained soil, and Wisconsin Horticulture says plants that are too wet or crowded can rot. UF/IFAS calls it generally pest free and reportedly deer resistant.
Sweet alyssum in the vegetable garden
Alyssum's low, spreading habit makes it an easy edging or living mulch between crops (Wisconsin Horticulture), and its small flowers feed the hover flies and tiny parasitic wasps that help control aphids. Tucking it along the edges of lettuce and broccoli beds copies that commercial practice on a small scale (GardenPal suggestion). For the research behind this, see flowers that attract beneficial insects and companion planting for pest control. Wisconsin Horticulture suggests combining it with other short annuals such as snapdragons and shorter zinnias and marigolds; see when to plant snapdragons and, for another low cool-season edging, when to plant pansies.
Frequently asked questions
When should I plant sweet alyssum?
Sow outdoors several weeks before your last spring frost, or start indoors 5 to 6 weeks before it (NC State). Hardened transplants can go out a week or two before the last frost (Wisconsin Horticulture).
Can I plant sweet alyssum in fall?
Yes. NC State suggests sowing in August for fall bloom, and in Florida UF/IFAS says you can plant from October until late spring.
How deep do I plant sweet alyssum seeds?
Barely cover them. Wisconsin Horticulture says the seed benefits from light to germinate.
How long until sweet alyssum blooms?
Plants generally start flowering within 6 to 8 weeks of sowing (Wisconsin Horticulture).
Why does my sweet alyssum look bad in summer?
Heat. Wisconsin Horticulture says growth and flowering slow in hot weather and plants look ragged, then revive as it cools. Shear lightly and water to help them recover.
Does sweet alyssum help with aphids?
Its small flowers attract hover flies and tiny wasps, and Wisconsin Horticulture says California organic lettuce and broccoli growers plant it to attract natural enemies for aphid control.
Check the vegetables you're edging with sweet alyssum, and spot any poor pairings before you plant.
Open itSources
- Virginia Cooperative Extension, "Companion Planting in Gardening" (SPES-620P)
- University of Maryland Extension, "Vegetable Succession and Companion Planting"
- Oregon State University Extension, "Companion Planting"
- Penn State Extension, "Herbs Make Good Plant Partners and Companions"
- University of Minnesota Extension, "Planting the vegetable garden"
- USDA Plant Hardiness Zone Map
- NOAA NCEI U.S. Climate Normals (frost/freeze data)
- NC State Extension Gardener Plant Toolbox, "Lobularia maritima" (sweet alyssum)
- University of Wisconsin-Madison Extension, Wisconsin Horticulture, "Sweet alyssum, Lobularia maritima"
- UF/IFAS Gardening Solutions, "Sweet Alyssum"
Pairings are summarized in our own words from extension-service guidance. Research support for many companion effects is limited, so treat them as good habits, not guarantees.