When to plant sweet alyssum in Billings, Montana
April 26 – May 3
Transplant outdoors · typical Billings yearClimate basis: NOAA station Billings Wtp, MT (USC00240802), 13 mi (21 km) from downtown Billings (ZCTA 59101 centroid), elevation 3097 ft, on the 1991–2020 climate normals. Transplant sweet alyssum outdoors from approximately the dates above, anchored to the median last spring 32°F freeze (May 10). One year in ten still freezes after May 24.
The Billings year
- indoors under lights
- in the garden
- frost-free season
- 1-in-10 years still freeze to May 24
Every method, with its sources
Start seeds indoors
Mar 29 – Apr 12Colorado State's seed-starting table lists sweet alyssum (Lobularia) at 6 weeks before the average frost-free date in the 'Indoors' column, the only indoor figure found that is anchored to frost rather than to the outdoor planting date, so indoor sowing opens 6 weeks before the median last freeze. Anchored to the median last spring freeze (May 10). Colorado State University Extension
Transplant outdoors
Apr 26 – May 3Wisconsin Horticulture puts hardened-off transplants in the garden a week or two before the average last frost, which starts later than Illinois' four-weeks-before hardy-annual rule, so the more frost-conservative Wisconsin interval governs. Anchored to the median last spring freeze (May 10). University of Wisconsin-Madison Division of Extension (Wisconsin Horticulture) · Clemson Cooperative Extension Home & Garden Information Center · Colorado State University Extension · University of Illinois Extension · Clemson Cooperative Extension Home & Garden Information Center
Direct-sow outdoors
Apr 12 – Apr 26Illinois Extension names sweet alyssum in its hardy-annual list and quantifies those at about four weeks before the frost-free date, and Clemson corroborates the direction by saying seed should be directly sown several weeks before the last frost date, so direct sowing opens 4 weeks before the median last freeze. Anchored to the median last spring freeze (May 10). Clemson Cooperative Extension Home & Garden Information Center · Clemson Cooperative Extension Home & Garden Information Center
Variety assumption: Assumes seed-grown Lobularia maritima, which Clemson notes flowers within 6 to 8 weeks of sowing and may stop blooming as summer heat rises; sources disagree on hardiness — Illinois and NC State call it a hardy annual while Clemson lists it as half-hardy, so the more cautious semi-hardy class is used.
These windows describe typical years (1991–2020 climate normals), not this year's forecast. Freezes happen outside them — especially in frost pockets, valleys, and urban edges. Watch the forecast before committing tender plants.
Crop rule sources (5)
- Colorado State University Extension: Growing Annual Plants from Seed (accessed 2026-07-25)"Table 1: Starting times for seeds grown indoors and in cold frames, shown as number of weeks to start seed before average frost-free date."
- Clemson Cooperative Extension Home & Garden Information Center: Sweet Alyssum (accessed 2026-07-25)"Sweet alyssum can easily be grown from seed and should be directly sown in the garden several weeks before the last frost date in spring."
- Clemson Cooperative Extension Home & Garden Information Center: Growing Annuals (accessed 2026-07-25)"Half-hardy annuals – such as allysum and dianthus -can tolerate light frost and are usually planted in early spring for spring and early summer color."
- University of Wisconsin-Madison Division of Extension (Wisconsin Horticulture): Sweet Alyssum, Lobularia maritima (accessed 2026-07-25)"Transplants that have been hardened off can be placed in the garden a week or two before the average last frost, otherwise tender transplants should be protected until add danger of frost is past."
- University of Illinois Extension: Annuals — Flowers (accessed 2026-07-25)"Ornamental cabbage and kale, pansy, snapdragon, sweet alyssum, calendula and dusty miller"