Sweet Alyssum in Zone 4a & 4b
Lobularia maritima
When to plant sweet alyssum in zone 4b
Zones 4a and 4b share the same average frost dates in our data, so this page covers both.
Method: Either: Start indoors for an earlier harvest, or direct-sow outside for less fuss. Sweet Alyssum tolerates either.
| Start indoors | Mar 27–Apr 3 |
|---|---|
| Direct sow / transplant outside | May 15–May 22 |
| Estimated bloom | Jun 24–Jul 1 (~40 days after planting) |
Spacing: 3 per square foot (square-foot-gardening spacing). Mature height: 3-10 in.
Sweet alyssum is one of the better-documented companion flowers in this whole list, backed by actual published field research on why it's planted alongside crops like lettuce and broccoli. It stays low, a 4- to 8-inch spreading mat, and disappears under dense clusters of tiny, four-petaled white or purple flowers.
How sweet alyssum fits zone 4's season
Zone 4b's frost-free window runs about 128 days, from May 15 to September 20. Sweet Alyssum needs about 40 days from sowing to first harvest.
Good companions
- Broccoli: Sweet alyssum intercropping is documented to attract aphid-eating hoverflies, but the published research behind this (Brennan 2013) specifically tested lettuce, not broccoli — treat the broccoli extension as plausible but unconfirmed by that study.
- Lettuce: A published field study (Brennan 2013, Biological Control) found strip-intercropping lettuce with sweet alyssum reduces aphid pressure via hoverflies, whose larvae can eat up to 150 aphids a day — real, measured pest control, not folklore.
Where to buy sweet alyssum seeds
See why we recommend these suppliers →
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See it growing
Search YouTube for "growing sweet alyssum" videos → (a live search, not a hand-picked video; results vary and aren't vetted by us)
← Back to the full zone 4b calendar · Full companion planting guide · Full plant profile · Plan your beds in the Garden Designer →
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Growing Sweet Alyssum in USDA zone 4b: when to start seeds indoors, when to direct-sow, and when to expect a bloom.
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