Sweet Alyssum in Zone 4a & 4b
Lobularia maritima
Yes, sweet alyssum grows in zones 4a and 4b. Start seeds indoors Mar 27–Apr 3 and plant out May 15–May 22.
When to plant sweet alyssum in zone 4b
Zones 4a and 4b share the same average frost dates, so this page covers both.
Method: Either: Start indoors for an earlier harvest, or direct-sow outside for less fuss. Either works.
| 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.
Plant alongside it in zone 4b
These go outside within two weeks of sweet alyssum in zone 4b, so one planting day can cover them.
- NasturtiumMay 22–May 29
- MarigoldMay 22–May 29
- African DaisyMay 15–May 22
- Amaranthus (Love-Lies-Bleeding)May 15–May 22
- Ammi (False Queen Anne's Lace)May 15–May 22
- Ammobium (Winged Everlasting)May 15–May 22
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 bloom.
Sweet Alyssum planting windows in every zone
Zone 4b is highlighted. Useful for comparing against a neighbouring zone, or for a garden somewhere else.
| Zone | Start indoors | Sow or plant out | Bloom |
|---|---|---|---|
| 2a & 2b | Apr 1–Apr 8 | May 20–May 27 | Jun 29–Jul 6 |
| 3a & 3b | Apr 11–Apr 18 | May 30–Jun 6 | Jul 9–Jul 16 |
| 4a & 4b (your zone) | Mar 27–Apr 3 | May 15–May 22 | Jun 24–Jul 1 |
| 5a & 5b | Mar 22–Mar 29 | May 10–May 17 | Jun 19–Jun 26 |
| 6a & 6b | Mar 7–Mar 14 | Apr 25–May 2 | Jun 4–Jun 11 |
| 7a & 7b | Feb 20–Feb 27 | Apr 10–Apr 17 | May 20–May 27 |
| 8a & 8b | Feb 4–Feb 11 | Mar 25–Apr 1 | May 4–May 11 |
| 9a & 9b | Dec 28–Jan 4 | Feb 15–Feb 22 | Mar 27–Apr 3 |
| 10a & 10bFall window | Aug–Jan | Oct–Mar | ~40 days after transplanting |
| 11a & 11b | Nov 13–Nov 20 | Jan 1–Jan 8 | Feb 10–Feb 17 |
Bloom dates are about 40 days after the sow or plant-out date, not after starting seeds indoors.
Zone 2 sows before zone 3 here, which looks wrong and is not. USDA zones rank winter cold, not season length, and zone 2 is mostly interior Alaska, where hard winters come with warm summers. More on what a zone measures.
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
High Mowing Organic Seeds (sweet alyssum seeds)
Varieties we recommend, and where to get them:
- 'Snow Crystals': University of Wisconsin-Madison Extension describes this tetraploid variety as vigorous with a mounding habit, large white flowers, and improved heat tolerance over standard sweet alyssum, useful if your bloom window needs to stretch through a hot summer to keep attracting hoverflies. Buy: True Leaf Market Full notes
- 'Easter Bonnet' series: An early-blooming series Wisconsin Extension lists in Deep Pink, Deep Rose and Violet, at 4-10 inches tall. Buy: True Leaf Market Full notes
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 · 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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