Cosmos in Zone 4a & 4b
Cosmos bipinnatus
Yes, cosmos grows in zones 4a and 4b. Start seeds indoors Apr 10–Apr 17 and plant out May 22–May 29.
When to plant cosmos 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 | Apr 10–Apr 17 |
|---|---|
| Direct sow / transplant outside | May 22–May 29 |
| Estimated bloom | Jul 31–Aug 7 (~70 days after planting) |
Spacing: 1 per square foot (square-foot-gardening spacing). Mature height: 2-6 ft.
Plant alongside it in zone 4b
These go outside within two weeks of cosmos in zone 4b, so one planting day can cover them.
- NasturtiumMay 22–May 29
- MarigoldMay 22–May 29
- Ageratum (Flossflower)May 22–May 29
- Annual Geranium (Pelargonium)May 22–May 29
- Butterfly Weed (Asclepias)May 22–May 29
- Campanula (Bellflower)May 22–May 29
Cosmos is one of the easiest flowers to grow from seed, and its main garden job here is pulling in pollinators and predatory insects near zucchini, a fine-textured, airy plant with feathery foliage and daisy-like pink, white, red, or orange blooms on thin stems that doesn't compete hard for space even at a full square foot per plant.
How cosmos fits zone 4's season
Zone 4b's frost-free window runs about 128 days, from May 15 to September 20. Cosmos needs about 70 days from sowing to first bloom.
Cosmos 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 15–Apr 22 | May 27–Jun 3 | Aug 5–Aug 12 |
| 3a & 3b | Apr 25–May 2 | Jun 6–Jun 13 | Aug 15–Aug 22 |
| 4a & 4b (your zone) | Apr 10–Apr 17 | May 22–May 29 | Jul 31–Aug 7 |
| 5a & 5b | Apr 5–Apr 12 | May 17–May 24 | Jul 26–Aug 2 |
| 6a & 6b | Mar 21–Mar 28 | May 2–May 9 | Jul 11–Jul 18 |
| 7a & 7b | Mar 6–Mar 13 | Apr 17–Apr 24 | Jun 26–Jul 3 |
| 8a & 8b | Feb 18–Feb 25 | Apr 1–Apr 8 | Jun 10–Jun 17 |
| 9a & 9b | Jan 11–Jan 18 | Feb 22–Mar 1 | May 3–May 10 |
| 10a & 10bSpring window | Not usually | Mar–Jun | ~70 days after planting |
| 11a & 11b | Nov 27–Dec 4 | Jan 8–Jan 15 | Mar 19–Mar 26 |
Bloom dates are about 70 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
- Zucchini: Attracts pollinators and beneficial predatory insects nearby.
Where to buy cosmos seeds
Botanical Interests · True Leaf Market · Burpee · High Mowing Organic Seeds (each supplier's cosmos seeds)
See why we recommend these suppliers →
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See it growing
Search YouTube for "growing cosmos" 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 Cosmos in USDA zone 4b: when to start seeds indoors, when to direct-sow, and when to expect a bloom.
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