Castor Bean in Zone 5a & 5b
Ricinus communis
Yes, castor bean grows in zones 5a and 5b. Plant it outside May 17–May 24.
When to plant castor bean in zone 5b
Zones 5a and 5b share the same average frost dates, so this page covers both.
Method: Direct sow: Direct-sow outside. It doesn't transplant well, or doesn't need the extra time indoors.
| Start indoors | Not typically started indoors |
|---|---|
| Direct sow / transplant outside | May 17–May 24 |
| Estimated bloom | Aug 25–Sep 1 (~100 days after direct-sowing) |
Spacing: about 10 square feet per plant (square-foot-gardening spacing). Mature height: 6-10 ft.
Plant alongside it in zone 5b
These go outside within two weeks of castor bean in zone 5b, so one planting day can cover them.
- NasturtiumMay 17–May 24
- MarigoldMay 17–May 24
- Ageratum (Flossflower)May 17–May 24
- Annual Geranium (Pelargonium)May 17–May 24
- Butterfly Weed (Asclepias)May 17–May 24
- Campanula (Bellflower)May 17–May 24
Castor bean is included here so this site can state plainly what it is, rather than leaving a gardener to find out from a packet that does not say.
How castor bean fits zone 5's season
Zone 5b's frost-free window runs about 148 days, from May 10 to October 5. Castor bean needs about 100 days from sowing to first bloom.
Castor Bean planting windows in every zone
Zone 5b is highlighted. Useful for comparing against a neighbouring zone, or for a garden somewhere else.
| Zone | Start indoors | Sow or plant out | Bloom |
|---|---|---|---|
| 3a & 3b | Not usually | Jun 6–Jun 13 | Sep 14–Sep 21 |
| 4a & 4b | Not usually | May 22–May 29 | Aug 30–Sep 6 |
| 5a & 5b (your zone) | Not usually | May 17–May 24 | Aug 25–Sep 1 |
| 6a & 6b | Not usually | May 2–May 9 | Aug 10–Aug 17 |
| 7a & 7b | Not usually | Apr 17–Apr 24 | Jul 26–Aug 2 |
| 8a & 8b | Not usually | Apr 1–Apr 8 | Jul 10–Jul 17 |
| 9a & 9b | Not usually | Feb 22–Mar 1 | Jun 2–Jun 9 |
| 10a & 10b | Not usually | Feb 6–Feb 13 | May 17–May 24 |
| 11a & 11b | Not usually | Jan 8–Jan 15 | Apr 18–Apr 25 |
Bloom dates are about 100 days after the sow or plant-out date.
Good companions
No well-established companions for castor bean.
Where to buy castor bean seeds
None of the seed suppliers we list carries castor bean seeds right now. A local garden center or a specialty seed house is the place to look.
See why we recommend these suppliers →
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See it growing
Search YouTube for "growing castor bean" videos → (a live search, not a hand-picked video; results vary and aren't vetted by us)
← Back to the full zone 5b calendar · Plan your beds in the Garden Designer →
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Growing Castor Bean in USDA zone 5b: when to start seeds indoors, when to direct-sow, and when to expect a bloom.
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