Zone 5a covers 27 days of last-frost dates. We measured Sioux City, IA at Apr 20 and Bozeman, MT at May 17. Give us your town or ZIP and we'll use ten years of daily lows recorded at your own coordinates instead of the middle of that range.
Why these dates work in zone 5a
Zone 5a's last spring frost averages Apr 30 and first fall frost Oct 20 — about 173 frost-free days. Rosemary is frost-sensitive, so the around last frost timing builds in a safety margin against a late cold snap — setting plants out too early risks losing them to frost. Starting seeds indoors 10 weeks ahead gives strong transplants ready the moment the soil warms.
Common questions
How long does rosemary take to grow in zone 5a?
Rosemary matures in about 80–100 days from transplanting. In zone 5a that means setting plants out around Apr 30 and first picking around Jul 19.
Will rosemary survive frost in zone 5a?
No — rosemary is frost-sensitive. In zone 5a, wait until after the last frost (around Apr 30) to plant, and harvest before the first fall frost (around Oct 20).
For soil, spacing, pests and companion planting, see the full rosemary growing guide →