Why these dates work in zone 4a
Zone 4a's last spring frost averages May 7 and first fall frost Oct 13 — about 159 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 4a?
Rosemary matures in about 80–100 days from transplanting. In zone 4a that means setting plants out around May 7 and first picking around Jul 26.
Will rosemary survive frost in zone 4a?
No — rosemary is frost-sensitive. In zone 4a, wait until after the last frost (around May 7) to plant, and harvest before the first fall frost (around Oct 13).
For soil, spacing, pests and companion planting, see the full rosemary growing guide →