Why these dates work in zone 4a
Zone 4a's last spring frost averages May 20 and first fall frost Sep 20 — about 123 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 20 and first picking around Aug 8.
Will rosemary survive frost in zone 4a?
No — rosemary is frost-sensitive. In zone 4a, wait until after the last frost (around May 20) to plant, and harvest before the first fall frost (around Sep 20).
For soil, spacing, pests and companion planting, see the full rosemary growing guide →