How to find your last frost date
Your average last spring frost is the date after which the chance of a killing frost drops low enough to plant tender crops such as tomatoes, basil and beans. National weather services, agricultural extension offices and many seed companies publish frost dates by postal code or nearest weather station. Because it is an average, experienced growers wait a week past it for the most cold-sensitive crops.
Why city gardens can plant earlier
Dense urban areas are typically warmer than surrounding countryside — the urban heat island effect — and a balcony against a brick wall stores heat overnight. Many city growers can plant a week or two ahead of regional dates. Exposed rooftops are the exception: wind chill can damage seedlings, so harden them off for 7–10 days first.
Succession sowing
Quick crops like radish, lettuce and cilantro are shown with two windows: spring and late summer. Sow small batches every two to three weeks within each window for a continuous harvest instead of a glut.