Most beginners look up their hardiness zone and expect it to tell them when to plant. It doesn’t. The right plant on the wrong date is one of the quiet killers of first gardens, and two dates for your area fix it.
What a hardiness zone is for
The USDA Plant Hardiness Zone Map is built on one number: the average coldest winter temperature at a location. It shows which perennial plants are likely to survive winter there. Most vegetables live for one season, so the zone says little about them. As University of Illinois Extension puts it, planting dates are determined by the first and last frost dates and by how long a crop takes to mature.
The two dates that matter
- Average last spring frost: the date after which the chance of another 32°F night drops below 50%.
- Average first fall frost: the date by which there is a 50% chance that the first 32°F night has already happened.
“Average” means a coin flip: in half of all years there is still a frost after the average last frost date. That is why plants that frost kills (basil, beans, tomatoes, peppers) should not go out on the average date. NOAA also publishes the date after which only 1 spring in 10 still has a frost. Across the NOAA stations behind our calendar, that safer date is typically about two weeks after the average one.
Cool-season and warm-season plants
- Cool-season plants (lettuce, radish, kale, green onions, cilantro) tolerate light frost and go out 2 to 5 weeks before the average last frost. They struggle once days pass 75–80°F, and they can be sown again in late summer for fall.
- Warm-season plants (basil, bush beans, cherry tomatoes, sweet peppers) are damaged by frost. Basil is hurt at 50°F. They go out after the frost danger has passed.
If frost is forecast after you planted
Bring the pots inside for the night: that is the great advantage of containers. Pots too big to move can be covered with a sheet, blanket or frost fabric before evening, and uncovered the next day once it is above freezing. Covers stop working below about 28°F.
Sources
Every fact in this guide comes from the publications below. None of them reviewed or endorsed Balcony Bounty.
- USDA Agricultural Research Service. USDA Plant Hardiness Zone Map. planthardiness.ars.usda.gov/
- University of Illinois Extension. When to Plant in Illinois (2025). extension.illinois.edu/sites/default/files/2025-05/accessibledocuments…
- NOAA Climate.gov. Interactive map: average date of last spring freeze across the United States. www.climate.gov/news-features/understanding-climate/interactive-map-av…
- NOAA National Centers for Environmental Information. U.S. Climate Normals 1991–2020, annual/seasonal (freeze dates at 32°F). www.ncei.noaa.gov/products/land-based-station/us-climate-normals
- University of Maryland Extension. Spring Frost or Freeze Dates in Maryland. extension.umd.edu/resource/spring-frost-or-freeze-dates-maryland
- Utah State University Extension. How to Grow Lettuce in Your Garden. extension.usu.edu/yardandgarden/research/lettuce-in-the-garden
- University of Illinois Extension. Herbs: Basil. extension.illinois.edu/herbs/basil
- University of New Hampshire Extension. Growing Vegetables: When to Plant Your Vegetable Garden. extension.unh.edu/resource/growing-vegetables-when-plant-your-vegetabl…
- Iowa State University Extension. How to Protect Plants from Frost and Freeze. yardandgarden.extension.iastate.edu/how-to/how-protect-plants-frost-an…
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