Your growing season is the stretch between the last frost in spring and the first frost in fall. This calculator counts those frost-free days, lets you add extra weeks for protection such as a cold frame or low tunnel, and then shows which warm-season crops fit comfortably, fit tightly, or need protection or a short-season variety. It is a quick way to check whether a crop is realistic before you buy seed.
How to use the growing season length calculator
- Enter your average last spring frost. Use the frost date tool or your local Cooperative Extension office.
- Enter your average first fall frost. It must come after the spring date.
- Add protection if you use it. If cold frames, low tunnels or a greenhouse give you extra weeks in spring or fall, enter them. A row cover is the simplest example.
- Read the result. You get your frost-free days, the total with protection, and three groups of warm-season crops.
How crops are sorted
The calculator compares your usable days with each warm-season crop’s days to maturity from our plant database:
- Comfortable fit: the slowest days to maturity fit with at least two weeks to spare.
- Tight fit: the fastest days to maturity fit with a week to spare, but the slow end does not. Choose fast varieties and start transplants early.
- Needs protection or a short-season variety: even the fastest days to maturity leave little or no margin.
Remember that some crops count days from transplanting and others from sowing. A tomato listed at 60 to 85 days from transplant also needs weeks of indoor seed starting beforehand, which the season length alone does not show. The seed starting calculator covers that.
Why cool-season crops are not listed
Lettuce, spinach, peas, kale and other cool-season crops tolerate light frost, so they can start before your last spring frost and keep going after the first fall frost. The frost-free window therefore does not limit them in the same way, and listing them would be misleading.
Ways to lengthen a short season
- Start seeds indoors so transplants go out on the first safe day.
- Choose short-season varieties. Seed packets and catalogs list days to maturity.
- Warm the soil with clear or black plastic or by using raised beds that warm earlier in spring.
- Cover plants on cold nights with row cover or frost cloth.
- Plant in the warmest spot with full sun and shelter from wind. The sunlight requirement finder helps you compare sites.
Limits of this tool
Frost dates are averages. Real last and first frosts shift from year to year and between nearby sites, so treat the numbers as a planning guide. Your local Cooperative Extension office can give the best dates for your area and tell you which varieties do well there.
Frequently asked questions
How long is the average growing season in the US?
It ranges from a few months in the far north and at high elevation to most of the year in the Deep South and coastal areas. Enter your own frost dates for an accurate count.
What counts as a frost-free day?
Here it means every day between your average last spring frost and your average first fall frost.
How much can a cold frame or row cover add?
It varies with the weather and materials, but a few extra weeks at each end is a common planning figure. Enter the number that matches your setup.
Why is my favorite crop listed as a tight fit?
Its fastest days to maturity fit your season, but the slow end does not. Pick a fast variety, start transplants early and protect plants in cold spells.
Where do I find my frost dates?
Use our frost date tool for an estimate, or ask your county Cooperative Extension office.