First frost date by zip code
Your frost dates control nearly everything in the garden. When to start seeds, when to transplant, when to harvest, when to cover things up in fall. Yet most gardeners either don't know their dates or are working off rough guesses. "Sometime in October" isn't specific enough when a single night of 30°F can wipe out your tomato plants.
Here's what frost dates actually mean, where the data comes from, and how to look up accurate dates for your specific location.
First frost vs. last frost
Two dates matter for gardening:
Your last spring frost date is the average date of the final frost in spring. This determines when you can safely plant warm-season crops outside. It's the date most seed packets and planting guides reference when they say "after last frost."
Your first fall frost date is the average date of the first frost in autumn. This tells you when to expect the growing season to end for frost-sensitive plants. The gap between these two dates is your frost-free growing season.
In zone 6 (much of the mid-Atlantic, Ohio Valley, and central plains), the last spring frost is typically around April 15 and the first fall frost around October 15. That's a 180-day growing season. In zone 4 (northern Minnesota, Montana), you might only get 120 days. In zone 9 (Gulf Coast, central Florida), frost is rare enough that you barely need to think about it.
Where the data comes from
Frost dates aren't someone's guess. They come from NOAA's climate records, specifically the 30-year climate normals published by the National Centers for Environmental Information. NOAA tracks daily temperature data from thousands of weather stations across the country. From that data, they calculate the median date of the last 32°F reading in spring and the first 32°F reading in fall for each station.
The USDA Plant Hardiness Zone Map adds another layer. It divides the country into zones based on average annual minimum winter temperatures. Your zone number gives a rough frost date range, but the NOAA station data is more precise because it's tied to a specific location rather than a broad zone.
Our planting calendar combines both datasets. When you enter a ZIP code, it maps to the nearest NOAA station and the corresponding USDA zone to give you frost dates and planting windows for 30+ crops.
Look up your frost dates now: enter your ZIP code here for free personalized frost and planting data.
"Average" doesn't mean "guaranteed"
This is the part that trips people up. An average last frost date of April 15 means that in about half of the years on record, the last frost came before April 15, and in half it came after. Some years you get a frost on May 1. Some years the last frost is March 28. The average is just the midpoint.
NOAA also publishes probability dates. A "90% probability" last frost date is the date by which there's a 90% chance the last frost has already occurred. This date falls 2 to 3 weeks later than the average. If you want to be safe, especially with expensive or hard-to-replace transplants, use the 90% date.
The 50% date (the average) is fine for hardy crops that can tolerate a light frost, like broccoli, peas, and kale. The 90% date is better for tender crops like tomatoes, peppers, and basil.
Microclimates change everything
Your ZIP code gets you close, but your actual yard might frost earlier or later than the official date. A few things that affect this:
Elevation. Cold air sinks. Gardens at the bottom of a hill or valley experience frost more often than gardens on hillsides. I've seen two gardens half a mile apart with a three-week difference in first fall frost because one was in a low-lying area.
Urban heat island. Cities are warmer than surrounding rural areas due to concrete, asphalt, and building heat. A garden in downtown Indianapolis might get its first frost two weeks later than a garden 20 miles outside the city.
Water proximity. Large bodies of water moderate temperatures. Gardens near the Great Lakes, the ocean, or large rivers tend to have later falls frosts and slightly later spring frosts too. The water acts as a thermal buffer.
Structures. A garden against a south-facing brick wall gets reflected heat during the day and radiated warmth at night. This can effectively shift your zone by half a step warmer. On the other hand, a garden in a shaded northern exposure will run cooler.
Tracking your own frost dates
After a couple of years of gardening in the same spot, your own observations become more valuable than any database. Keep a simple log: date of last visible frost in spring, date of first frost in fall, and what the low temperature was. After three or four years, you'll have your own averages that account for your specific microclimate.
An outdoor min/max thermometer is useful for this. The cheap ones from the hardware store work fine. Place it at plant height, not on a wall or post above the garden. Cold air settles at ground level, so the temperature at 6 inches off the ground can be 3-5 degrees colder than what a thermometer at eye level reads.
What happens at frost
A "frost" occurs when the air temperature at ground level drops to 32°F (0°C). Ice crystals form on plant surfaces. For tender plants like tomatoes, basil, and peppers, this ruptures cell walls and kills the tissue. The plant turns black and mushy within hours.
Hardy plants handle frost differently. Kale, spinach, and Brussels sprouts have higher sugar concentrations in their cells, which acts as antifreeze. Some of these crops actually taste better after a frost because the cold triggers additional sugar production.
A "hard freeze" (below 28°F) kills most garden plants, including some that tolerate light frost. At this point, the growing season is definitively over for everything except the hardiest greens.
Using frost dates for planning
Once you know your frost dates, garden planning becomes straightforward. Count backward from your last spring frost to schedule seed starting. Count forward from your last spring frost to schedule warm-season transplanting. Count backward from your first fall frost to figure out the latest you can plant something and still get a harvest before cold weather arrives.
For fall planting (which more gardeners should do), take the days to maturity listed on the seed packet, add two weeks (because shorter fall days slow growth), and count backward from your first fall frost. That gives you the latest planting date for a fall crop.
The planting calendar does all of this math for you. But understanding the logic behind it helps you make better decisions when conditions don't match the textbook.