About the Frost & Planting Planner
The frost date on a seed packet chart is a single “average” — a date with a hidden 50% chance of frost after it. This planner shows the real thing: the full distribution of last-spring and first-fall frosts computed from 30 years of daily minimum temperatures for your exact location, so you can choose your risk instead of inheriting a coin flip. It also derives your USDA hardiness zone from the same record and measures your true frost-free growing season.
The planting side does the arithmetic gardeners do on the back of an envelope every spring, for 40 common vegetables, herbs, and flowers: start-indoors dates counted back from transplant time, direct-sow windows, and — the one everyone forgets — the last safe sowing date before the fall frost ends the season.
Your location is detected from your connection only to fetch the local history and is never stored. Dates are area baselines from ~6-mile gridded data; cold hollows frost later into spring and low spots earlier in fall, so adjust for your site. Both hemispheres are handled.
How to Use the Frost & Planting Planner
Confirm your location — or search for the garden’s town if you’re planning remotely — and the tool computes your frost statistics from 30 years of daily minimum temperatures. The hero date is when it’s safe to plant tender crops at the current risk setting; the distribution table beneath shows the honest spread, from the earliest last-frost on record to the latest. Then pick a crop and read its dates: when to start seeds indoors, when to transplant, when to direct-sow, and when the fall frost closes the window.
The 50% Problem With Every Frost Chart You’ve Seen
Almost every extension chart, almanac, and seed packet prints the average last frost — which is, by definition, the date with a 50% chance of frost still to come. Planting $100 of tomato transplants on a coin flip is how Mother’s Day freezes become folklore. Statistically minded growers use percentiles instead: the 30%-risk date for cheap, replaceable sowings, and the 10%-risk date for expensive transplants and long-season crops that can’t be restarted. At most mid-latitude locations the gap between the median and the safe date is two to three weeks — the exact stretch when the garden centers are busiest and the frost damage happens.
Hardy, Half-Hardy, Tender: What Actually Dies at 32°F
Frost tolerance is a spectrum, and using it well adds a month to your season on both ends. Hardy crops — peas, spinach, kale, onions, radishes — germinate in cold soil and shrug off frosts to around 28°F; sow them 4–6 weeks before the last frost. Half-hardy crops — lettuce, chard, potatoes, cauliflower — handle a light touch of frost but not a hard freeze. Tender crops — tomatoes, peppers, beans, basil, squash, and nearly every summer flower — are damaged at 32°F and sulk in soil below 60°F, so planting them early gains nothing even when frost spares them. The crop table encodes these offsets crop by crop.
The Fall Deadline Nobody Calculates
Spring gets all the attention, but the season’s other bookend is just as mathematical: a crop needs its days to maturity plus a couple of weeks of buffer (growth slows as days shorten) before the first fall frost. A 60-day bush bean sown 90 days before your median first frost is comfortable; sown 65 days before, it’s a race you’ll usually lose. That arithmetic is the Pro countdown: for every crop, the last safe sowing date and the days remaining — the difference between a productive fall garden and a bed of frost-killed seedlings.
USDA Zones vs Frost Dates — Two Different Questions
Your USDA zone answers one question only: how cold does winter get, averaged over 30 years of each winter’s single coldest night. It decides which perennials, shrubs, and fruit trees survive to spring. It says nothing about when frost ends — Seattle and Oklahoma City share similar zones with wildly different frost calendars. Annual vegetable timing runs on frost dates and season length; perennial shopping runs on zone. This tool computes both from the same local record so the two questions stop getting conflated.
Microclimates: Adjusting the Baseline to Your Yard
- Low spots and valley floors: cold air pools — assume frost lingers 3–7 days later in spring and arrives earlier in fall.
- South-facing walls and slopes: soak up heat — often half a zone warmer, with earlier safe dates.
- Urban cores: heat-island effect can push frost dates a week or more in your favor versus the surrounding countryside.
- Raised beds and containers: soil warms faster in spring (earlier sowing) but roots freeze harder in winter (subtract a zone for overwintering perennials in pots).
Building out the garden itself? Size the beds with the Landscape Material Calculator and keep new trees alive with the Tree Watering Calculator. Browse all Home & Real Estate tools for more.
Frequently Asked Questions
What is the difference between the median frost date and the safe frost date?
The median (50 percent) date is the coin-flip: half of the last 30 years had frost after it. The safe (10 percent risk) date is later — only 1 year in 10 frosted after that point. Extension charts usually print a single average date without telling you it carries 50/50 odds, which is how tomatoes get killed in May. Plant tender crops on the safe date; gamble only with cheap, replaceable seedlings.
What temperature counts as a frost?
This tool uses 32°F (0°C) air temperature as a light freeze, which damages tender plants like tomatoes, basil, and peppers, and 28°F (-2.2°C) as a hard freeze, which kills most semi-hardy crops. Frost can form on plants at air readings a few degrees above 32°F on calm, clear nights, so low-lying gardens should pad the dates by a few days.
How is the USDA zone calculated here?
The USDA defines hardiness zones by the 30-year average of each winter's single coldest temperature. The tool computes that directly from the same station history used for frost dates, so the zone reflects your actual local record rather than a coarse national map cell. Zones guide which perennials survive winter — they say nothing about frost timing, which is why both numbers are shown.
Why do my frost dates differ from my neighbor's a few miles away?
Cold air drains downhill and pools in valleys, so a garden 200 feet lower can frost two weeks later into spring. Urban heat islands, large lakes, and south-facing slopes all shift dates by days to weeks. The data here is interpolated to roughly a 6-mile grid — treat it as your area's baseline and adjust for your specific site's microclimate.
When should I start seeds indoors?
Count backward from your transplant date, not the frost date itself. Tomatoes need 6 to 8 weeks indoors before going out about a week after the last frost; peppers and eggplants need 8 weeks and warmer soil, going out two weeks after. Starting too early is the classic mistake — leggy, root-bound transplants establish worse than younger ones.
