Planting calendar by frost date

Most US planting guides time crops from a single reference point: the average date of the last spring frost. KidsGardening's planting basics describe the method directly: find that date, then count sowing and transplanting dates in weeks before or after it, adjusting for real weather and soil conditions. For fall crops, Montana State University Extension's Can I Grow That Here? guide counts back from the average first fall frost instead. This tool does both arithmetic steps for you. It needs frost dates, not a hardiness zone; the hardiness zones page explains the difference, and the zone 5 page shows how far frost dates can differ within one zone.

Why the anchor date matters

An "average" frost date is a probability, not a promise. NC State Extension explains that NOAA's freeze tables give dates at several probability levels; the 50% date is the median, meaning a later frost still happens in about half of years. Planting tender crops right on that date is a moderate-risk choice; a later, 10% date is the conservative one. NOAA's Climate.gov describes the last-freeze map in the same way, as historical odds rather than a forecast. If a crop is frost-tender, the safer move is to enter a later spring date.

The offsets used

Negative numbers are weeks before the last spring frost; positive numbers are weeks after. "Latest planting" is weeks before the average first fall frost. "Indoors" is UNH's seed-to-transplant time in weeks.

Colorado State University's GardenNotes #720 groups the same crops by frost tolerance: broccoli, cabbage, lettuce, peas, radishes and spinach are "hardy"; beets, carrots, cauliflower, potatoes and Swiss chard "semi-hardy"; beans, corn, cucumbers and summer squash "tender"; and eggplant, melons, peppers, tomatoes and winter squash "very tender", planted two or more weeks after the last frost. That grouping explains why the warm-season crops in the table all land after frost and the brassicas before it.

Limits of a frost-date calendar

• Soil, not air. UNH times beets, carrots, corn, cucumbers and squash by soil temperature (45°F, 60°F or 70°F). A soil thermometer beats a calendar for those.
• Season length. Montana State suggests counting the days between your two frost dates and comparing them with a crop's days to maturity; if the season is shorter, the crop may not finish outdoors.
• Regional bias. UNH writes for New Hampshire, where Memorial Day is called a conservative last-frost estimate for the central part of the state; Montana State writes for a short, high-elevation season. Southern gardens with mild winters often grow cool-season crops through winter instead (see the zone 9 page), a pattern these northern tables do not cover.