Frost Date Planting Calculator

Frost dates are the foundation of garden planning in temperate climates. The last spring frost date and first fall frost date define your growing season, and knowing these dates helps you determine when to start seeds indoors, when to transplant outdoors, and when to harvest.

How planting dates are calculated: Each plant type has specific timing requirements relative to the last frost date:

Transplant Date = Last Frost Date + Safety Buffer (based on cold hardiness) Indoor Seed Start Date = Transplant Date - Indoor Growing Weeks Direct Sow Date = Last Frost Date + Days After Last Frost (based on soil temperature)

The indoor start counts back from the day the seedling goes in the ground, not from the frost date. That distinction is easy to lose and it costs a month: hardy crops go out four weeks before the frost date, so counting eight weeks back from the frost leaves the seedling only three weeks old on transplant day. Far too young for a broccoli.

Plant cold hardiness categories:

Hardy plants (can tolerate light frost, 28-32°F / -2 to 0°C): Plant outdoors 4 weeks before last frost. Examples: peas, lettuce, spinach, kale, broccoli, radishes, onions, potatoes. Start seeds indoors 8 weeks before transplanting.

Semi-hardy plants (can tolerate brief cool temps but not frost): Plant outdoors 2 weeks before last frost. Examples: beets, carrots, Swiss chard, parsley, celery. Start seeds indoors 8 weeks before transplanting.

Tender plants (damaged by any frost): Plant outdoors only after all danger of frost has passed, a week past the last frost date here. Examples: tomatoes, peppers, cucumbers, squash, beans, corn, basil. Start seeds indoors 8 weeks before transplanting.

Warm-loving plants (need warm soil, above 60°F / 16°C): Plant 2 weeks after last frost when soil has warmed. Examples: melons, okra, sweet potatoes, eggplant, pumpkins. Start seeds indoors 4 weeks before transplanting, and no earlier: these resent having their roots disturbed and a pot-bound melon never catches up.

Growing season calculation: Growing Season Length = First Fall Frost Date - Last Spring Frost Date

This determines which crops you can grow to maturity. Tomatoes typically need 60-90 days, peppers need 60-80 days, and pumpkins need 90-120 days from transplant to harvest.

The two frost dates do not move together. Going one zone colder pulls the spring date later and the autumn date earlier, so the season shortens from both ends at once. That is why a single zone step costs far more growing time than the shift in either date alone suggests.

These are the averages this calculator uses. Your own dates can sit a week or two either side depending on elevation, how close you are to water, and whether your garden is a frost pocket.

If you already know your two dates, or your extension office publishes them for your zip code, type them straight into the frost date planner instead of picking a zone here. It takes both dates and skips the averages. The two pages give the same seed-start, planting and harvest dates for hardy, semi-hardy and tender crops.

Finding your frost dates: Frost dates are based on historical weather data and represent the date when there is typically a 50% chance of frost occurring. Sources include the USDA Plant Hardiness Zone Map, local agricultural extension offices, and weather services.

Important note: Frost dates are averages and probabilities, not guarantees. Microclimates (south-facing walls, hilltops, valley bottoms) can shift actual frost dates by 1-3 weeks. Always monitor local weather forecasts near your frost dates.