How the calculator works

Everything is counted from one date: the average last spring frost where you live. Each crop has a window for planting out (or sowing outdoors) based on how much cold it can take. The sow-indoors dates are then worked back from the plant-out dates.

Plant out or direct sow: frost date plus the crop's window from the table below.
Sow indoors, first date: first plant-out date minus the longest time seedlings need indoors.
Sow indoors, last date: last plant-out date minus the shortest time indoors.
Dates only: the calculator works in whole calendar days, so a week is always exactly 7 days and clock changes can't shift a date.

If you sow early in the sow-indoors window, plant out early in the plant-out window, so seedlings don't sit in their pots for too long.

Cold tolerance	Plant out or sow outdoors	Examples
Hardy	4 to 2 weeks before the frost date	Peas, spinach, broccoli, cabbage, onions, pansies
Semi-hardy	2 weeks before to the frost date	Carrots, beets, potatoes, cauliflower, parsley
Tender	The frost date to 2 weeks after	Snap beans, sweet corn, celery, marigolds, zinnias
Very tender	1 to 3 weeks after the frost date	Tomatoes, peppers, eggplant, melons, winter squash, basil

Colorado State University Extension sorts vegetables into these four groups: hardy crops can go in as early as 2 to 4 weeks before the average last frost, semi-hardy crops 0 to 2 weeks before, tender crops around the frost date and very tender crops typically 2 or more weeks after. Utah State University Extension uses the same groups and says very tender crops should wait at least a week after the frost date, so the very tender window here starts at 1 week. Utah State also says hardy crops can be planted 45 to 60 days before the frost date once the soil can be worked; the calculator uses Colorado State's more cautious 2 to 4 weeks. For annual flowers, Illinois Extension plants hardy types about four weeks before the frost date, half-hardy types about two weeks before and tender types on or after it, and Penn State gives similar groups.

The windows are not deadlines. Colorado State gives the earliest safe times ("as early as"), and planting calendars carry on for weeks after them: Maryland's calendar, based on an April 15 frost date, sows peas from mid-March through April and beets from April through July. Cool-season crops should still mature before hot weather, because they lose quality in the heat (Colorado State, Minnesota).

How long seedlings need indoors comes from seed-starting guides by Maryland, Maine, Colorado State, New Hampshire, Minnesota, Iowa State, Penn State and Utah State extension services. Where they disagree, the calculator uses a range that covers most of them. For example, Colorado State and Maine give peppers 6 to 8 weeks and Maryland gives 8 to 10, so peppers get 6 to 10 weeks.

Worked example

Say your average last frost is April 15, 2027 (the example date in the calculator).

Tomatoes are very tender, so they go out 1 to 3 weeks after the frost date: April 22 to May 6. They need 5 to 7 weeks indoors, so sow from March 4 (April 22 minus 7 weeks) to April 1 (May 6 minus 5 weeks).
Broccoli is hardy, so it goes out 4 to 2 weeks before the frost date: March 18 to April 1. With 4 to 7 weeks indoors, sow from January 28 to March 4.
Peas are sown straight into the garden, 4 to 2 weeks before the frost date: March 18 to April 1.

With this frost date, the first job of the season is sowing pansies indoors, from December 3, 2026.

Your frost date is an average, not a promise

An average last frost date comes from past weather records, so in some years frost arrives later. Penn State Extension describes it as a date based on historical patterns and advises checking the local forecast before planting: forecast frost, strong wind or heavy rain should delay planting. Utah State points out that frost dates can differ by many days within a few miles: low spots where cold air collects have a shorter frost-free season than hillside benches and town gardens. University of Maryland Extension suggests keeping row covers, light blankets, tarps, baskets or paper bags ready to cover tender plants on a cold night.

This site doesn't have a ZIP code lookup yet. To find your date:

Ask your local Cooperative Extension office. The Extension Foundation keeps a directory of extension services by state.
In 25 states of the Midwest, Northeast and Appalachia, the Freeze Date Tool from the Midwestern Regional Climate Center at Purdue and the USDA Midwest Climate Hub shows historical freeze dates for your county.
Outside the US, your national weather service or a local gardening club may be able to tell you the usual last frost date.

Tips for strong seedlings

Don't sow too early. University of Maryland Extension calls this a common mistake: seedlings get tall and root-bound and don't do well in the garden.
Harden seedlings off. For one to two weeks before planting out, set them outside for a few hours in a sheltered spot and leave them out a little longer each day (University of Maine and University of Minnesota).
Give them strong light. A window is rarely enough. University of Minnesota Extension suggests keeping lights no more than 4 inches (10 cm) above the seedlings and giving 12 to 16 hours of light a day.
Check soil temperature as well as the date. Colorado State calls soil temperature one of the best guides to planting time: snap beans need soil of at least 55°F (13°C), and squash, cucumbers and melons 60°F (16°C).
Keep squash-family transplants young. Colorado State notes that cucumber, squash and melon roots hate being disturbed, so use small plants no more than 2 to 3 weeks from seeding, or sow directly. Maine and New Hampshire allow up to 4 weeks, which is why the calculator shows 2 to 4.

Why seed-starting charts disagree

Charts count from different starting points. The University of Maine chart says to sow tomatoes indoors 6 to 8 weeks before the last frost, and New Hampshire gives tomato seedlings 6 to 8 weeks. Colorado State and Utah State plant tomatoes out a week or more after the frost date, and Colorado State and Maryland give tomato seedlings 5 to 7 and 5 to 6 weeks indoors. This calculator counts back from the plant-out date, so its tomato sowing window (6 to 2 weeks before the frost date) is later than the Maine chart's. Maryland warns that sowing too early gives tall, root-bound plants.

Minnesota's calendar is the outlier for the cabbage family: it keeps broccoli indoors 10 to 11 weeks, where Maryland, New Hampshire, Colorado State and Maine suggest 4 to 7. The calculator follows the majority.

Common questions

When should I start tomato seeds indoors?: Count back from the day you plan to plant them out. Tomatoes go out 1 to 3 weeks after your average last frost and need 5 to 7 weeks indoors. With an April 15 frost date that means sowing between March 4 and April 1 and planting out between April 22 and May 6.
My last frost is in mid-May. How do the dates change?: Every date moves by the same amount. May 15 is 30 days after April 15, so tomatoes would be sown from April 3 to May 1 and planted out from May 22 to June 5. Enter your own date and the whole table updates.
Which seeds should I not start indoors?: Root crops such as carrots and beets don't transplant well (University of Minnesota), and peas, beans, sweet corn and spinach are normally sown straight into the garden (University of Maine). Cucumbers, squash and melons can be started indoors, but only 2 to 4 weeks ahead, and Colorado State notes that their roots dislike being disturbed.
How does the calendar download work?: The "Download calendar (.ics)" button makes a file with an all-day event on the first day of every window for the crops in the table, plus one on your frost date. Each event says when its window ends. Open or import the file in Google Calendar, Apple Calendar or Outlook. The file is made in your browser; nothing is sent to us.
Why are the dates different from my seed packet?: Varieties differ, and charts count from different starting points. The University of Minnesota and Iowa State both say to follow the packet for how long seedlings need indoors. Use the calculator to turn those weeks into dates for your frost date.

Sources

Last checked and updated: October 31, 2026.