Seed Starting Date Calculator: When to Sow

This calculator works out the best date to start seeds indoors (or sow directly outside) based on your last spring frost date, the crop’s growth speed, and how you plan to transplant. You enter your average last frost date, choose the crop from the list, and pick whether you transplant or direct sow; you get back the exact calendar date to sow, plus the number of weeks before frost. The result is a planning date, not a guarantee—weather and local conditions always matter.

Formula

The core formula is simple: Sow date = Last frost date − (Weeks before frost × 7 days). The variable Weeks before frost comes from the crop’s typical growth lead time—how many weeks a plant needs to grow indoors before it can safely go outside (or how many weeks before frost to sow directly). Each crop in the dropdown has a preset number of weeks: tomatoes 8, peppers 10, cabbage 6, lettuce 4, broccoli 7, cucumbers 3, squash 3, basil 6, marigolds 8, zinnias 6. If you choose “Direct sow,â€� the same weeks are subtracted from the frost date, but the result is the date to plant seeds in the ground, not indoors.

Worked Example

Using the defaults: last frost date = April 15, crop = Tomato (8 weeks), method = Transplant.

1. Convert the frost date to a day-of-year: April 15 is day 105 (non-leap year).
2. Multiply weeks by 7: 8 × 7 = 56 days.
3. Subtract from frost date: day 105 − 56 = day 49.
4. Day 49 is February 18 (31 days in January + 18).

So the calculator returns February 18 as the sow date. The “weeks before frost� output shows 8, confirming the lead time.

What the Result Means

The date you get is a starting point for planning. It tells you when to sow seeds indoors so that transplants are ready to go outside around the last frost date. If you direct sow, it tells you the earliest safe date to put seeds in the ground—usually a few weeks before the last frost for cool-season crops, but for warm-season crops like cucumbers and squash, it’s often safer to wait until after the frost date. The calculator assumes a typical frost date; your local microclimate may shift it by a week or two.

How to Choose Your Frost Date

The single most important input is your last spring frost date. If you don’t know it, you can find it from:

• Local extension service or gardening club websites
• USDA hardiness zone maps (though they give zones, not frost dates)
• Online frost date databases that use your ZIP code or city
• Your own garden records—if you’ve gardened for a few years, note the last date you saw frost each spring

If you’re unsure, use a date 1–2 weeks later than the average to be safer for warm-season crops.

Common Mistakes and Misconceptions

One big mistake is using the first frost date in fall instead of the last frost in spring. They are different. Another is ignoring the difference between “transplant� and “direct sow.� Tomatoes are almost always transplanted; cucumbers are often direct sown. The calculator handles both, but you must choose the right method.

A common misconception: “Start seeds 8 weeks before frost� means you can plant them outside 8 weeks before frost. No—it means you sow them indoors 8 weeks before, and the plants go outside after the frost date. The weeks are a lead time, not a planting window.

Also, some gardeners think the date is a hard deadline. It’s not. If you’re late, you can often start seeds a week later and still get a crop, especially for fast-growing crops like lettuce or radishes.

Limits of the Formula

This calculator gives a general guideline, not a precise agricultural recommendation. It doesn’t account for:

• Your specific microclimate (urban heat, altitude, proximity to water)
• Soil temperature (some seeds need warm soil, not just warm air)
• Day length and light intensity indoors
• Variety differences (some tomato varieties need more or less time)

For best results, cross-check with a local planting calendar from a nearby university extension. They often provide dates for your region that are more accurate than a generic formula.

Alternatives to This Calculator

If you don’t have a frost date, you can use the soil temperature method: buy a soil thermometer and plant when the soil reaches the recommended temperature for that crop (e.g., 60°F for beans, 70°F for tomatoes). Another alternative is the phenology method—plant when certain wild plants bloom (e.g., when lilacs bloom, it’s safe to plant cool-season crops). Many gardeners also use a paper calendar and count back weeks manually, which is exactly what this calculator automates.

Frequently Asked Questions

What if I don’t know my last frost date?

Use a frost date lookup tool with your ZIP code, or ask a local garden center. As a rough fallback, use April 15 for many northern US regions and March 15 for southern regions, but this is very approximate.

Can I start seeds earlier than the calculator says?

Yes, but you risk leggy, overgrown transplants that may struggle after planting. Starting a week earlier is usually fine; starting a month earlier often causes problems.

Why do some crops have different weeks?

Because they grow at different speeds. Peppers and eggplants are slow, so they need 10 weeks. Cucumbers and squash grow fast, so 3 weeks is enough. The weeks are based on typical transplant size.

Does this work for fall planting?

No, this calculator is for spring planting. Fall planting uses the first frost date in fall and counts backward differently (e.g., plant 12 weeks before first fall frost for broccoli).

What if the date falls on a weekend or holiday?

It doesn’t matter. You can sow any day that works for you. The date is a guideline, not a strict rule.

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in frost | Last spring frost date (MM/DD) | - | 04/15 | 0
sel crop | Crop | Tomato=8, Pepper=10, Cabbage=6, Lettuce=4, Broccoli=7, Cucumber=3, Squash=3, Basil=6, Marigold=8, Zinnia=6
sel method | Sowing method | Transplant=1, Direct sow=0
out sow_date | Sow seeds (date) | - | frost - (crop * 7) * method + (1 - method) * (crop * 7) | 0
out weeks_before | Weeks before frost | - | crop * method | 0