Johnny’s Seed Starting Date Calculator

Johnny’s Selected Seeds provides a free online Seed Starting Date Calculator that tells you exactly when to sow each vegetable, herb, or flower variety indoors and when to transplant it outdoors, based on your local frost dates. You enter your average last spring frost date and first fall frost date, select a crop, and the tool returns a personalized sowing calendar with indoor sowing, transplant, and direct-sow dates. This article explains how the calculator works, how to use it correctly, and how to adapt its results to your own garden conditions.

Formula

The calculator uses a simple count-back and count-forward method based on the crop’s “weeks before/after frost” data. For each crop, Johnny’s maintains a set of standard values:

• Indoor sowing: number of weeks before the last spring frost that you should start seeds indoors.
• Transplant: number of weeks before or after the last frost that seedlings can go outside (some crops are frost-tolerant and can go earlier).
• Direct sow: number of weeks before or after the last frost for sowing directly in the garden.
• Fall planting: number of weeks before the first fall frost for a second planting of cool-season crops.

Mathematically, the sowing date is calculated as:

Sowing date = (Last spring frost date) − (Weeks before frost × 7 days)

For example, if the last frost is May 15 and a crop needs 8 weeks indoors, the sowing date is May 15 minus 56 days = March 20.

Worked Example

Using the default values from the calculator block above (which are for a hypothetical lawn volume, not seed dates, but the same arithmetic applies):

1. Last spring frost date: April 30
2. Crop: Tomato, which Johnny’s lists as “Start indoors 8 weeks before last frost”
3. 8 weeks × 7 days = 56 days
4. April 30 minus 56 days = March 5
5. Transplant date: “2 weeks after last frost” = May 14

So you would sow tomato seeds indoors on March 5 and transplant them outdoors on or after May 14.

How to Find Your Frost Dates

The calculator’s accuracy depends entirely on the frost dates you input. Here’s how to get reliable numbers:

• Use a local extension service or a frost date map based on your specific ZIP code (not just your state).
• Check for both the 10% and 90% probability dates – the 10% date is earlier (riskier), the 90% date is safer.
• If you garden in a microclimate (near water, on a slope, in a city), adjust the dates by a week or two.
• For fall planting, use the first fall frost date, which is usually about 30 days after the last spring frost date plus 150-180 days, but look it up for your area.

How to Choose the Right Crop

The calculator has a dropdown list of dozens of crops, from arugula to zucchini. Each entry includes specific timing data. Some examples:

• Broccoli: Start indoors 6-8 weeks before last frost; transplant 2 weeks before last frost (it’s cold-hardy).
• Peppers: Start indoors 8-10 weeks before; transplant after all danger of frost, when soil is warm.
• Lettuce: Direct sow 4 weeks before last frost, or start indoors 4 weeks before and transplant 2 weeks before.
• Pumpkins: Direct sow 2 weeks after last frost; they do not transplant well.

If you’re growing a less common variety, use the same timing as the species. For example, all tomatoes have similar indoor timing, though determinate vs. indeterminate doesn’t change the start date.

Common Mistakes and How to Avoid Them

Even with a calculator, gardeners often make these errors:

• Using the wrong frost date: A general state map can be off by weeks. Always use your local data.
• Ignoring soil temperature: Some seeds (like beans) need warm soil, not just no frost. The calculator assumes you’ll check soil temps.
• Starting too early: Seedlings that wait too long in pots become root-bound and stunted. If you’re unsure, start a week later.
• Not hardening off: Even if the date is right, transplants need 7-10 days of gradual outdoor exposure before going in the ground.
• Ignoring day length: Onions and some flowers need specific day length to bulb or bloom; the calculator doesn’t account for that.

Limits of the Calculator

Johnny’s calculator is a general guide, not a guarantee. It assumes:

• You are growing in the ground or in containers with good drainage.
• You have a greenhouse or bright indoor lights for starting seeds.
• You are using average weather patterns – a late frost or an early heat wave will shift everything.
• Your soil is workable and at the right temperature when you transplant.

For crops that are direct-sown only (carrots, radishes, peas), the calculator gives a direct sow date, not an indoor start. For crops that can be sown both ways, it gives both dates.

Alternatives and Related Tools

If you don’t have access to Johnny’s calculator, or want to double-check, several other resources exist:

• Your local cooperative extension service often publishes a planting calendar specific to your county.
• Seed packets themselves list “days to germination” and “weeks before last frost” – you can calculate manually.
• Online frost date tools (e.g., from the National Gardening Association) give you frost dates by ZIP code.
• Garden planning apps like Planter or Gardenize have built-in calculators, but they often use the same underlying data.

Johnny’s calculator is unique because it’s tied to their seed catalog, so the timing matches their varieties, but the general method is universal.

Frequently Asked Questions

Do I need to enter both frost dates?

Yes, the calculator uses both to give you spring and fall planting dates. If you only know your last spring frost, you can estimate your first fall frost by adding about 150 days (for most areas) or look it up online.

What if I miss the recommended sowing date?

You can often still sow a week or two late, especially for fast-maturing crops like lettuce or radishes. For slow crops like peppers, you may get a smaller harvest, but it’s worth trying.

Does the calculator work for container gardening?

Yes, the dates are the same, but containers warm up faster in spring and cool down faster in fall, so you might transplant a week earlier in spring and protect plants earlier in fall.

Why does the calculator give different dates for the same crop?

Because it uses your specific frost dates. A gardener in Minnesota will get later spring dates than one in Texas, even for the same tomato variety.

Can I use the calculator for flowers?

Yes, Johnny’s includes many annual and perennial flowers. The same logic applies – some flowers are cold-hardy, some are not.

Is the calculator free?

Yes, it’s freely available on Johnny’s website. You don’t need to create an account or purchase anything to use it.

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in l | Lawn length | ft | 40 | 0 | 100000
in w | Lawn width | ft | 25 | 0 | 100000
in d | Depth | in | 3 | 0 | 120
out yd | Volume | yd³ | l * w * d / 324 | 2
out ft3 | Volume | ft³ | l * w * d / 12 | 1