Seed Starting Calculator

This seed starting calculator turns your average last spring frost date into a personal sowing schedule. Enter your frost date and it lists when to start each common crop indoors and when to move it outside, from onions and peppers in late winter to cucumbers and melons a few weeks before planting time. An optional safety margin pushes back transplant dates for frost-tender crops, for gardeners who would rather wait an extra week or two than risk a late cold snap.

How to use the seed starting calculator

1. Find your average last spring frost date. Your state Cooperative Extension service, many seed companies and frost date lookups by ZIP code publish this. Use a weather station close to your garden and similar in elevation.
2. Enter it in the calculator. The schedule updates immediately.
3. Choose a safety margin if you want one. This only moves the outdoor dates for frost-tender crops such as tomatoes, peppers, basil and squash. Cool-season crops keep their normal dates.
4. Copy the dates to a calendar. Writing them down in January is the easiest way to make sure nothing gets started late.

For a step-by-step guide to growing healthy seedlings once your dates are set, see our article on when to start seeds indoors.

How the dates are calculated

Every crop has a typical indoor growing period before it is ready to go outside, and a typical outdoor planting time relative to the last frost. The calculator counts backward from your frost date for sowing, and forward or backward for transplanting.

These are typical ranges for home gardens, consistent with our plant database. Seed packets sometimes give slightly different timing for particular varieties, and a fast early tomato and a slow late pepper do not need identical schedules. When the packet is specific, follow it.

The earlier date in each range suits slower-growing varieties, cooler growing conditions indoors, or gardeners who like larger transplants. The later date suits fast varieties and avoids seedlings outgrowing their pots before planting time.

Example schedules for three climates

To show how much timing shifts across the country, here are the key dates for three different average last frost dates.

Last frost around March 15

Typical of parts of the South and lower-elevation West. Peppers and eggplant are started around January 4 to 18, tomatoes around January 18 to February 1, and tomatoes go outside around March 22. Gardeners in these regions often have time for a second planting of tomatoes for fall, and some cool-season crops do better planted in fall than in spring.

Last frost around April 20

Typical of much of the Mid-Atlantic, lower Midwest and Pacific Northwest. Peppers and eggplant are started around February 9 to 23, tomatoes around February 23 to March 9, broccoli and cabbage go out around April 6, and tomatoes around April 27.

Last frost around May 20

Typical of northern states and higher elevations. Peppers and eggplant are started around March 11 to 25, tomatoes around March 25 to April 8, and tomatoes go outside around May 27. With a shorter frost-free season, choosing faster-maturing varieties and starting transplants indoors makes a big difference here.

Understanding frost dates

An average last frost date is exactly that: an average. It comes from decades of weather records and marks the point in spring where a later frost is roughly as likely as not. In some years the last frost comes two weeks earlier. In others it comes two weeks later.

That is why the calculator offers a safety margin. Frost-tender crops such as tomatoes, peppers, eggplant, basil, cucumbers, squash and melons can be badly damaged or killed by a single frost, and they also grow poorly in cold soil even without frost. Adding a week or two to their outdoor dates lowers the risk considerably. Cool-season crops such as broccoli, cabbage, kale, lettuce and onions tolerate light frosts, so their dates do not change.

Do not confuse frost dates with USDA hardiness zones. Zones describe how cold your winters get and matter most for perennials. Frost dates describe when your growing season starts and ends, which is what matters for vegetables.

What to sow directly instead

Not every crop belongs indoors. Root crops, beans, peas and corn do better sown directly in the garden. Their timing also comes from your last frost date:

Soil temperature matters as much as the calendar for warm-season seeds. Beans and corn rot in cold, wet soil, so wait until it has warmed, even if the date has passed.

Planning a fall garden

The same idea works in reverse for fall. Instead of counting back from your last spring frost, count back from your average first fall frost. Take the crop’s days to maturity, add about two weeks because growth slows as days shorten, and add the indoor growing time if you start transplants. That gives the latest sensible sowing date.

For example, if your first fall frost is around October 15 and a broccoli variety takes about 60 days from transplant, add two weeks for slower fall growth and the broccoli should be transplanted by around early August. Starting it indoors six weeks before that means sowing seeds in late June. Many cool-season crops, including broccoli, kale, cabbage and lettuce, are often better in fall than in spring, because they mature in cooling weather rather than summer heat.

How many seeds to start

Once the dates are set, decide how many plants you need. The plant spacing calculator and square foot gardening calculator show how many plants fit in your beds. Then use the seed quantity calculator to work out how many seeds to sow, allowing for germination rate. Many gardeners start 20 to 30 percent more seedlings than they need, then plant the strongest.

Keeping a seed-starting log

The calculator gives a good first schedule. Over time, your own records make it better. Keep a simple log with:

• The sowing date for each variety.
• How many days it took to germinate.
• The date it went outside, and how big it was.
• The actual last spring frost in your garden that year.
• Notes such as “too leggy, start a week later next year” or “too small, start earlier.”

After two or three seasons, you will have a schedule tuned to your own garden, light setup and favorite varieties.

Common timing mistakes

• Starting too early. The most common mistake. Seedlings kept indoors too long become tall, root-bound and stressed, and often grow more slowly after transplanting than younger plants.
• Starting everything on one day. Peppers need far longer than cucumbers. Follow the crop-by-crop dates.
• Planting tender crops on the frost date. Tomatoes and peppers planted into cold soil sit and sulk. Waiting a week or two often gives a faster harvest, not a slower one.
• Forgetting to harden off. Seedlings need a week or so of gradual outdoor exposure before planting.
• Ignoring the forecast. Averages are averages. Keep row cover handy in case a late frost is predicted.

Seed starting by region

The calculator handles timing automatically once you enter your frost date, but each part of the country has its own quirks worth knowing.

Northern states and high elevations

Short frost-free seasons make indoor starting essential for tomatoes, peppers and eggplant. Choose varieties with shorter days to maturity, and consider adding the one- or two-week safety margin, because late frosts are common. Cool-season crops such as broccoli, cabbage and lettuce can go out a few weeks before the last frost and often do very well in long, cool summers.

Mid-Atlantic, lower Midwest and Pacific Northwest

These regions follow the standard schedule closely. Spring can swing between warm spells and cold snaps, so harden off seedlings carefully and keep row cover handy. In the Pacific Northwest, cool summers mean heat-loving crops benefit from extra warmth, such as black plastic mulch, a sunny wall or a cold frame, after transplanting.

The South and Gulf Coast

Early last frost dates mean seed starting begins in winter. Summer heat arrives quickly, so cool-season crops planted in spring can bolt before maturing. Many Southern gardeners grow their best broccoli, cabbage, lettuce and carrots in fall and winter instead, and start a second round of tomatoes in midsummer for a fall crop.

The desert Southwest

Extreme summer heat creates two main growing seasons, spring and fall, with a difficult stretch in midsummer. Transplants need to be in the ground early enough to set fruit before temperatures soar. Local extension planting calendars for desert regions are especially valuable, because the general rules do not fit well.

Adjusting for your indoor setup

The calculator assumes typical indoor conditions. Your setup can shift the ideal dates.

• Cool rooms slow growth. If your seed-starting space is below normal room temperature, start toward the earlier end of each range.
• Warm rooms and strong lights speed growth. If your seedlings regularly outgrow their pots before planting time, start toward the later end.
• Windowsill growing gives less light, which makes seedlings stretch. Starting a little later, when days are longer, often gives sturdier plants.
• Larger pots let seedlings grow longer indoors without becoming root-bound, so you can start a little earlier if you pot up.

After your first season, compare how your transplants looked at planting time with how you wanted them to look, and shift next year’s dates by a week in whichever direction makes sense.

When to buy seeds

The earliest dates in your schedule may arrive sooner than you expect. If your first sowings fall in January or February, order seeds in early winter, when catalogs arrive and popular varieties are still in stock. Check last year’s leftover packets first, and run a quick germination test on anything more than a couple of years old: roll ten seeds in a damp paper towel, keep them warm, and count how many sprout. Having seeds, mix, containers and lights ready a week or two before your first sowing date means nothing gets started late because of a missing supply.

Frequently asked questions