When to Start Seeds for Spring: A Complete Timing Guide

Quick Answer

The right time to start seeds for spring depends on your last frost date, not the calendar. Count backward from that date: 10-12 weeks for onions and leeks, 8-10 weeks for peppers and eggplant, 6-8 weeks for tomatoes, and 4-6 weeks for brassicas and lettuce. Beans, squash, corn, and cucumbers skip indoor starting entirely and go straight into the ground once the soil warms.

Every seed packet gives you several weeks, not a fixed date, and that’s on purpose. A gardener in Minneapolis and a gardener in Atlanta both start tomatoes 6 to 8 weeks before their last frost, but those two dates can sit 2 months apart. Knowing when to start seeds for spring means knowing your local last frost date first and building every other date around it.

This guide walks through how to find that date, how to count backward for each type of crop, and which vegetables skip the indoor step altogether.

Step 1: Find Your Last Frost Date

Your last frost date is the average date when the risk of a freeze drops to about 50%. It’s not a guarantee; a late frost can still happen after this date, but it’s the standard planning point extension services use.

Look it up through a frost date calculator using your zip code, or check with your local cooperative extension office. Rough zone averages look like this:

• Zones 3-4: Early to late May
• Zones 5-6: Mid-April to early May
• Zones 7-8: Late March to mid-April
• Zones 9-10: Late February to mid-March

If you garden in the Southern Hemisphere, this whole calendar shifts. Spring seed starting runs from August through October, with the frost-free window opening between October and December.

Step 2: Count Backward From That Date

Once you have your last frost date, subtract the number of weeks listed on your seed packet. If your last frost falls on May 15 and your tomato packet says “start 6 weeks before last frost,” you’d sow tomato seeds around April 3.

Do this for each crop separately. A garden with tomatoes, peppers, and lettuce needs 3 different start dates, not one.

Seed Starting Timeline by Weeks Before Last Frost

10-12 Weeks Before: Slow Growers

Onions, leeks, and celery take a long time to reach transplant size, so they go in first. Some slow-growing herbs, like parsley and chives, also fall into this window.

8-10 Weeks Before: Peppers and Eggplant

Peppers and eggplant germinate slowly and grow slowly at first, so they need this extra runway. Many long-season herbs and artichokes also start here.

6-8 Weeks Before: Tomatoes

Tomatoes germinate faster than peppers but still need 6 to 8 weeks indoors to reach a sturdy transplant size. Starting them earlier than that usually backfires; more on that below.

4-6 Weeks Before: Brassicas and Lettuce

Broccoli, cabbage, and other brassicas, along with lettuce, grow fast enough that they only need a month or so indoors before they’re ready to move outside, often while there’s still a light frost risk.

2-3 Weeks Before: A Few Fast Starters

Cucumbers and some summer squash can be started indoors in this narrow window if you want a small head start, though most gardeners direct-sow these instead.

Crops That Skip Indoor Starting Entirely

Some vegetables don’t transplant well because their roots resent disturbance. These go straight into the ground as seed, either right at your last frost date or a few weeks before it, depending on whether they’re cool-season or warm-season crops.

What Happens If You Start Too Early or Too Late

Starting too early is the more common mistake. Seedlings started weeks ahead of schedule outgrow their pots before it’s safe to plant them outside. They get leggy, root-bound, and stressed, and often perform worse than seeds started on time or even a bit late.

Starting too late costs you part of the growing season. A tomato plant started 2 weeks late still needs its full 6 to 8 weeks indoors, which pushes back your first ripe fruit by the same 2 weeks.

If you’re not sure which way to round, err a few days later rather than earlier. A slightly smaller transplant handles the shock of moving outside better than an overgrown one.

Hardening Off: The Step Before Transplanting

Seedlings started indoors need a transition period before they go into the garden permanently. This process, called hardening off, means setting trays outside for a few hours a day in a sheltered spot, then gradually increasing their time outdoors and exposure to direct sun and wind over 7 to 10 days.

Skipping this step is a common reason transplants wilt or die within days of being planted out, even when the timing was otherwise correct.

Common Mistakes When Timing Seed Starting

Using one start date for the whole garden. Onions, tomatoes, and squash all need different lead times. Write out a separate date for each crop instead of starting everything the same weekend.

Ignoring soil temperature for direct-sow crops. Beans and squash seeds rot in cold, wet soil even after the air temperature warms up. Wait until soil consistently reads above 60°F before direct sowing warm-season crops.

Forgetting your region’s frost data can be wrong for your yard. A frost date calculator gives you an average for your broader area. A low spot in your yard, or a spot next to a north-facing wall, can run several degrees colder and see frost later than your zip code average suggests.

Not accounting for indoor light. Seedlings started under weak windowsill light grow leggy no matter how well you time the sowing date. A grow light held 2 to 3 inches above the seedlings fixes most of this.

Final Thoughts

Find your last frost date, then count backward using the weeks listed on each seed packet: 10-12 weeks for onions and leeks, 8-10 for peppers, 6-8 for tomatoes, 4-6 for brassicas and lettuce. Skip the indoor step for beans, squash, corn, and root crops, and harden off every transplant before it goes in the ground for good. Get the frost date right, and the rest of the schedule falls into place on its own.

FAQs

How do I find my exact last frost date? 

Enter your zip code into a frost date calculator through your local extension service or a site like the National Gardening Association. This gives you a 50% probability date specific to your area.

Can I start seeds before I know my last frost date?

It’s better to look it up first. Starting from a guess risks sowing tomatoes or peppers too early, which leads to leggy, stressed seedlings by the time it’s actually safe to plant outside.

What if I missed my ideal start window?

 Start as soon as you can. A tomato started 2 weeks late still needs its full 6 to 8 weeks indoors; your harvest shifts later, but the plant isn’t ruined.

Do all vegetables need to be started indoors? 

No. Beans, corn, squash, cucumbers, melons, peas, carrots, and radishes all grow better when sown directly into garden soil, since their roots don’t transplant well.

Is the timing different in a greenhouse or high tunnel? 

Yes, a protected structure can push your effective last frost date earlier by several weeks, which lets you start and transplant sooner. Check the structure’s actual internal temperature rather than assuming it matches outdoor zone averages.