When to Start Seeds Indoors: Timing Chart for Every Vegetable and Herb

Most seed-starting failures aren’t about technique. They’re about timing. Start tomatoes too early and you’re stuck babying leggy, root-bound seedlings for weeks past when they should’ve gone in the ground. Start them too late and you lose a chunk of your growing season before it even begins.

The fix is simple math once you know two things: your last frost date and how many weeks each plant needs indoors before it’s ready to go out. This guide covers both, the timing chart and how to adjust it whether you’re growing in soil trays or a hydroponic system.

Find your last frost date first

Every timing decision in this guide works backward from one number: the average date of your last spring frost. If you don’t already know yours, check our full guide to finding and using your last frost date, it takes about two minutes to look up by zip code.

Once you have that date, everything below is just counting backward.

Seed starting timing chart

Weeks are counted before your last frost date. A plant listed at “6-8 weeks before” means start it indoors 6 to 8 weeks ahead of that date, not 6-8 weeks from today.

Plant	Start indoors	Notes
Onions, leeks	10-12 weeks before	Slowest starters on this list, don’t skip the head start
Celery, celeriac	10-12 weeks before	Slow and finicky to germinate, keep soil consistently moist
Peppers (sweet & hot)	8-10 weeks before	Wants warm soil to germinate, use a heat mat if your space runs cool
Eggplant	8-10 weeks before	Similar heat needs to peppers
Tomatoes	6-8 weeks before	Earlier than this and they outgrow their pots before it’s safe to plant out
Broccoli, cabbage, kale	6-8 weeks before	Cold-hardy once established, can go out before last frost
Lettuce	4-6 weeks before	Also direct-sows well; starting indoors just gets you an earlier harvest
Basil	6-8 weeks before	Do not plant out until nights stay above 50°F, it sulks in cold soil
Cucumbers, squash, melons	3-4 weeks before	Grows fast, resents root disturbance, use larger cells or biodegradable pots
Beans, peas, corn, carrots, radishes	Direct sow only	Root disturbance sets these back further than starting late would

Adjusting for hydroponic growers

If you’re germinating for a hydroponic system rather than soil trays, the weeks-before-frost math still applies for anything headed outdoors eventually, but a few things change for plants staying in the system:

• Germination media matters more than soil timing: In rockwool, oasis cubes, or germinating without rockwool, most greens and herbs sprout 1-3 days faster than they would in soil, since the media holds consistent moisture without the risk of drying out between waterings. Shift your start date a few days later than the chart above for lettuce, basil and other fast-germinating greens.
• Slow-flowering fruiting crops don’t shift much: Tomatoes and peppers still need their full 6-10 weeks regardless of media, the bottleneck there is plant maturity, not germination speed.
• If your system runs indoors year-round: the last-frost math only matters for anything that’s eventually transplanting outside or into an unheated greenhouse. Plants staying in a climate-controlled hydroponic setup permanently can be started on a rolling schedule instead, start a new tray every 2-3 weeks for continuous harvest rather than timing around frost at all.

Working backward from your last frost date: an example

Say your last frost date is April 15.

• Onions: start by January 21 - February 4
• Peppers: start by February 4 - 18
• Tomatoes: start by February 18 - March 4
• Cucumbers: start by March 18 - 25
• Beans: direct sow after April 15, once soil has warmed

Set calendar reminders for each start date the moment you know your frost date. This is the step most people skip and it’s the entire reason seed starting feels chaotic instead of routine.

Common timing mistakes

• Starting everything on the same day: The chart above exists because different plants need wildly different lead times. Tomatoes and cucumbers started on the same date will be badly mismatched by the time either is ready to go out.
• Not accounting for hardening off: Add 7-10 days to your indoor timeline for gradually acclimating seedlings to outdoor conditions before transplanting. Skip this and even correctly-timed seedlings can go into shock.
• Restarting after a failed germination without adjusting the schedule: If your first tomato tray fails at week 2, you’re now behind, recalculate from today’s date rather than trying to force the original timeline.

What to do while you wait

The weeks before your earliest start dates are a good time to get equipment ready rather than sitting idle: clean and sanitize trays or net pots, check your grow light timer and if you’re growing hydroponically, run a clean water cycle through your system to flush out any residue from its last use.

For the full spring planting sequence this fits into, see the spring gardening guide.

Editor’s Note: Parts of this guide were structured and optimized with the assistance of AI, then thoroughly reviewed, edited and expanded with first-hand growing experience by our author Raymond to ensure practical, real-world accuracy.