When to Start Seeds Indoors: A Zone-by-Zone Guide

⬇ Download the printable zone-by-zone schedule

Why indoor seed timing matters more than the seed packet

Seed packets give a generic answer: "start indoors 6–8 weeks before your last frost." But that only works if you actually know your last frost date — and if you have the right plan for what to do with seedlings once they outgrow the tray. Start too early and your tomatoes turn into leggy, root-bound vines weeks before it's safe to plant them out. Start too late and you lose a month of harvest.

The fix is to anchor every sowing date to two numbers: your USDA hardiness zone (which sets your average last spring frost) and the number of weeks each crop needs indoors before transplant. Get those right, and the rest of the season runs smoother.

Step 1: Find your last frost date

Your average last spring frost is the date your area typically stops getting freezing temperatures. It's an average, not a guarantee — but it's the best starting point for indoor sowing math.

• Zone 3: late May to early June
• Zone 4: mid May
• Zone 5: early May
• Zone 6: mid to late April
• Zone 7: early to mid April
• Zone 8: mid to late March
• Zone 9: late February to early March
• Zone 10: frost-free (sow on a year-round schedule)

For a precise local date, BotaNotes auto-detects your zone from your location and stores it so every planting reminder is timed to your garden, not someone else's.

Step 2: Count backward from your last frost

Once you have a last frost date, indoor seed starting becomes simple arithmetic. Subtract the number of weeks each crop needs indoors and that's your sowing date.

Crops to start 8–10 weeks before last frost

• Onions, leeks, and shallots — slow to size up; start early or buy sets
• Celery and celeriac — germinate slowly and benefit from a head start
• Artichokes — need vernalization plus a long indoor run

Crops to start 6–8 weeks before last frost

• Tomatoes — the classic 6-week crop; longer if you have grow lights
• Peppers and eggplant — slow germination, want warmth (80°F)
• Broccoli, cauliflower, cabbage, brussels sprouts — cool-season brassicas

Crops to start 4–6 weeks before last frost

• Lettuce, kale, swiss chard — quick and forgiving
• Basil and most annual herbs — sow once soil and air both warm

Crops to start 2–4 weeks before last frost

• Cucumbers, zucchini, summer squash, melons, pumpkins — they hate transplant shock; only start indoors if your season is short

Crops not worth starting indoors

• Carrots, beets, radishes, parsnips, turnips — root crops resent transplanting; direct-sow when soil works up
• Beans, peas, corn — germinate fast outdoors and rarely benefit from an indoor start

Step 3: Give seedlings the right conditions

Timing is half the battle. The other half is light, warmth, and airflow. Most seed-starting struggles — leggy stems, damping off, pale leaves — come from too little light, too much heat after germination, or stagnant air.

• Light: Sunny windows almost never give enough. A simple shop light with full-spectrum LED bulbs, 2–3 inches above the seedlings, on 14–16 hours a day, produces dramatically stockier plants than any window.
• Heat: Use a heat mat to germinate (70–80°F for most crops, 80–85°F for peppers and eggplant), then move trays off the mat once seedlings emerge. Cooler air after germination prevents stretching.
• Airflow: A small fan on low strengthens stems and prevents fungal issues. Run it a few hours a day once true leaves appear.
• Watering: Bottom water by setting trays in a shallow pool of water for 10–15 minutes. It keeps leaves dry and roots reaching downward.

Step 4: Harden off before transplant

Indoor seedlings cannot go straight into the garden, no matter how warm it gets. They need a week to ten days of hardening off — gradual exposure to outdoor sun, wind, and temperature swings. Start with an hour in dappled shade, add an hour each day, and avoid moving them outside on windy or freezing days. Skipping this step is the single most common reason home-grown transplants fail.

Step 5: Track what worked and adjust next year

The best indoor seed-starting calendar is the one you build from your own garden over time. Note which varieties germinated best, which trays got leggy, which crops transplanted cleanly, and which sat too long indoors. Two or three seasons of records will beat any generic planting guide.

That's exactly what BotaNotes is built for. Log your sowing dates, photograph your trays, and the app will remind you when each crop is ready to transplant — calibrated to your zone, not a national average.

Quick zone-based starting schedule

Here's a one-glance summary, using a Zone 6 garden (last frost ~April 25) as the example:

• Mid February: onions, leeks, celery
• Late February: peppers, eggplant
• Early March: tomatoes, brassicas (broccoli, cabbage, cauliflower)
• Mid March: lettuce, kale, swiss chard, herbs
• Early April: cucumbers, squash, melons (only if season is short)
• Mid April: start hardening off the earliest trays
• Late April / early May: transplant frost-hardy crops; wait until mid-May for tomatoes, peppers, and squash

Shift the entire calendar two weeks earlier for each zone warmer than 6, and two weeks later for each zone colder. The pattern stays the same; only the dates move.