Seed starting indoors, timing backwards from last frost
The question I get every January, usually from someone who's about to buy their first packets of tomato seeds: "When should I start these?"
Answer: count backwards from your last frost date. For tomatoes, that's 6-8 weeks. For peppers, 8-10 weeks. For brassicas, 4-6. For squash, 2-3. Every crop has a "how early indoors" window that's pretty well established.
The tricky part is knowing your actual last frost and having enough grow-light space for everything you want to start. Here's how I run the indoor nursery every year.
Finding your last frost date
The Old Farmer's Almanac frost calculator is the single best tool for this. Type in your ZIP code, it gives you the "50% probability of last frost" date, the "10% probability" date (the conservative safe date), and the historical range.
For my Zone 6a location in central Indiana:
- Average last frost: April 28
- 10% probability last frost: May 15
- I plant warm-season crops after May 15 (conservative)
- I plant cold-hardy transplants outdoors starting April 1 under row cover
The 10% date is what matters for transplants. Average dates tell you the mean, but they don't protect you from the 1-in-10 year that destroys your garden.
The backward calendar
From last frost, counting back:
10 weeks before (March 6 for me)
- Peppers (sweet and hot): slow to germinate, slow to grow, need a head start
- Onions from seed: if you're doing Walla Walla or Copra from seed instead of sets
- Eggplant: similar timing to peppers, heat-loving, slow-developing
8 weeks before (March 20)
- Tomatoes (all varieties)
- Celery: slow to germinate, benefit from extra time
- Leeks: thin seedlings, need weeks to develop
6 weeks before (April 3)
- Brassicas: broccoli, cabbage, cauliflower, Brussels sprouts, kale
- Lettuce (for transplant, not direct sow)
- Basil: can go this early if you keep it warm and sunny
4 weeks before (April 17)
- Cucumbers, summer squash, zucchini: fast-growing, don't start too early
- Melons: only if you have a long season
2-3 weeks before (April 24)
- Winter squash (if you're getting a jump)
- Okra (heat-loving, needs soil at 70F+ to transplant)
Don't start indoors
- Root crops: carrots, radishes, beets, parsnips. Direct-sow only.
- Beans and peas: direct-sow.
- Corn: direct-sow after last frost.
- Dill, cilantro: direct-sow, they hate transplanting.
My indoor setup
Basement rack, 4 feet wide, 4 feet tall. Three shelves. Each shelf has a Barrina T8 LED grow light (about $30 on Amazon, 4 feet long, full spectrum). Lights run 14 hours a day on a timer, noon to 2 AM works for me because that's when my electricity is cheapest.
Seedling trays are standard 72-cell trays from Bootstrap Farmer. Reusable, thick plastic, fit in standard 1020 solid trays for bottom-watering.
Heat mat for peppers and eggplant only, those need soil temps of 80F+ to germinate reliably. Everything else germinates fine at basement temperature (around 65F).
I cap seedlings about 2 inches under the lights and raise the lights as they grow. Too far from the light and seedlings get leggy.
Soil mix for seedlings
Not garden soil. Not potting mix. A real seed-starting mix: Pro-Mix HP or Fox Farm Light Warrior. These are peat-based, light, and sterile. Seedlings don't have enough root mass to push through dense soil.
Fill cells, tamp lightly, drop 1-2 seeds per cell, dust with vermiculite, water gently from the bottom by pouring into the 1020 tray.
The 2023 late-freeze disaster
May 2023. I'd planted out my tomato transplants on May 10 because the 7-day forecast looked clean. May 14, Indiana got a surprise overnight freeze (29F). I lost 18 tomato transplants overnight. That was the year I learned: don't trust the forecast; trust the statistical date.
Now I always wait until Memorial Day weekend for tomatoes and peppers. Even if the forecast says 60F at night. Zone 6a safety margin is real, and a single freeze in May erases six weeks of indoor nursery effort.
Hardening off
Seedlings you started indoors can't go directly into full sun. They've never seen UV or wind. Move them outside gradually over 7-10 days:
- Day 1: 1 hour in shaded, protected spot
- Day 2: 2 hours same spot
- Day 3-4: 3-4 hours, partial sun
- Day 5-6: 4-6 hours, more direct sun
- Day 7-10: full day outside
Skip this and you'll watch healthy seedlings bleach white and die from sunburn in a single afternoon. I did exactly this in 2020 with 24 pepper seedlings. Lost all of them.
Transplanting day
Dig a hole deeper than the root ball. For tomatoes specifically, bury the stem up to the first set of leaves, they root along the buried stem and produce stronger plants. Water deeply at transplant. Mulch immediately.
If there's any frost risk in the 10-day forecast, hold off or use frost cover.
What to buy vs start
Seed starting makes sense for:
- Unusual varieties not available as transplants (most heirlooms)
- Volume: 24+ transplants of the same variety
- Cost: a packet is $3-4, transplants are $4-6 each
Buying transplants makes sense for:
- 2-6 plants of common varieties (Celebrity tomato, Better Boy, Jalapeno)
- Your first year gardening, when you don't have grow lights
- Last-minute additions you didn't plan for
I do a mix. Heirloom tomatoes (Black Krim, Cherokee Purple, Brandywine) I start from seed, because nobody sells those transplants locally. Peppers I buy from a local greenhouse because starting peppers is finicky and the local grower does a better job than my basement.
Seed storage and viability
A packet you don't use doesn't expire in one year. Most vegetable seeds stay viable 3-5 years if stored cool and dry. My protocol: original seed packets go into a labeled mason jar with a silica desiccant pack, jar lives in the fridge from harvest season through the following March.
Viability by crop (rough rule of thumb):
- Onion, leek, parsley: 1 year, buy fresh every season
- Carrot, pepper, corn: 2-3 years
- Tomato, bean, pea, radish, lettuce: 4-5 years
- Cucumber, squash, melon, brassicas: 5+ years
If you're unsure about a packet, do a quick viability test. Dampen a paper towel, place 10 seeds on it, fold, seal in a plastic bag, keep at room temperature. Check in 7-10 days. If 7+ sprouted, the packet's fine. If fewer than 5, buy fresh.
The one-paragraph calendar
Eight weeks before last frost, sow tomatoes. Six weeks, brassicas. Four weeks, cucumbers and squash. Direct-sow everything else after last frost. Harden off for 7-10 days before planting out. Don't trust the weekly forecast, trust the statistical last-frost date plus 2 weeks.
Plan how many transplants you need with the calculator. Enter your bed size, pick your plants, and the calculator tells you exactly how many seeds and transplants to prepare.