How to Start Vegetable Seeds Indoors (Dates, Lights and Hardening Off)

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• Also see: Top 10 Thanksgiving Side Dishes, Ranked
• Also see: The Holiday Season, Sorted

Quick answer: find your last frost date, count back each crop’s weeks-before-frost number, and sow into a fine starting mix under strong light 2 to 4 inches above the seedlings, 14 to 16 hours a day. Grow cool with airflow, feed only after true leaves, and harden off 7 to 10 days before transplanting.

The two numbers that decide everything

Sow date and light distance. Everything else is refinement. Peppers need the longest head start; squash need almost none. A bright window a foot away reads as darkness to a seedling; a cheap LED bar inches above reads as noon.

Last frost date: your anchor · every sowing date counts back from it
Peppers: 8 to 10 weeks before · slowest to germinate and size up
Tomatoes and basil: 6 to 8 weeks before · the classic indoor crop
Onions: 10 to 12 weeks before · tiny seeds, long runway
Squash and cucumbers: 3 to 4 weeks before · sow late or direct
Light: 14 to 16 hours a day · 2 to 4 inches above the tops
Harden off: 7 to 10 days · shade first, then sun and wind

type: bar
title: Weeks before last frost to sow indoors
x: Onions, Peppers, Tomatoes, Basil, Squash, Cucumbers
Weeks: 11, 9, 7, 6, 3.5, 3

The shape is the whole plan: slow crops cluster at two months out, fast vines at under a month. Squash sown with the peppers becomes a monster.

The seed-start date calculator

Enter your average last frost date and it counts back each crop for you.

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  <h4>Seed-Start Date Calculator</h4>
  <div><label>Average last frost date</label><input type="date" id="frost" value="2026-05-10" oninput="calc()"></div>
  <table id="t"></table>
  <script>
    var crops = [['Onions', 11], ['Peppers', 9], ['Tomatoes', 7], ['Basil', 6], ['Squash', 3.5], ['Cucumbers', 3]];
    function fmt(d) { return d.toISOString().slice(0, 10); }
    function calc() {
      var f = new Date(document.getElementById('frost').value + 'T12:00:00');
      if (isNaN(f)) return;
      var html = '<tr><th>Crop</th><th>Sow indoors</th></tr>';
      crops.forEach(function (c) {
        var d = new Date(f.getTime() - c[1] * 7 * 864e5);
        html += '<tr><td>' + c[0] + '</td><td class="s">' + fmt(d) + '</td></tr>';
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Soil, water, warmth, light

• Mix: fine seed-starting mix, pre-moistened to a wrung-sponge feel. Garden soil compacts and carries damping-off fungi.
• Depth: cover seeds about twice their own diameter; press tiny seeds onto the surface and mist.
• Water: from below by setting trays in a shallow bath. A humidity dome helps until germination, then comes off.
• Warmth: most vegetables germinate fastest at 65 to 75F. Peppers want the warm end; lettuce prefers it cooler.
• Light: hang on chains so you can raise it, keeping 2 to 4 inches of clearance, 14 to 16 hours on a timer. A small fan on low builds thicker stems.

Potting up, feeding, hardening off

True leaves mean the seed’s packed lunch is spent: feed at quarter to half strength weekly at most. Move seedlings into bigger pots when roots circle the cell — tomatoes go deeper, everything else at the same depth. Handle by the leaves, never the stem.

Harden off over 7 to 10 days: an hour of outdoor shade first, adding time, sun and breeze daily, nights out only at the end above 50F. Plant out after frost danger, on a cloudy afternoon if you can choose.

Harden off over 7 to 10 days: an hour of outdoor shade first, adding time, sun and breeze daily, nights out only at the end above 50F. Plant out after frost danger, on a cloudy afternoon if you can choose.

Gear up the windowsill

The calculator gives the dates; these are the purchases that change outcomes. Light first, then mix, then seed — that order is the priority order.

Seed-Starting Kit

Light is the strongest lever. Everything else is refinement.

Strongest lever

Barrina T5 Grow Lights (4-pack)

Linkable LED bars that hang inches above trays; the fix for leggy seedlings.

$50.99

Easiest start

Ferry-Morse Indoor Seed Starting Kit

Tray, dome and cells in one box for a first sowing season.

$29.95

Fine texture

Burpee Seed Starting Mix

Fine, low-nutrient mix that keeps small seeds in moist contact.

$10.99

Gentle feed

Espoma Organic Indoor Plant Food

Mild organic feed, easy to dose at quarter strength after true leaves.

$14.99

As an Amazon Associate, Eric Varney earns from qualifying purchases. It costs you nothing extra, and it does not change which products I recommend or what I say about them.

Watch it explained

A complete walkthrough of starting seeds under lights — trays, mix, light height and timing.

The one rule for indoor sowing

Light close, dates counted back, hardening off unskipped. Get those three right and the rest is tidying. When the harvest lands, the Thanksgiving sides ranking is where half of it ends up — and starting your own herbs pairs well with How to Make a Herbal Tincture.

Frequently asked questions

When should I start vegetable seeds indoors?

Count back from your average last frost date: peppers 8 to 10 weeks, tomatoes and basil 6 to 8, onions 10 to 12, squash and cucumbers only 3 to 4.

Do seedlings need a grow light or is a windowsill enough?

A windowsill is rarely enough: seedlings stretch tall and weak reaching for side light. An LED bar hung 2 to 4 inches above them, on 14 to 16 hours a day, grows stocky transplants.

What soil should seeds start in?

A fine, low-nutrient seed-starting mix, not garden soil and not rich potting compost.

What is hardening off and why does it matter?

About 7 to 10 days of gradually increasing outdoor exposure, shade first then sun and breeze. Skipping it causes sunscald and transplant shock.

Why are my seedlings tall and floppy?

Almost always too little light, too far away. Lower the light, run it longer, add gentle airflow, and grow them slightly cooler.

• Also see: How to Make a Herbal Tincture — and Work Out the Dose
• Also see: The Holiday Season, Sorted: Every Top 10 in One Place