Starting seeds indoors means sowing seeds in containers under controlled light, warmth, and moisture several weeks before your area’s last spring frost, then transplanting the resulting seedlings outdoors once conditions allow. Done right, it gives you earlier harvests, a wider choice of varieties than nursery six-packs offer, and full control over how your plants are grown from day one.
Below is the full process — containers, soil, sowing depth, watering, light, and heat, through to hardening off and transplanting — followed by a timing guide and answers to the most common per-plant and technique questions.
Starting indoors isn’t the only way to grow from seed — plenty of crops do fine sown directly into the garden once the soil warms. The reasons to start indoors instead come down to three things: a longer effective growing season in areas with a short frost-free window, a much wider variety selection than what nurseries stock as transplants, and lower cost per plant when you’re filling more than a handful of pots. Slow-growing, cold-sensitive crops like peppers and onions benefit the most; fast, transplant-resistant crops like cucumbers and sunflowers benefit the least, and are often better off sown straight into the ground.
What You’ll Need
The core list is short: containers with drainage, a seed-starting mix, water, and a warm, bright spot. Depending on what and how much you’re starting, many indoor growers also add a heat mat to speed germination, a tent or clear dome to hold in humidity, and a small grow light to replace or supplement window light. We cover verified options for each of those on our dedicated gear pages — seedling heat mats, grow tents, and complete seed starting kits that bundle trays, domes, and growing medium together. None of that gear is required to get started — plenty of successful seed-starting happens on a sunny windowsill with nothing more than trays and a heat source from the room itself — but it’s worth knowing what exists before you decide what your setup needs.
Step 1: Choose Your Containers
Any container with drainage holes works: seed trays with individual cells, small pots, or even reused yogurt cups with holes punched in the bottom. Cell trays (usually 6, 12, or 24 cells per tray) are the standard choice because each seedling gets its own root space, which reduces transplant shock later. A clear humidity dome over the tray holds in moisture during germination and comes off once seedlings emerge. Whatever you use, make sure it’s been cleaned — reused pots can carry fungal spores that cause damping-off, a common cause of seedling collapse.
Cell size matters more than it looks like it should: a too-small cell dries out fast and restricts root growth, while an oversized cell holds more moisture than a young seedling’s small root system can use, which raises the risk of rot. For most vegetables and flowers, a standard 72-cell or 128-cell tray strikes a reasonable balance for the first few weeks; slower growers like onions and peppers can stay in smaller cells longer than fast growers like squash, which often need bumping up to a larger pot before they even go outside.
Step 2: Use a Seed-Starting Mix, Not Garden Soil
Garden soil is too dense for containers and often carries pathogens or weed seeds. Use a dedicated seed-starting mix — typically a light, sterile blend of peat or coconut coir, perlite, and vermiculite with little to no added fertilizer. Fine texture matters here: a seedling’s first roots are delicate, and a heavy potting mix or garden soil can crust over and block emergence entirely. Moisten the mix before filling containers so it’s evenly damp, not soggy, before you sow.
Step 3: Sow at the Right Depth
The general rule is to plant a seed about twice as deep as its own diameter — most vegetable and flower seeds end up somewhere between a quarter-inch and half-inch deep. Some seeds, including many lettuce varieties and a number of flowers such as snapdragon, need light to germinate and should sit on the surface of the mix, pressed in gently rather than covered. Always check the seed packet first; depth and light requirements vary enough by species that a packet’s specific instructions override any general rule. Sow two or three seeds per cell as insurance against poor germination, then thin to the strongest single seedling once true leaves appear.
Step 4: Water From Below and Keep the Mix Evenly Moist
Bottom watering — setting the tray in a shallow reservoir of water and letting the mix absorb moisture upward — keeps the surface from staying wet, which is the single biggest factor in preventing damping-off. Top watering with a gentle mist works too, but avoid a heavy stream that dislodges seeds or compacts the mix. The goal is consistently damp, never waterlogged or bone-dry; a humidity dome helps maintain this during the germination window, but should come off as soon as seedlings emerge to allow airflow.
Step 5: Give Seedlings Strong, Consistent Light
The most common seed-starting failure isn’t germination — it’s what happens after: seedlings that stretch into thin, pale, leaning stems because they didn’t get enough light. A south-facing window rarely provides enough intensity on its own in late winter and early spring, which is why most indoor growers add a small grow light positioned just a few inches above the seedlings, raised as they grow, and run for roughly 14–16 hours a day. Our guide to grow lights for seedlings covers verified picks sized for standard seed-starting trays.
Step 6: Keep the Root Zone Warm
Most vegetable and flower seeds germinate fastest with a soil temperature in the roughly 70–80°F range — often warmer than typical indoor room temperature, especially near a drafty window. A seed-starting heat mat placed under the tray raises soil temperature a controlled amount above ambient and can meaningfully speed up and even out germination, particularly for warm-season crops like tomatoes and peppers. It’s most useful during germination; most growers remove the mat once seedlings are up, since established seedlings don’t need the extra warmth and prefer slightly cooler air to stay compact.
Step 7: Harden Off Before Transplanting
Seedlings grown indoors haven’t experienced direct sun, wind, or temperature swings, and moving them outdoors without preparation can shock or kill them. Over 7–10 days, gradually introduce them to outdoor conditions: start with an hour or two in a shaded, sheltered spot, then increase both duration and sun exposure a little each day, bringing them in if a cold snap threatens. Transplant on an overcast day or in the evening once your area’s frost risk has passed for that crop, and water well immediately after planting to settle the roots.
Skipping this step is one of the most common ways a batch of otherwise healthy seedlings gets lost right at the finish line — a seedling that’s spent six or eight weeks under stable indoor light and temperature has no tolerance built up for direct midday sun or a sudden gust of wind, and can wilt, scorch, or snap within hours of being set out unprepared. The week of hardening off costs nothing but a little attention and is cheap insurance against losing an entire tray’s worth of work.
When to Start: Zones and Timing
Two different pieces of data answer two different questions, and it’s worth keeping them separate. The USDA Plant Hardiness Zone Map — updated in November 2023 using 1991–2020 climate data — tells you the average annual minimum winter temperature where you live, which is mainly useful for choosing perennials that can survive your winters. Your average last spring frost date, tracked by NOAA and regional extension offices, is the number that actually drives a seed-starting schedule: it’s the date you count backward from to know when to sow.
As a rough guide (check your own zone and frost date at the links above, since both vary by local microclimate, not just by zone): colder zones (3–4) typically see their last frost in mid-to-late May or later; mid-range zones (5–6) usually clear frost by early-to-mid May; warmer zones (7–8) are often frost-free by late March or April; and the warmest zones (9–10) may see little or no frost at all. Once you know your date, work backward using the per-crop windows below.
- 8–10 weeks before last frost: onions, leeks, and other slow starters, plus long-season flowers like snapdragon
- 6–8 weeks before last frost: tomatoes, eggplant, and peppers
- 4–6 weeks before last frost: broccoli and other brassicas, lettuce, basil, marigolds, and zinnias
- 2–4 weeks before last frost (or direct-sown after frost): cucumbers, melons, pumpkins, and nasturtiums — crops that transplant poorly and are often better off started in biodegradable pots or sown directly outdoors once soil has warmed
These windows are general starting points drawn from consistent guidance across university extension programs, not a substitute for the specific date your own extension office publishes for your area.
Frequently Asked Questions
When should I start tomato seeds indoors?
Start tomato seeds about 6–8 weeks before your area’s average last frost date. Tomatoes are warm-season plants and transplant well, so starting too early just means leggy, overgrown seedlings waiting for the ground to warm up.
When should I start basil seeds indoors?
Start basil about 4–6 weeks before your last frost date. Basil is cold-sensitive and grows quickly once warm, so there’s little advantage to starting it much earlier — it can catch up fast under good light and heat.
When should I start pepper seeds indoors?
Start peppers 8–10 weeks before your last frost date. Peppers are slower to germinate and grow than tomatoes, and they want consistently warm soil (a heat mat helps considerably here), so give them the longest head start of the common warm-season vegetables.
When should I start onion seeds indoors?
Start onions from seed 8–10 weeks before your last frost date. Onions are slow growers early on, and a longer indoor head start produces larger bulbs by the time the growing season ends.
When should I start broccoli seeds indoors?
Start broccoli 4–6 weeks before your last frost date. Broccoli is a cool-season crop and tolerates a slightly earlier transplant outdoors than warm-season vegetables, as long as a hard freeze isn’t imminent.
When should I start lettuce seeds indoors?
Start lettuce 4–6 weeks before your last frost date. Many lettuce varieties need light to germinate, so press seeds into the surface of the mix rather than burying them.
When should I start cucumber seeds indoors?
If starting indoors at all, sow cucumbers only 2–4 weeks before your last frost date, ideally in biodegradable pots, since cucumbers have sensitive roots and transplant poorly. Many gardeners skip indoor starting entirely and direct-sow cucumbers outdoors once the soil has warmed.
When should I start watermelon seeds indoors?
Start watermelon 2–4 weeks before your last frost date, in biodegradable pots to minimize root disturbance at transplant. Like cucumbers, watermelon is often direct-sown outdoors instead once the soil is reliably warm.
When should I start pumpkin seeds indoors?
Start pumpkins 2–4 weeks before your last frost date if you want a head start, again in biodegradable pots. Pumpkins are frequently direct-sown outdoors instead, since their vigorous growth quickly catches up to transplanted starts.
Can I grow strawberries from seed indoors, and when should I start?
Yes, though most home growers start strawberries from bare-root crowns or runners rather than seed, since seed-grown strawberries germinate slowly and unevenly. If starting from seed, sow 10–12 weeks before your last frost date and expect germination to take several weeks.
When should I start marigold seeds indoors?
Start marigolds 4–6 weeks before your last frost date. They germinate and grow quickly, so there’s little benefit to starting them any earlier, and many gardeners direct-sow them outdoors instead once frost has passed.
When should I start zinnia seeds indoors?
Start zinnias 4–6 weeks before your last frost date. Like marigolds, zinnias are fast growers and transplant reasonably well, but they also do well direct-sown once the soil has warmed.
When should I start sunflower seeds indoors?
Sunflowers are usually direct-sown outdoors after the last frost, since their long taproot resents transplanting. If you do start them indoors, sow only about 2–3 weeks ahead in biodegradable pots to minimize root disturbance.
When should I start snapdragon seeds indoors?
Start snapdragons 8–10 weeks before your last frost date. They’re slow starters and, like some lettuce varieties, need light to germinate, so press seeds onto the surface of the mix rather than covering them.
When should I start hollyhock seeds indoors?
Start hollyhocks 6–8 weeks before your last frost date. Hollyhocks are often grown as biennials, meaning plants started this year may not flower heavily until their second season.
When should I start nasturtium seeds indoors?
Start nasturtiums only 3–4 weeks before your last frost date, since they grow quickly and transplant poorly. Many gardeners skip indoor starting altogether and direct-sow nasturtiums outdoors after frost risk has passed.
What is the paper towel method for starting seeds?
The paper towel method pre-germinates seeds before they go into soil: dampen a paper towel, spread seeds on one half, fold the other half over, and seal it in a plastic bag in a warm spot. Check daily; once you see a root emerge, transfer the seed carefully into soil. It’s most useful for checking whether old seed is still viable, or for slow, uneven germinators, and it isn’t necessary for most common vegetables and flowers.
Can I use egg cartons to start seeds?
Cardboard egg cartons can work for a small batch of seeds, but they hold very little soil volume and dry out quickly, so they need more frequent watering than a standard cell tray and generally need transplanting into a larger container sooner. They work best as a low-cost way to try a few extra varieties, not as a full-scale setup.
Can I start seeds indoors without a grow light?
Yes, with a caveat: a very bright, unobstructed south-facing window can be enough for some crops, but most homes don’t get consistent enough light in late winter and early spring to prevent the leggy, stretched growth that comes from insufficient light. If seedlings are leaning hard toward the window or look thin and pale, that’s the sign to add supplemental light rather than push through on a windowsill alone.
Ready to Choose Your Setup?
The steps above work with whatever equipment you already have on hand, but if you’re deciding what to buy, our three seed-starting gear guides go deeper on each category: seedling heat mats for faster, more even germination, grow tents for a controlled, enclosed setup as your growing space expands, and complete seed starting kits if you’d rather start from one box that already includes trays, a dome, and growing medium. If you’re specifically starting herbs, see our dedicated guide to growing herbs indoors for herb-specific timing and care.
General growing information above is provided for educational purposes. Always confirm your local last frost date and check individual seed packets, since timing and depth vary by variety.