How To Start Seeds Indoors Before Spring

Getting a Head Start on the Growing Season

Starting seeds indoors is one of the most satisfying things a home gardener can do. Germinating seeds weeks before the last frost gives you more time to grow, lets you choose from varieties most nurseries don’t carry, and helps you get to know your plants from day one. You’ll need to plan ahead, pick up a few basic supplies, and learn what each type of seed needs early on.

When to start seeds indoors depends on your local last frost date — and that changes a lot depending on your USDA Plant Hardiness Zone. In Zone 5, where the last frost usually falls between April 15 and May 15, many cool-season annuals go in trays as early as late February. In Zone 8, where the last frost often comes around March 15, gardeners might not start those same crops until mid-January. Knowing your zone is the first thing to figure out before you plant anything.

Choosing the Right Seeds for Indoor Starting

Not all plants do well started indoors. Carrots, parsnips, and other root vegetables don’t like being moved and grow best when sown straight into the garden. But plenty of flowers — annuals, perennials, and even some biennials — take well to an indoor start.

Snapdragons (Antirrhinum majus) are a longtime favorite. They need 10 to 12 weeks indoors before going outside and do best in soil with a pH between 6.0 and 7.0. Petunias also need 10 to 12 weeks and are slow to sprout, so starting them inside is usually the only way to get full blooms by summer. Lisianthus (Eustoma grandiflorum), popular with florists and tested regularly at the Chicago Botanic Garden, needs 16 to 20 weeks indoors — meaning gardeners in colder zones often start seeds in December or January for summer flowers.

Perennials like columbine (Aquilegia spp.) — which blooms late spring to early summer and grows in Zones 3 through 9 — and coneflower (Echinacea purpurea) — which benefits from 8 to 10 weeks indoors before hardening off — can also be started from seed. Many perennial seeds need cold and moisture first (a process called stratification) to break dormancy and improve germination. The American Horticultural Society notes this helps (American Horticultural Society, 2022).

Seeds That Benefit Most from Indoor Starting

• Snapdragon (Antirrhinum majus) — 10 to 12 weeks before last frost; full sun; pH 6.0–7.0
• Petunia — 10 to 12 weeks before last frost; full sun; pH 6.0–7.0
• Lisianthus (Eustoma grandiflorum) — 16 to 20 weeks before last frost; full sun; pH 6.5–7.0
• Coneflower (Echinacea purpurea) — 8 to 10 weeks before last frost; full sun to part shade; pH 6.0–7.0
• Columbine (Aquilegia spp.) — 8 to 10 weeks before last frost; part shade; pH 6.0–7.0
• Impatiens — 10 to 12 weeks before last frost; shade to part shade; pH 6.0–6.5
• Lobelia — 10 to 12 weeks before last frost; full sun to part shade; pH 6.0–7.0

Equipment and Setup

You don’t need a greenhouse to start seeds indoors. A spare room, basement corner, or sunny south-facing windowsill can work — though adding lights usually makes a big difference. Even a bright south window doesn’t give seedlings enough intensity during the short days of late winter. Most need 14 to 16 hours of light daily to grow compact and strong.

Full-spectrum LED grow lights have mostly replaced fluorescent shop lights. Modern LED panels use about 30 to 50 watts per square foot and hang just 2 to 4 inches above the trays — you raise them as the plants grow. Light intensity matters as much as how long they’re on: most seedlings need at least 2,000 foot-candles (around 21,500 lux) to develop well, according to the Royal Horticultural Society (Royal Horticultural Society, 2023).

Essential Equipment Checklist

• Seed-starting trays with cell inserts (72-cell or 128-cell for small seeds)
• Soilless seed-starting mix — not garden soil or regular potting mix
• Full-spectrum LED grow lights on a timer
• Heat mats for bottom warmth (most seeds germinate best at 65–75°F / 18–24°C)
• Humidity domes to hold moisture while seeds sprout
• Spray bottle or gentle watering can for surface watering
• Labels and waterproof marker
• Liquid fertilizer diluted to quarter-strength, used after the first true leaves appear

Seed-starting mix is worth paying attention to. Regular potting soil is too heavy and often too rich for tiny seeds. A good seed-starting mix is fine, light, and nearly sterile — which lowers the chance of damping-off, a fungal problem that makes seedling stems collapse right at the soil line. Many commercial mixes include perlite and vermiculite for better drainage and aeration, and most sit between pH 5.5 and 6.5.

Sowing Technique and Germination

How deep you sow matters more than most people think. A simple rule: plant seeds about twice as deep as their diameter. Very fine seeds — like petunias, snapdragons, and lobelia — should just rest on the surface of moistened mix and stay uncovered, since they need light to sprout. Even a thin layer of mix can cut germination by half or more.

Keeping the mix evenly moist — but not soggy — is key during germination. Bottom watering (setting trays in a shallow pan of water and letting the mix soak it up from below) works better than pouring water on top, which can wash away small seeds and encourage fungus. Once seedlings pop up and you’ve taken off the humidity dome, let the top layer dry a bit between waterings to help prevent damping-off.

Germination speed depends more on soil temperature than air temperature. Most annual flower seeds sprout fastest when the soil stays between 70 and 75°F (21–24°C). A heat mat under the tray raises the soil temp about 10 to 20°F above room temperature, which often cuts germination time. Impatiens, for example, sprout in 7 to 14 days at 70°F but may take three weeks or longer at 60°F.

Troubleshooting Common Germination Problems

If seeds don’t sprout within the expected time, check a few things. Old seeds lose viability — most flower seeds last 2 to 3 years if kept cool and dry, but drop off quickly after that. Soil that’s too cold is another common issue, as is planting too deep. Some seeds, especially perennials, won’t sprout without cold treatment (stratification) or scarification first.

Damping-off shows up as healthy-looking seedlings suddenly collapsing at the base. It’s caused by soilborne fungi like Pythium and Rhizoctonia. Prevention works better than trying to fix it later: use sterile seed-starting mix, don’t overwater, keep air moving with a small fan, and water in the morning so everything dries before nightfall.

Transplanting and Hardening Off

Seedlings grown indoors haven’t felt wind, direct sun, or outdoor temperature swings. Moving them straight from a warm, sheltered space into the garden can shock them — leading to scorched leaves, wilting, or worse. Hardening off is the gradual shift that helps them adjust.

Start hardening off about 7 to 10 days before you plan to plant outside. On day one, set seedlings in a shady, sheltered spot for 1 to 2 hours, then bring them back in. Add 1 to 2 hours of outdoor time each day, slowly introducing more sun and breeze. By day 7 or 8, most can stay out overnight — as long as temperatures stay above 50°F (10°C) for tender plants.

The Missouri Botanical Garden suggests planting on cloudy days or in the late afternoon to reduce stress. Water seedlings well before lifting them from their cells, and water again right after planting. A weak dose of liquid seaweed or fish emulsion at transplant time can help roots settle in.

"The single greatest mistake home gardeners make with transplants is rushing the hardening-off process. A seedling that has spent ten weeks growing under artificial lights needs at least a week of gradual outdoor exposure before it can handle a full day of direct sun. Patience at this stage pays dividends for the entire growing season." — Missouri Botanical Garden, Plant Science Division

Timing Reference by Plant and Zone

The following table shows when to start common ornamental plants indoors in three USDA Hardiness Zones, based on average last frost dates.

Plant	Weeks Before Last Frost	Zone 5 (Start Date)	Zone 7 (Start Date)	Zone 9 (Start Date)
Snapdragon	10–12 weeks	Early February	Late December	Early November
Petunia	10–12 weeks	Early February	Late December	Early November
Lisianthus	16–20 weeks	Late November–December	Mid-October	Late August
Coneflower	8–10 weeks	Late February	Mid-January	Late November
Impatiens	10–12 weeks	Early February	Late December	Early November
Columbine	8–10 weeks	Late February	Mid-January	Late November

These dates assume a last frost of May 1 for Zone 5, March 15 for Zone 7, and January 15 for Zone 9. Check your local extension office or the USDA Plant Hardiness Zone Map (updated in 2023) for your exact last frost date — climate patterns have shifted across much of the U.S.

With timing that matches your zone, the right tools, and attention to each plant’s needs, starting seeds indoors turns late winter from a waiting game into real gardening work. The seedlings you tend under lights in February will be blooming in your garden by June — thanks to the time and care you put in while the ground outside is still cold.