Spring seed starting is where the growing season begins. A seed sown indoors in early spring becomes a tomato plant by summer; a seed sown outdoors in mid-spring becomes a row of lettuce by early summer. It is also where the most seedlings are lost, because seed needs warmth, moisture and light in the right balance, and spring provides them unevenly — warm days and cold nights, bright sun and grey weeks, wet soil and dry winds. This guide explains how to start seed in spring, indoors and out, and how to keep the seedlings alive until they are ready to plant.
What to start indoors, and when
Tender crops that need a long season — tomatoes, peppers, aubergines, courgettes, cucumbers, and tender flowers such as cosmos and zinnias — are started indoors six to eight weeks before the last frost, so they are ready to plant out as soon as the weather warms. Hardy crops — lettuce, peas, beans, carrots and radishes — are usually sown directly outdoors, because they germinate in cool soil and resent transplanting. Starting a hardy crop indoors gains little and risks a weak, leggy seedling.
Work backwards from the last frost average for your area: if the average last frost falls in mid-spring, tomatoes are sown in early spring, not in winter, because a tomato sown too early becomes a tall, leggy plant in a small pot long before it can go outside. The seed starting calculator helps you work out sowing dates from your frost average, and your local extension service publishes frost averages for your area. In zones 11 to 13, where there is no frost, the sowing calendar is governed by heat rather than cold, and tender crops can be sown for most of the year.
Sowing indoors: mix, containers and conditions
Use a sterile, fine seed-starting mix, not ordinary potting mix, which is too coarse and may carry damping-off fungi — see our seed starting mix guide. Use clean containers with drainage holes, and label every one with the variety and the sowing date, because seedlings look alike for weeks. Sow seed at the depth on the packet, usually about twice the seed's own depth, and water gently from below or with a fine mist so the seed is not washed away or buried.
Most vegetable and flower seed germinates at warm room temperatures, roughly 65 to 75 degrees Fahrenheit, and a heat mat helps in a cool room. Light matters from the moment seedlings emerge: a bright windowsill is often not enough in early spring, when days are short, so a simple grow light hung a few inches above the seedlings for 12 to 14 hours a day prevents the leggy, pale growth that ruins so many spring sowings. The germination time calculator gives the expected germination time for common crops. Keep the mix evenly moist but never soggy, and ventilate daily — still, damp air is the main cause of damping off.
Sowing outdoors in spring
Outdoor sowing starts when the soil can be worked and has warmed enough for the seed to germinate — use the squeeze test from our spring soil preparation guide, and a soil thermometer if you have one. Sow hardy crops first: peas, broad beans, lettuce, spinach, radishes and carrots, in rows marked with labels. Sow thinly, because thinning crowded seedlings wastes seed and disturbs the survivors; sow a little extra and thin to the spacing on the packet once the seedlings have their first true leaves.
Cover early sowings with fleece or cloches on cold nights, and remove the cover during the day so the seedlings do not overheat. Water gently after sowing, and keep the seed bed moist until germination — a dry seed bed in a windy spring is the most common reason outdoor sowings fail. Succession sow every two weeks, so the harvest is spread out and a single cold night does not wipe out the whole crop. Our spring planting guide explains the order of spring sowings, and our spring watering guide explains how to water seed beds.
From seedling to planting out
Thin seedlings once they have two true leaves, keeping the strongest and snipping the rest at the base rather than pulling them. When seedlings have several true leaves and roots are visible at the drainage holes, pot them on into larger pots of ordinary potting mix, handling them by a leaf rather than the stem. Feed very lightly, if at all, until they are growing strongly — fresh mix contains nutrients, and a seedling's roots burn easily.
Harden seedlings off over seven to fourteen days before planting them out: put them outside in a sheltered, shady spot for a few hours on the first day, bring them in at night, and increase the time outdoors and the exposure to sun and wind each day. Plant them out on a cloudy day or in the evening, water them in well, and protect them with fleece if a cold night is forecast. Our spring transplanting guide covers the move in detail.
Common seed starting problems
Damping off — seedlings collapsing at soil level — is caused by fungi in damp, still air. Prevent it with sterile mix, clean containers, air movement and not overwatering, and discard affected seedlings. Leggy seedlings mean too little light; move them closer to a window or under a grow light. Seed that never germinates is usually old, sown too deep, kept too cold or kept too dry — sow fresh seed, check the packet, and keep conditions steady. Seedlings that stop growing after potting on are usually cold or root-bound; check the temperature and the pot size. If problems persist, check your local extension service for advice.
Seed starting indoors for houseplants and flowers
Spring seed starting is not only for vegetables. Flower seed started indoors in spring — cosmos, zinnias, marigolds, nasturtiums, sweet peas and sunflowers — gives you plants weeks ahead of direct sowing, and a longer flowering season. Sow them in the same way as vegetables: sterile mix, clean containers, warm temperatures and plenty of light, and harden them off before planting out. Sweet peas can be sown in autumn or in early spring in deep pots, because they make a long tap root, and they need a cane to climb from the start.
Houseplant seed is a different project, and a slower one: most houseplants are propagated by cuttings, not seed, because seedlings rarely match the parent — see our houseplants seed starting guide for the honest picture. But a few houseplants, such as coleus and African violets, are rewarding from seed in spring, and the same indoor sowing skills apply: fine seed on the surface, steady warmth, bright light and patience. Whatever you sow, keep a notebook of what you sowed, when, and how it did — the notes are worth more than any book, because they record what works in your garden, in your climate, with your soil and your light. Next spring you will know exactly what to sow, when to sow it, and what to do differently.
Frequently asked questions
When should I start seeds indoors in spring?
Six to eight weeks before the average last frost for tender crops such as tomatoes and peppers. Hardy crops are usually sown directly outdoors.
Why are my seedlings leggy?
Too little light. Move them closer to a window or under a grow light hung a few inches above them for 12 to 14 hours a day.
What is damping off?
A fungal disease that makes seedlings collapse at soil level in damp, still air. Prevent it with sterile mix, clean containers, ventilation and not overwatering.
How do I harden off seedlings?
Put them outside for a few hours a day in a sheltered spot, increasing the time and sun exposure over seven to fourteen days until they spend the whole day outside.
Can I sow seeds directly outdoors in early spring?
Yes, for hardy crops such as peas, lettuce, spinach and radishes, as soon as the soil can be worked. Tender crops must wait until after the last frost. If you are unsure, check your local extension service for a sowing calendar.