Starting seeds: containers, mix, warmth, light and feeding
Most seed-starting failures come from four things: a heavy or dirty mix, the wrong temperature, too little light after emergence and uneven watering. Get those right and the species-specific numbers in the plant database do the rest.
Open the sowing calendar Germination troubleshooter
Containers and mix
Use clean trays, cells or pots with drainage holes. Wash reused containers and let them dry; dirty containers are a common route for damping off. Fill with a fine, soilless seed-starting mix rather than garden soil or coarse potting compost, and wet it thoroughly before sowing so it is damp like a wrung-out sponge.
Peat-free mixes work well; coir, compost and perlite blends are all used. Organic seed-starting mixes are sold ready-made, or you can sift your own compost into a mix with perlite for drainage. Biodegradable pots and soil blocks avoid root disturbance at transplanting.
Sowing depth and spacing
A common rule is to cover a seed with soil about two to three times its width. Very fine seed (many herbs, poppies, snapdragons) is pressed onto the surface, and seeds that need light to germinate are never covered. The database lists a depth and a light requirement for every plant.
Temperature and humidity
Soil temperature matters more than room temperature. Each plant has a germination range, and warm-season crops such as tomatoes, peppers and cucumbers germinate much faster on a heat mat than in a cool room. Cool-season crops such as lettuce and spinach can fail when the mix is too warm. Cover trays with a clear dome or film until the first seedlings show, then remove it to let air move.
Keep the mix evenly moist, not soggy. Watering from below with a tray avoids washing seed about and keeps the surface dry enough to discourage damping off.
Light
The moment seedlings emerge they need strong light. A south window is rarely enough; leggy, pale seedlings are the sign. Put full-spectrum LED or fluorescent grow lights a few inches above the leaves and raise them as the plants grow, running them for roughly 14 to 16 hours a day.
Feeding, thinning and potting up
Seed carries its own food, so no fertiliser is needed until the first true leaves appear. Then use a dilute organic liquid feed such as fish or seaweed emulsion at about half the label strength. Thin to one seedling per cell by snipping the extras with scissors rather than pulling. Pot up when roots fill the cell and the plants have two or three sets of true leaves.
Hardening off and transplanting
Seedlings raised indoors need a week to ten days of gradual exposure to sun, wind and cooler nights before planting out: an hour or two of shade at first, then longer and brighter each day. Transplant on a mild, overcast day and water in well. Count back from your last frost date to know when to start: the sowing calendar does that for every plant in the database.
Perennials and seed that needs a chill
Many perennials, wildflowers and fruit trees will not germinate until the seed has been through a period of cold, damp conditions (cold-moist stratification). Either sow outdoors in autumn and let winter do it, or mix the seed with damp sand or vermiculite in a bag in the fridge for the time given on the plant page. Others have a hard coat that benefits from nicking or soaking (scarification).