Seed starting mix is a fine, light, low-nutrient blend designed for one job: helping a seed germinate and grow its first roots without rotting, drying out or being overwhelmed. It is not the same as potting mix, and the difference matters: potting mix is richer, coarser and heavier, which is fine for an established plant but too much for a seedling. Seeds carry their own food for the first days of growth, so they do not need fertiliser at sowing — they need even moisture, air around the seed, and a clean environment. This guide gives the best homemade seed starting recipe, explains how to use it, and covers the seedling problems that a good mix prevents.
A good seed starting mix holds water evenly, drains freely, is fine enough for tiny seeds to sit in contact with it, and is free of weed seeds, pests and disease. Ordinary garden soil fails on all four counts: it compacts, it crusts, it carries weed seeds, and it can carry the fungi that cause damping off. Even good potting mix is often too coarse and too rich for the smallest seeds.
The best homemade seed starting mix
The standard recipe is:
- 2 parts fine coco coir or sieved leaf mould — holds moisture evenly and keeps the mix light.
- 1 part fine, sieved compost — adds a small amount of nutrition and biology.
- 1 part fine vermiculite — holds moisture and keeps the mix open.
Sieve each ingredient through a 5 mm mesh before mixing, so the blend is fine and even. Mix the ingredients dry on a tarp, then wet the mix lightly until it is damp like a wrung-out sponge. Do not add fertiliser, manure or lime to seed starting mix — seedlings do not need it yet, and it can burn young roots or encourage the fungi that cause damping off. Our coco coir for plants guide covers preparing coir, including rinsing it first, and our perlite vs vermiculite guide explains why vermiculite is the better mineral for seeds. Use the potting mix calculator to work out quantities for your trays.
A simpler alternative that works well is equal parts fine coir and fine vermiculite with a little sieved compost — roughly 2 parts coir, 1 part vermiculite and half a part compost. The exact ratio matters less than the qualities: fine, even, moist, clean and low in nutrients. Commercial seed starting mixes are also fine if you prefer to buy one, but check the bag: a good one feels light and spongy, not heavy and woody, and it should not smell sour.
How to use seed starting mix
1. Fill the container. Fill seed trays, modules or pots with the damp mix, firming it very gently — press it down just enough to remove large air pockets, not so hard that it compacts. Fill to about 1 cm below the rim so there is room for watering and for a covering layer.
2. Sow at the right depth. The rule is to sow a seed at a depth roughly twice its own width: tiny seeds such as lettuce sit on the surface and are barely covered, while large seeds such as beans go 2–3 cm deep. Check the packet for the exact depth. Sow thinly — crowded seedlings compete and become weak and leggy.
3. Cover lightly. Cover the seed with a thin layer of fine vermiculite or sieved mix. Vermiculite is the better covering for small seeds because it holds moisture on the surface and lets light through, and it discourages the algae and mould that grow on a damp compost surface.
4. Water gently. Water from below where possible: stand the tray in a shallow tray of water for a few minutes until the surface darkens, then drain it fully. Watering from above with a can can wash small seeds away or compact the surface. Keep the mix evenly damp, never soaking and never dry.
5. Give warmth and light. Most seeds germinate best at 18–24°C. A warm windowsill, a propagator lid, or a heat mat helps. As soon as seedlings appear, give them as much light as possible — a bright windowsill or a grow light 10–15 cm above the seedlings for 12–14 hours a day. Seedlings grown in dim light stretch tall, pale and weak, and no mix can fix that.
6. Feed and pot on. Start feeding with a half-strength balanced liquid feed once the first true leaves appear — the seed leaves do not need feeding, the true leaves do. Pot seedlings on into our general potting mix recipe when they have two or three true leaves and their roots fill the module. Handle seedlings by a leaf, never by the stem, which bruises easily.
Damping off and other seedling problems
Damping off is the most common seedling killer: the stem pinches in at soil level, the seedling topples over, and it dies within a day. It is caused by fungi that thrive in wet, airless, cool conditions. A good seed starting mix prevents much of it, because it drains and holds air, but you also need clean trays, fresh mix each year — never reuse old seed starting mix — gentle watering from below, good air circulation, and not sowing too thickly. If damping off appears, remove affected seedlings at once, improve air flow, let the surface dry slightly between waterings, and sow a fresh batch in new mix and clean trays.
Other common problems: leggy seedlings mean too little light — move them closer to the window or under a grow light. Seedlings that germinate then stop growing usually need feeding or potting on. Mould on the surface means the mix is too wet and the air too still — ease off watering, improve ventilation, and cover the surface with vermiculite. Seeds that do not germinate at all may be too old, sown too deep, too cold, or kept too wet or too dry. Check the packet for the temperature and depth each variety needs, and check your sowing dates on our zone pages, such as when to plant tomatoes in zone 7.
Which seeds to start indoors
Start indoors the plants that need a long season or an early start: tomatoes, cucumbers, peppers, aubergines, broccoli and cabbage all benefit from indoor sowing. Sow direct the plants that resent transplanting or grow fast: carrots, peas, beans, spinach and most root crops. Work backwards from your last frost date to set sowing dates, and harden seedlings off — putting them outside for a few hours a day, increasing gradually over a week — before planting them out, or the shock of cold wind and sun can check their growth for weeks.
Common mistakes
The most common mistake is using potting mix or garden soil for seeds, which is too coarse, too rich and too dirty. The second is sowing too deep, so small seeds never reach the surface. The third is overwatering, which causes damping off — keep the mix damp, not wet, and water from below. The fourth is too little light, which produces leggy seedlings that never recover properly. The fifth is reusing old seed starting mix, which carries disease. The sixth is feeding too early or too strong, which burns young roots — wait for true leaves and use half-strength feed. And the seventh is planting out without hardening off, which shocks seedlings and sets them back for weeks.
Safety notes
Wear gloves and a mask when handling dry coir, vermiculite and compost, which raise dust. Wash your hands after handling compost. Use fresh mix each year and clean trays with hot soapy water before sowing, to reduce disease. Keep seed starting mix away from children and pets, and discard any mix that smells sour or has visible mould. If you are unsure about a product or a persistent seedling disease, check your local extension service for advice.
Frequently asked questions
Can I use potting mix for starting seeds?
You can, but it is not ideal. Potting mix is coarser and richer than seed starting mix, which can bury small seeds, burn young roots, and encourage damping off. Sieve potting mix finely and mix it with extra vermiculite if you must use it, but a dedicated seed starting mix gives better results.
How wet should seed starting mix be?
Damp like a wrung-out sponge: moist enough to hold together when squeezed, with no water dripping out. Keep it evenly damp until germination, then let the surface dry slightly between waterings. Water from below where possible.
When should I start feeding seedlings?
Start with a half-strength balanced liquid feed once the first true leaves appear. The seed leaves feed the seedling at first, so feeding before true leaves appear is unnecessary and can burn roots. Pot on into general potting mix when seedlings have two or three true leaves.
Why are my seedlings tall and leggy?
Too little light. Seedlings stretch towards the light when it is dim or far away. Move them to the brightest windowsill or under a grow light 10–15 cm above them for 12–14 hours a day, and give them air movement so stems grow sturdy.
What is damping off and how do I prevent it?
Damping off is a fungal disease that pinches seedling stems at soil level and kills them quickly. Prevent it with fresh seed starting mix, clean trays, thin sowing, watering from below, good air circulation, and not overwatering. Remove affected seedlings immediately.