To start seeds indoors, you need clean pots or trays with drainage, suitable compost, labels, gentle watering and a bright place for the seedlings. A heated propagator is useful when the seeds need more warmth than the room provides. Grow lights become worthwhile when the available daylight cannot keep the young plants growing compactly.
Start with the space you can give the plants after germination. A small tray of seeds can soon become a windowsill full of individual pots. The most useful first purchases are sturdy containers and a peat-free seed compost; add heat or lighting to solve a specific problem with your setup.
Buy for the number of plants you can grow on
Count the pots you can accommodate before opening the seed packets. If you want six tomato plants, sow a few extra tomato seeds to allow for losses rather than emptying a full packet simply because the tray has room.
A packet’s sowing window helps you work backwards from the conditions the plants will need outdoors. Early sowing means keeping seedlings indoors longer, often during the weakest light of the year. Later sowings can be easier to manage and still produce useful plants.
Set aside a bright growing-on position as well as a germination spot. A warm cupboard may suit the initial stage for some seeds, but emerged seedlings need light promptly. Seeds that require light to germinate need a bright position from the start; check the packet before choosing their position.
Choose trays you can carry without bending them
Reusable rigid trays are worth having if you sow each year. They are easier to move when wet, and a sound base reduces the chance of the compost cracking or spilling as you carry it to the window.
Use individual modules or small pots for larger seeds and plants you want to move with minimal root disturbance. Shallow trays suit small seeds that will be pricked out later. Reuse clean nursery pots where they fit the job; every container needs drainage holes.
A clear cover helps hold moisture during germination. Separate lids are convenient when different batches emerge on different days. Open vents and remove covers as seedlings establish, rather than keeping a whole collection enclosed for the slowest tray.
In a warm room, the unheated Super 7 self-watering propagator provides separate covered trays and a capillary watering base. Choose the electric version only if you also need added warmth.
Use a fine compost and sow at the right depth
A seed compost makes it easier to create an even surface around small seeds. Fill the tray, break up lumps and settle it gently. Water before sowing if the mix is dry, then let excess water drain.
Follow the packet for sowing depth and covering. Some fine seeds need light and should stay on the surface; larger seeds often need a covering. A pencil or small dibber helps make consistent holes in individual modules.
Label each batch immediately with the plant and variety. Add the sowing date to the label or your notebook so you can compare progress with the packet’s expected germination period.
The peat-free seed compost guide compares a straightforward seed mix, a moisture-retaining wool compost and a formulation for both sowing and young plants.
Add heat when the seeds need it
A heated propagator warms the compost, which is useful for seeds needing warmer germination conditions than a cool windowsill can provide. Check the required range on the packet and measure the actual conditions if germination is unreliable.
Simple heated bases raise the temperature above the room; thermostatic models let you choose a target and control the heat more closely. A sunny window can become hot even in cool weather, so check the propagator during the day as well as overnight.
For several small batches, the electric Super 7 is a practical narrow-windowsill option. For adjustable temperature control and more growing space, look at the heated propagator comparison before buying.
Give emerged seedlings light and space
Move emerged seedlings into good light promptly. Pale stems stretching towards a window are a sign to improve the growing conditions. More warmth can accelerate that stretching when light remains poor.
A bright windowsill may be enough for a modest spring sowing. Rotate trays where growth leans strongly, keep leaves clear of cold glass and move plants away from curtains that trap them against a chilly window overnight.
If your only space is a shaded shelf, a grow light lets you place light directly above the plants. Measure the tray area and available height first. Our grow lights for seedlings guide compares an all-in-one unit, shelf bars and a bulb for an existing suitable fitting.
Water gently, then let the trays drain
Water small seedlings with a fine rose, or stand the tray briefly in shallow water until moisture reaches the compost. Lift it out and allow excess water to drain. A waterproof tray protects the sill but should not leave ordinary pots permanently standing in water.
Capillary systems are designed to draw water from a reservoir through matting. Follow their setup instructions and check that the compost is moist rather than assuming the reservoir guarantees the right amount. Different composts and room temperatures change how quickly water is used.
Clean trays between batches, avoid overcrowding and improve ventilation as growth develops. These habits reduce the damp, stagnant conditions in which young seedlings can collapse.
Keep a few small pots ready for the next stage
When tray-sown seedlings have developed their first true leaves and are large enough to handle, move them into individual pots or modules. Support the roots with a small tool and hold a leaf gently; a crushed stem is much harder for the plant to recover from.
Choose a pot only moderately larger than the root system. Water it in, return it to good light and leave space between plants as they grow. Before planting outdoors, gradually acclimatise tender plants to outside conditions and wait until the weather suits the crop.
The RHS indoor sowing guide covers the sowing and transplanting sequence.
