Seed Starting Basics: Light, Warmth, Soil and Timing

Starting seeds indoors can feel like a technical hobby, but the basics are friendly. Get the soil, warmth, light and timing right, and most vegetable and flower seeds will do the work for you.

Choose a good seed-starting mix

Use a light, fine-textured mix made for seed starting rather than heavy garden soil. Good mixes drain well, hold some moisture and are free from weed seeds and disease organisms. Dampen the mix before filling your containers so it is evenly moist like a wrung-out sponge. Almost any clean container with drainage holes will do, from purchased trays to yogurt cups or egg cartons with holes punched in the bottom.

Check the packet for timing

The seed packet usually tells you how many weeks before the last frost to sow. Slow growers like peppers and onions start earlier, while squash and cucumbers need only three or four weeks. Find your average last frost date from local sources and count backwards. Starting too early leads to leggy, root-bound plants that struggle outside.

Warmth for germination

Most seeds germinate best at around 18 to 24 degrees Celsius, with peppers and tomatoes sometimes preferring a little more warmth. A seedling heat mat can speed things up, or you can use the top of a warm, not hot, appliance with care. Cover the tray with a clear lid or plastic wrap until sprouts appear, then remove it to let air circulate.

If you do not have a heat mat, a spot on top of a warm room's shelf, away from draughts, often works well enough. Check under the lid daily, because once the first green tips appear the seedlings need light immediately. Seeds that sit in the dark for even a day or two after sprouting can become pale and stretched, and they rarely recover their strength fully.

Light after sprouting

Once seedlings emerge, they need strong light right away. A sunny window is often not enough, especially in winter, and seedlings stretch and flop. A simple LED or fluorescent grow light kept just a few centimetres above the plants for twelve to sixteen hours a day gives sturdy growth. Raise the light as seedlings grow. A small fan nearby strengthens stems and reduces damping-off, a fungal problem that causes seedlings to collapse at the base.

Thinning, feeding and hardening off

Thin crowded seedlings to one per cell by snipping the extras. Once true leaves appear, feed lightly with a diluted fertiliser. About one to two weeks before planting out, begin hardening off. Place seedlings outdoors in a shady, sheltered spot for an hour or two, and gradually increase the time and sunlight over several days. Skipping this step is a common cause of sunburned and shocked plants.

Label every tray, keep notes, and do not be discouraged by a few failures. Each season teaches you something new about your space and your plants.

This article is general information, not medical advice.