How do you make a homemade seedbed?

By region

Tomato months come from the guide when to plant tomatoes: February to March in the north from Valle d'Aosta, the same window in the centre from Umbria's calendar, and January in the south only with warmth and light. Basil's 20 °C and nights above 10 °C are from the RHS, in when to plant basil. French beans in the open ground, and why one seed to a pot, are in when to plant French beans.

Containers, including reused ones

Each seedling gets its own cell, so the roots do not tangle. There must be holes in the base: without them, water sits and the roots rot. Cell trays, small pots, clean food containers, or a box cut from scrap cardboard all work. The RHS, for a plastic-free version, cuts squares of about 7 cm (3 in) from waste cardboard and tapes them with paper masking tape. University of Minnesota Extension also mentions newspaper pots for plants that resent transplanting, such as cucumber and squash.

A container you have used before gets washed and soaked for 30 minutes in a disinfectant, at the dilution the label gives for surfaces, then rinsed. That is how University of Minnesota Extension avoids carrying last year's diseases forward. Fibre pots go into the ground with the plant, but the rim is cut level with the soil, or it wicks water away from the roots.

Compost and sowing

Seed compost, peat-free if you can, not garden soil: it is light, free of weed seeds, and low in fertiliser. A strong feed, the RHS writes, damages seedlings. For the same reason you do not pour nettle feed on a seed tray.

Fill, firm, and leave about 1 cm (½ in) below the rim. Sow thinly. Large seeds go in one by one. The packet states the depth. If it is silent, University of Minnesota Extension says about twice the seed's width, and the RHS usually covers with a layer no deeper than the seed. Write the name and the date on a label. Water from below, or with a fine rose, and tip out any water left in the tray.

Most seeds, the RHS says, germinate at about 18 °C (64 °F). Basil wants about 20 °C (68 °F). If the packet gives another figure, the packet wins. Feeding waits: University of Minnesota Extension gives it only after several true leaves, at a quarter of the stated rate, once a week, and plain water on the other days.

Light, warmth, planting out

A windowsill is the coldest place in the house at night and can overheat by day. Seedlings grow thin. University of Minnesota Extension keeps fluorescent or LED lights a few centimetres above the leaves, on for 12–16 hours, with a dark period each night, and a mat under the pots: bottom heat helps germination and cuts damping off. The method is the same as in starting seeds indoors.

Hold a seedling by a leaf, never by the stem. Extra seedlings in the same cell are cut off at compost level, not pulled.

Before they go outside, two or three weeks of hardening off. The RHS puts them out by day, in shelter, and brings them in at night, or sets them in a cold frame and opens the lid by day, a little further each time. University of Minnesota Extension starts at about two weeks, in shade and out of the wind, then lengthens the sun and the hours. An open lid lets humidity out. Hardening off does not make a heat-loving plant hardy: if nights still freeze, it stays inside. The dates for your garden are in the planting calendar.