How to sow seeds: a step-by-step guide for South African gardeners

Growing from seed is cheaper than buying seedlings, and it opens up varieties no nursery stocks, but that is not really why people do it. They do it because there is something quietly satisfying about a tray of bare mix that turns green overnight. Then two weeks later half of it has collapsed at the stem and they decide seed sowing is not for them.

Almost every seed failure we hear about traces back to one of four things. Sown too deep, kept too wet, kept too dark, or moved outside too suddenly. Get those four right and the rest is detail. Here is how we do it, timed for a South African year rather than a northern one, because the calendar on the back of an imported seed packet will send you badly wrong.

Trays or straight into the ground

The decision is mostly about roots. Anything that forms a taproot resents being lifted, so carrots, beetroot, radish, parsnips and turnips go directly into their final position and stay there. Large seed with vigorous roots, like beans, peas, mielies, pumpkin and sunflowers, also does better sown where it will grow, because the seed is big enough to push through on its own and the plant grows away faster without a transplant check.

Trays make sense for fine seed you would otherwise scatter uselessly, for anything expensive, and for crops where you want a head start on the season. Tomatoes, peppers, brinjals, lettuce, brassicas and most annual flowers all suit a tray. The NC State Extension chapter on propagation sets out the reasoning for each group in more detail than we can here. If you are sowing directly, the RHS guide to runner beans is a good model for how large seed is handled outdoors.

The mix matters more than the container

People agonise over trays and then fill them with garden soil, which is the single worst choice available. Garden soil carries fungal spores, weed seed and insect eggs, it compacts under repeated watering, and it holds water in a way that suffocates a young root. Use a fine, free draining mix instead.

For seed trays indoors we use Indoor Potting Soil sieved to remove the coarse pieces, and for direct sowing outdoors we prepare the top few centimetres with Bark Outdoor Potting Soil and a little Bark Compost worked in. The seed carries its own food for the first stage, so a rich mix is not the goal. An open, crumbly texture that lets water through is. There is more on this in our article on whether garden soil can be used in pots, and the same logic applies with more force to seed.

How deep, really

The old rule holds up. Sow a seed at about twice its own width deep. A bean the width of a fingernail goes in around two centimetres. Lettuce seed the width of a pinhead goes barely under the surface, pressed in and dusted over. Some seed needs light to germinate and should sit on the surface entirely, and the packet will say so, which is one of the few things the packet always gets right.

Sowing too deep is the most common mistake by a wide margin. The seed germinates, uses up its reserves pushing towards a surface it never reaches, and dies underground where you cannot see the evidence. If you are unsure, err shallow. A seed sown slightly too shallow will usually still make it. Firm the mix gently after sowing so the seed is in contact with damp material rather than sitting in an air pocket, and water with a fine rose or from below so you do not wash everything into one corner.

Warmth gets them up, light keeps them honest

Most vegetable seed germinates fastest somewhere between eighteen and twenty five degrees. On the Highveld in early spring, an indoor windowsill or a spot on top of the fridge will beat an outdoor tray by a week or more. The University of Maryland Extension guide to starting vegetable seeds indoors is a solid reference on temperature and timing.

The moment green appears, the priority flips. Warmth without light produces pale, stretched seedlings that flop over and never recover, so get the tray into the brightest position you have the same day. A north facing window in South Africa is the best indoor light you will find, and even that is dimmer than an overcast day outdoors, so turn the tray every day or two to stop everything leaning. Seedlings that are already leggy will not straighten out, and it is faster to sow again than to nurse them.

Damping off, and how to avoid it

Damping off is the collapse you find one morning where healthy seedlings have toppled at soil level with a thin, water soaked stem. It is caused by soil borne fungi that thrive in cold, wet, still conditions, and once it moves through a tray you will not save it. The RHS page on damping off covers the organisms involved and the control measures.

Prevention is entirely about conditions. Use fresh mix rather than the leftovers from last season, clean your trays properly before reuse, water from below and let the surface dry slightly between waterings, sow thinly so air can move between the seedlings, and crack a window or run a fan for a while each day if the tray is indoors. Warmth helps too, since the fungi do their worst damage when a tray is cold and saturated at the same time.

Thinning feels harsh and is not optional

Once the first true leaves appear, which are the second set and look like miniature versions of the adult leaf rather than the rounded seed leaves, thin to the spacing on the packet. Everybody hates doing this. Everybody who skips it ends up with a crowded tray of weak plants competing for the same light and the same water, and crowding is also what invites damping off in the first place.

Snip the extras off at the base with scissors rather than pulling them, so you do not disturb the roots of the ones you are keeping. With direct sown carrots and beetroot, thinning is doubly important, because a crowded row gives you a hundred pencil thin roots instead of thirty good ones.

Hardening off before they meet the real weather

A seedling raised on a windowsill has never felt wind, direct sun or a cold night, and moving it straight into a Highveld garden will scorch it within a day. Harden it off over roughly a week. Start with an hour or two in dappled shade outdoors, bring it back in, and extend the time and the exposure each day until it is out overnight. Then plant.

Plant in the late afternoon rather than at midday, water in well, and expect a few days of sulking before growth resumes. Our article on when and how to plant seedlings goes through the transplant itself, including how deep to set a tomato and why you should firm around the root ball rather than leaving it loose.

Timing it for a South African year

Inland, the last frost usually clears in early to mid September, so start warm season seed indoors from late July or August and plan to plant out once the risk has passed. Cool season crops go the other way, sown from late January through March so they establish before winter and grow through it. In the winter rainfall areas of the Western Cape the pattern inverts, and the productive sowing window is autumn rather than spring.

Sow in small batches every two or three weeks rather than emptying the packet at once. Successional sowing spreads the harvest, and it also means one bad week of weather does not wipe out the whole crop. Our free Garden Planner will help you set the sequence out, and our beginner guide to vegetable gardening in South Africa covers what to grow once the seedlings are in the ground.

When the seedlings are ready, give them somewhere decent to live. Our herb and vegetable pots and the trough range are cast from our Fibre Concrete Mix, which holds up to summer rain and Highveld frost, and a trough in particular lets you plant a proper row rather than a single lonely lettuce. Stand them on concrete pot feet so the drainage stays free, and the hard part of the season is already behind you.

Frequently asked questions

How deep should I sow seeds?

About twice the width of the seed. Large seed like beans goes in around two centimetres, fine seed like lettuce sits just under the surface, and a few types need light and should rest on top of the mix.

Can I use garden soil for seed trays?

No. Garden soil carries fungal spores, weed seed and insect eggs, and it compacts under watering until young roots suffocate. Use a fine, free draining potting mix instead.

Why do my seedlings fall over at the base?

That is damping off, a fungal collapse caused by cold, wet, still conditions. Use fresh mix and clean trays, water from below, sow thinly and give the tray some air movement to prevent it.

When should I start seeds in Gauteng?

Start warm season seed indoors from late July or August, since the last frost inland usually clears in early to mid September. Cool season crops are sown from late January through March.

What is hardening off and do I need to do it?

Hardening off is a gradual week of increasing outdoor exposure before planting out. It is necessary, because a seedling raised indoors has never felt wind or direct sun and will scorch if moved straight into the garden.