When and how to plant seedlings in South Africa

Every spring we get the same question at the counter, usually from someone holding a tray of tomato seedlings that have gone a bit leggy on a windowsill. When do these go outside? The honest answer is that the calendar matters less than the weather in your own street, but there are a few dates worth trusting and a handful of habits that make the difference between a seedling that sulks for a month and one that never looks back.

Planting out is the moment where most of the effort you put into seeds is either banked or lost. Seedlings are small, their roots are shallow, and they have almost no reserves to fall back on. Handle that one week well and the rest of the season is mostly watering and picking. Here is how we do it, and what we tell people planting into pots on a patio rather than into open beds.

Waiting out the last frost

On the Highveld we work on frost being finished around the middle of September, and we still watch the forecast into the first week of October, because a late cold snap in Centurion is not unusual. Gardens that sit in a low spot or at the bottom of a slope catch frost well after the houses up the hill have stopped getting it, so trust your own garden over a regional average. The South African Weather Service publishes historical rainfall and climate records by region, which is worth a look if you have just moved and have no feel yet for the local pattern.

Coastal gardeners have it easier. Along the KwaZulu-Natal coast tender seedlings can go out from August. In Cape Town the limiting factor is not usually cold but wet, so the sensible move is to wait until the winter rain eases and the soil has drained and warmed, which is often late September. In the Free State and the Karoo, hold your nerve until October. Warm season crops like tomatoes, beans, brinjals and peppers care about soil temperature as much as air temperature, and cold soil simply stalls them.

Hardening off, the step most people skip

A seedling raised on a windowsill or under cover has thin leaves, a soft stem and no experience of wind. Move it straight into full Highveld sun and it can scorch in a single afternoon. The fix is boring and it works. Over seven to ten days, stand the tray outside in morning sun and bring it back under cover each afternoon, adding an hour or two of exposure each day, until it is happily spending a full day and then a full night outdoors. Clemson's home garden service sets out how to raise and toughen up seedlings before transplanting, and the principle travels perfectly well to our conditions.

You can watch the plants change while this happens. Leaves thicken a little, the colour deepens, stems stiffen. Ease off the water slightly during that week too, not to the point of wilting, just enough that the plant is not soft and swollen when it goes out. Seedlings bought from a nursery where they were already standing outdoors have done most of this work, though we still give them two or three days to settle in their new spot. If you are still at the sowing stage, our step by step seed starting guide covers everything that happens before this point.

Plant in the cool of the evening

Late afternoon and early evening is the best time to plant, and it is free. The seedling then gets twelve cool hours to push out new root hairs before it has to face sun and wind again. Planting at eleven in the morning in October asks a plant with a disturbed root ball to transpire hard on day one, which is exactly when it is least able to. An overcast, still day works just as well if you get one.

Water the tray about an hour before you start, so the root ball is moist and holds together when you tip it out. Never pull a seedling up by the stem. Push it up gently from underneath, or squeeze the cell, and hold it by a leaf if you need to steady it. A crushed stem never recovers, while a torn leaf hardly matters.

Getting a seedling into a pot properly

Dig or scoop a hole a bit wider than the root ball and no deeper, then set the plant so the soil surface of the root ball sits level with the surrounding soil. Tomatoes are the one common exception, since they root from the buried stem and can go in noticeably deeper. Firm the soil around the root ball with your fingers rather than stamping it down, because compacted soil around a young root system is worse than slightly loose soil.

In a pot, start with fresh growing medium rather than tired soil from last season. Our Bark Outdoor Potting Soil holds moisture without turning to sludge, and a couple of handfuls of Bark Compost mixed through gives the seedlings something to grow into. For a big pot, a layer of Pot Filler at the base saves a surprising amount of soil and keeps the weight down without blocking drainage. Whatever you do, keep the drainage hole clear, since a waterlogged pot after a Highveld thunderstorm kills more seedlings than frost ever does.

Spacing, and why crowding costs you a crop

Almost everyone plants too close. A punnet of six lettuce looks lonely in a trough, so in they all go, and six weeks later you have six small plants competing for the same water and light instead of three good ones. Read the spacing on the packet and take it seriously. Lettuce and spinach want roughly twenty centimetres between plants, bush beans about fifteen, a cherry tomato wants a pot to itself unless the pot is very large.

Air movement between plants matters as much as root space here, because our humid summer evenings are perfect for downy mildew and blight. The Royal Horticultural Society's guidance on growing tomatoes makes the same point about airflow and spacing, and it applies just as much to a pot on a Johannesburg patio. If you are planning a whole bed or a row of pots, our free Garden Planner is a quick way to sketch out how many plants actually fit.

Watering in, and the fortnight that follows

Water each seedling in properly the moment it is planted, slowly and at the base, until the water has soaked well past the bottom of the root ball. This is not just about moisture. It settles the soil into contact with the roots and closes the air pockets that dry a root ball out from the inside. A light sprinkle over the leaves does none of that.

For the first two weeks the roots are still mostly inside the old root ball, so the plant cannot reach water two centimetres away. Check daily with your finger and water little and often until you see fresh new growth, which is the signal that roots have moved out into the surrounding soil. After that, shift to less frequent but deeper watering, which is what trains roots downwards and gets you through a dry week.

Pots or beds

Pots win when your soil is heavy clay, when you rent, when the only decent sun is on paving, or when you want the herbs within reach of the kitchen door. You control the growing medium completely, you can move a pot out of a hailstorm, and you use far less water than a bed because none of it drains away to feed weeds. The trade off is that pots dry out faster and need feeding sooner. Our guide to growing vegetables in containers goes into the sizes that work for each crop, and Clemson has a solid rundown of container vegetable gardening basics if you want a second opinion.

Beds win for anything that wants to run, like pumpkins and mealies, and for deep rooted crops. Plenty of people do both, keeping the fussy quick crops in troughs near the house and the sprawlers out in the garden. There is more on how we approach the whole question in our guide to growing food in pots.

Hail, wind and the first real heatwave

A Highveld hailstorm in November can shred a bed of young seedlings in four minutes. Anything in a pot can be dragged under the patio roof when the sky goes green, which is one of the quiet advantages of container growing. For seedlings in beds, an upturned crate or a length of shade cloth held down with bricks is unglamorous and effective, and it only needs to be there for the storm itself.

Heat is the other threat. When the forecast says thirty five degrees, rig shade cloth over anything planted in the last fortnight and water early in the morning rather than at midday. Wind is worth thinking about too, since a seedling rocking in a strong afternoon southwester tears its own new roots. A stick pushed in next to a tomato at planting time, and a soft tie, costs nothing and saves the plant.

Get the timing and the first fortnight right and the rest is genuinely easy. If you are planting into pots this season, the sizes that suit herbs and vegetables are a little deeper and a little more generous than most people expect, and having the right pot from the start means never having to disturb the roots again.

Frequently asked questions

When can I plant seedlings out on the Highveld?

From about the middle of September, once the risk of frost has passed. Keep an eye on the forecast into early October, because low lying gardens can still catch a late cold snap.

Do I really need to harden seedlings off?

Yes, if they have been raised indoors or under cover. Give them seven to ten days of gradually longer spells outdoors, otherwise the first day of full sun can scorch the leaves badly.

What time of day is best for planting seedlings?

Late afternoon or early evening. The seedling then gets a cool night to settle its roots before it has to cope with sun and wind again.

How deep should a seedling be planted?

At the same depth it was growing in the tray, with the top of the root ball level with the surrounding soil. Tomatoes are the exception and can be planted deeper, because they root along the buried stem.

How often should I water newly planted seedlings?

Check daily for the first two weeks and water lightly whenever the top few centimetres are drying out. Once you see fresh new growth, switch to deeper watering less often.