The Highveld: frost, rain and what to plant when

Frost and rain

The Highveld is the high interior plateau of South Africa, taking in Gauteng, the Free State and the higher parts of Mpumalanga and the Eastern Cape. SANBI describes it as having sunny summers with summer rain, and dry winters with frost. For a gardener that means two seasons that matter more than the four on a calendar: a wet, warm half of the year when everything grows, and a dry, frosty half when the garden is dormant, the lawn turns brown and the only things moving are the hardy winter crops and anything planted bare root.

Frost

The dates above are medians, so in half of all years the first frost comes earlier and in half it comes later. They come from Free State records of daily minimum temperature, in which a light frost is a reading of 2 °C or lower in the weather station screen, a medium frost 0 °C or lower and a heavy frost minus 2 °C or lower. The first light frost typically falls between mid April and early May and the first medium frost between the end of April and late May. The last medium frost typically falls between early and late September, and light frost can run on into mid October. At the cold end of the Free State, about one year in five still has a light frost in the last third of October.

The spread inside the zone is wide. SANBI’s Johannesburg garden records an average July night of 1.7 °C, with frost on occasional mornings. Its Pretoria garden may often have frost, with nights of about -3 °C in June and July. At Bloemfontein very cold nights reach -5 °C, and black frost there is always a killing frost.

Frost forms on clear, still nights, and cold air drains into low ground, so the bottom of a plot frosts weeks before the top. A cloudless sky at dusk, no wind and a quick fall in temperature after sunset are the signs. Protect tomatoes, peppers, basil, beans, citrus and clivia by covering them before dusk, and take the cover off in the morning.

Rain and watering

Rain falls mainly from November to February, after a spring that is usually still dry, and it comes as short thunderstorms in the late afternoon. Autumn and winter are clear and dry. That leaves two stretches that the hose has to cover: the long dry winter, and September and October, when the soil is warm, the seedlings are going in and the storms have not yet started. Water deeply twice a week rather than daily, and mulch straight afterwards. In midsummer let the storms do most of the work and check the soil before you water.

Indoors the plants slow down in the cold and the compost stays damp for longer, so water houseplants less often from May to August, as the southern winter watering guide explains. A brown winter lawn is dormant rather than dead, as the Highveld lawn guide shows.

Month by month

Each month below adds the local frost and rain timing to the matching southern hemisphere page, which has the general list.

January

Midsummer, in the middle of the rainy season, with storms most afternoons. Sow carrots, bush beans, lettuce, radish, Swiss chard and kale for autumn, into a drill watered the night before. Take cuttings of pelargonium, lavender and rosemary, and watch for mildew and black spot in the damp heat. The general list is on January, south of the equator.

February

Late summer, and the autumn sowing starts with the first frost still two months away at the earliest. Sow beetroot, carrots, onions, lettuce, rocket, parsley and chard, and sow sweet peas. Divide agapanthus just after it flowers. The general list is on February, south of the equator.

March

The heat breaks and the storms tail off. Plant daffodils, hyacinths and garlic, sow spinach and keep the winter vegetables going. Write down the date of your own first frost this year, because it will serve you better than any average. The general list is on March, south of the equator.

April

Autumn, with the first light frost possible before the month ends. Plant garlic, onion sets and strawberries, and sow lettuce, spinach, kale and carrots. Keep frost cloth by the back door and bring the tender pots in on clear, still evenings. The general list is on April, south of the equator.

May

Frost is reliable from now. Plant pre-chilled tulips, finish the other bulbs, and plant hardy hedging such as box and photinia while the roots can settle. Mulch thickly, move tender pots against a warm wall, and water only if the dry spell is long. The general list is on May, south of the equator.

June

Winter proper, with dry, sunny days and frosty mornings. Prepare planting holes for the bare-root roses and apples that go in next month, and plant dormant blackcurrants, rhubarb and raspberries now, watering them by hand because no rain is coming. Sow carrots, kale, parsley and spinach. The general list is on June, south of the equator.

July

The coldest month, and the traditional time to prune roses, deciduous fruit trees and vines. Leave frost-damaged growth in place, because it shelters the crown from the next frost. Keep watering the new plantings. The general list is on July, south of the equator.

August

The light returns but the frost has not finished. Sow peas, beetroot, carrots and lettuce, plant seed potatoes, and sow tomatoes and peppers in trays under cover. Plant lavender, rosemary and thyme, and get the irrigation working before the heat. The general list is on August, south of the equator.

September

The biggest month of the year, with the last medium frost typically between early and late September. Sow tomatoes, peppers, courgettes and cucumbers in trays and plant them out only after the last frost. Plant dahlias and cover the shoots if frost is forecast, and sow in short rows a fortnight apart. The general list is on September, south of the equator.

October

Frost has mostly gone, though a cold hollow can still catch a late one. Plant out tomatoes, peppers, beans and cucumbers, and plant clivia and agapanthus. Water by hand until the storms arrive. The general list is on October, south of the equator.

November

The rains are under way. Sow basil, coriander and dill, plant the heat-tolerant annuals such as zinnia and salvia, and mulch everything. Stake the tomatoes now, while the roots are still small. The general list is on November, south of the equator.

December

Peak storm season, so the risk is fungal rather than dry. Space plants, water the soil rather than the leaves, and pick off marked foliage. Sow broccoli, cabbage and cauliflower in trays for autumn. The general list is on December, south of the equator.

Plants that belong here

• Agapanthus is a South African plant that flowers in summer and takes a light frost, though an evergreen kind can be damaged in a cold winter.
• Trailing ice plant comes from the dry interior and survives hard frost on dry, gritty ground, but rots in wet soil.
• Cape leadwort is frost tender, and South African growers cut it back hard at the end of winter, once the cold has passed.
• Bird of paradise survives a light frost with damage in a sheltered, sunny spot, and regrows from the base after a hard one if the ground does not freeze.
• Clivia is frost tender, so plant it in shade against a wall or under a tree, out of cold wind.
• Jade plant is a South African succulent that does best indoors on the Highveld, where winter nights are too cold for it outside.
• Rose is planted bare root and dormant in winter, which is why July is the traditional month.
• Lavender takes hard frost and wants sun, grit and dry winters, which the Highveld provides.
• Tomato is killed by frost, so the whole calendar is built backwards from the last frost.

More South African natives are on the South African plants shelf.

Questions from the margin

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Planting almanac for this region

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South African plants

Plants native to South Africa, checked against the SANBI PlantZAfrica database.