The arid interior: heat, frost and a narrow window to plant

Frost and rain

The Karoo, the Northern Cape and the dry continental interior are the driest and most extreme part of South Africa. Summers are very hot, winter nights are cold and frosty, and rain is scarce and unreliable. SANBI’s zones divide the area in two: the summer rainfall Karoo, which gets its rain in the summer half of the year, and the winter rainfall Karoo to the west. For a gardener that means two short windows to plant, one at each end of the year, between a summer that stops most things and a winter that is hard on anything tender.

Frost

No first or last frost date is published for the interior, so this page gives none. SANBI classes the summer rainfall Karoo and Highveld zone as having severe frost in winter, with a coldest-month mean minimum that runs from below -5 °C to 5 °C. The winter rainfall Karoo has light frost, and the northern and coastal parts of Namaqualand have little or none.

Two SANBI gardens show the range. At Worcester, winter sunrise lows are around 2 °C and fall to -2 °C in July and August, with strong north-westerly winds. At Bloemfontein very cold nights reach -5 °C, black frost mornings average -2 °C, and the garden covers its cold-sensitive plants with frost cloth. Black frost, SANBI notes, is always a killing frost.

Frost comes on clear, still nights, and the swing in the interior is large: at Bloemfontein winter days warm up to about 18 °C and the nights fall to -5 °C. Cold air pools in low ground, so keep tender plants out of hollows. Cover them before dusk and take the cover off in the morning.

Rain and watering

Rain is scant. The Botanical Society of South Africa says most of the Nama-Karoo receives 200 mm or less every year. In the east it falls in the summer half of the year, and SANBI’s Bloemfontein garden gets most of its rain from November to February, with a dry autumn. In the west it falls in winter and spring, as on the Bokkeveld plateau, and the Karoo Desert garden at Worcester has almost no rain in summer and unpredictable winter rain, with only one or two thunderstorms a year.

Summer reaches 44 °C at Worcester, so water goes on early in the morning or late in the evening, and goes on deeply and less often. Mulch every bed, group plants by how much water they need, and use drip lines where you can. Indoors the hot dry summer dries pots quickly, so check them often, and in the cold winter keep succulents away from the glass.

Month by month

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

January

Very hot, and growth slows or stops. Sow only kale and Swiss chard, with shade cloth over the row, and water before dawn. Mulch everything and leave planting for later. The general list is on January, south of the equator.

February

Still hot, so use the month to prepare. Sow rocket and chives, divide agapanthus, and dig compost into the beds that will take the autumn crops. Set up drip lines before the cold arrives. The general list is on February, south of the equator.

March

The heat eases, and SANBI’s Karoo Desert garden calls the autumn days comfortable, at 25 to 29 °C. Plant daffodils, hyacinths and garlic, and keep sowing rocket and chives. The window to sow is opening. The general list is on March, south of the equator.

April

The best sowing month of the year, after the worst of the heat and before the frost. Sow beetroot, carrots, kale, lettuce, onions, parsley, peas, radish, spinach and Swiss chard, and sow sweet peas and pot marigolds. Keep frost cloth ready for the first clear night. The general list is on April, south of the equator.

May

Frost arrives, and the rains may too in the western Karoo, where early showers can come in May. Plant pre-chilled tulips and finish the other bulbs, and mulch thickly. Cover tender plants on still, clear evenings. The general list is on May, south of the equator.

June

Cold, with frosty nights and winter days that warm to about 18 °C at Bloemfontein. Sow carrots, radish and spinach in sheltered beds, and plant dormant raspberries, rhubarb and gooseberries. Water by hand when the weather stays dry. The general list is on June, south of the equator.

July

The coldest month, with lows of -2 °C at Worcester and black frost at Bloemfontein. Plant bare-root fruit trees and roses, prune the dormant ones, and leave frost-damaged growth in place until spring. The general list is on July, south of the equator.

August

The light returns but the frost has not finished. Sow peppers and chillies under cover, plant thyme, oregano and sage, and plant strawberries. Mend the irrigation before the heat. The general list is on August, south of the equator.

September

The spring window opens. Sow tomatoes, lettuce, carrots, beetroot and Swiss chard, plant seed potatoes, and plant out only after the last frost. Mulch after watering. The general list is on September, south of the equator.

October

Plant out the summer vegetables while the soil is warm and the days are not yet at their hottest. Sow beans, cucumbers, courgettes, sunflowers and basil, and begin deep watering twice a week. The general list is on October, south of the equator.

November

The heat arrives. Plant the fast, heat-tolerant crops such as green beans and courgettes, and plant in the evening. Water the hole before the plant goes in, and shade anything small. The general list is on November, south of the equator.

December

Very hot again. Sow basil, coriander, dill and courgettes, mulch deeply, and water before dawn. Do not start a new bed you cannot water through January and February. The general list is on December, south of the equator.

Plants that belong here

• Trailing ice plant comes from the dry South African interior, survives hard frost on gravel and dies of wet rather than cold.
• Century plant wants full sun, poor gritty ground and no water through the cold months, because it dies of winter wet rather than winter cold.
• Geranium comes from dry rocky places and wants sun, sharp drainage and a proper dry spell between waterings.
• Lavender takes hard frost and thrives in poor, stony ground and full sun.
• Rosemary comes from dry coastal hills and does better with less water than with more.
• Thyme wants poor soil, full sun and long dry spells, and is properly hardy.
• Oregano is grown hard, dry and in full sun, and the flavour improves for it.
• Agapanthus flowers in summer and takes a light frost, and wants deep water in spring.
• Jade plant comes from the dry eastern parts of South Africa, and is best grown in a pot that can come indoors for the frosty nights.

The South African plants shelf lists more natives.

Questions from the margin

Read on

Planting almanac for this region

Opens the almanac with this region and its hemisphere already chosen.

South African plants

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