Southern New Zealand: late frosts, a short season and what to plant when

Frost and rain

Southern New Zealand takes in Canterbury, Otago and Southland and the inland high country. It is the coldest and most southerly part of the country, with a short growing season and frost that runs late into spring. NIWA says Southland and Otago have temperatures lower than the rest of the country, with frosts and snowfalls occurring relatively frequently each year, though summer days above 30 °C are possible inland. For a gardener the season is short, the last frost is the date that matters, and tender crops go in last.

Frost

NIWA’s regional climatologies give the average number of frosts in each month, as air frosts, which are readings below 0 °C in a screen 1.3 m above the ground, and ground frosts, which are -1 °C or lower at the surface of clipped grass. Ground frosts occur more often than air frosts, and air frosts are most frequent inland. Christchurch averages about 33 air frosts a year, mostly from May to September. Invercargill has about 41, Queenstown 45, Alexandra about 88 and Lake Tekapo about 100. Dunedin’s Musselburgh, by the sea, has only about eight.

The season is long at the edges. Air frosts begin in April at Christchurch, Invercargill and Queenstown, from March at Alexandra, and in every month at Lake Tekapo. They continue into October and November: Alexandra averages two in October, Invercargill one, Lake Tekapo five in October and two in November.

Frost forms on clear, still nights, and cold air pools in valleys and basins, so the bottom of a plot frosts first. Hoar frost, a feathery deposit on fences and leaves, is typically limited to the inland basins. Cover tender plants before dusk and take the cover off in the morning.

Rain and watering

Rain is spread through the year, though not evenly across the region. Southland’s rain is fairly evenly distributed, with the eastern lowlands receiving between 800 and 1,200 mm a year and some summer maxima around Gore. In Canterbury the plains have low rainfall, and NIWA says irrigation is necessary in most parts of them during the growing season. North-westerly days bring exceptional evaporation. The inland basins have low rainfall with a summer maximum, below 500 mm a year in the Mackenzie Country.

Central Otago is the driest place in New Zealand, with median rainfall below 400 mm, and NIWA says no area of New Zealand has hot dry summers and cold dry winters that more closely approach a semi-arid climate. Dry spells of more than two weeks are relatively frequent there. Mulch heavily, water deeply and shelter young plants from wind. In the house the cold season runs long, so water houseplants less often from June to August and keep them off cold glass.

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. Sow carrots, beetroot and kale for autumn, and plant out the last brassicas. Water deeply, particularly on the Canterbury plains and in Central Otago. The general list is on January, south of the equator.

February

Late summer. Sow quick crops such as lettuce, radish and spinach, and sow the winter brassicas in trays. Divide perennials while the soil is warm. The general list is on February, south of the equator.

March

Autumn, with the first air frosts possible in the high basins at Alexandra and Lake Tekapo. Pick the tender crops before the cold, and plant spring bulbs. Sow broad beans and spinach. The general list is on March, south of the equator.

April

Air frosts begin across the region. Plant bulbs and shallots, sow spinach and broad beans, and bring tender pots under cover. Mulch the beds before the cold. The general list is on April, south of the equator.

May

Frosts are common. Plant garlic and hardy hedging, and mulch against the cold. Cover tender plants on clear, still evenings. The general list is on May, south of the equator.

June

Winter, with frost almost nightly inland: Alexandra averages 20 air frosts this month. Plant garlic and dormant blackcurrants and rhubarb, and prepare planting holes for roses and apples. Keep drainage clear. The general list is on June, south of the equator.

July

The coldest month, and the peak for frost. Prune roses, fruit trees and vines, and keep planting bare-root stock. Leave frost-damaged growth in place until spring. The general list is on July, south of the equator.

August

Late winter, with frost still frequent: Alexandra averages 19 air frosts. Chit seed potatoes in a cool, light place, and plant bare-root stock while it is still dormant. The general list is on August, south of the equator.

September

Spring, but frost is far from over inland, with Lake Tekapo averaging ten air frosts. Sow peas, broad beans, spinach and lettuce, and start tomatoes and capsicums under cover. Do not plant out tender crops. The general list is on September, south of the equator.

October

Plant potatoes in October and November, in sheltered ground, and earth them up if frost is forecast. Start tomatoes, cucumbers and courgettes under cover. Keep fleece handy, because the last frosts are close. The general list is on October, south of the equator.

November

The last frosts in the inland basins. Plant out tomatoes, capsicums, beans, cucumbers and courgettes from the middle of the month in a sheltered spot, and sow beans. Mulch and water. The general list is on November, south of the equator.

December

Early summer, with the season at its peak. Finish planting the tender crops, stake the tomatoes, and water deeply. Sow brassicas in trays for autumn. The general list is on December, south of the equator.

Plants that belong here

• Apple is planted bare root in the dormant season and the entry gives a minimum of -25 °C.
• Blackcurrant is planted bare root, deliberately deep, so that buds below the soil make new shoots from the base.
• Raspberry canes are sold bare root through the dormant season and go in as soon as they arrive.
• Rhubarb is bought as a dormant crown and planted through the cold months.
• Potato is planted deep once the frosts are ending and earthed up as it grows.
• Garlic needs a cold period to split into cloves, so plant it in May or June.
• Kale stands through frost and snow, and the flavour improves after a hard frost.
• New Zealand broadleaf takes salt-laden coastal wind, which suits the exposed Southland coast.
• Daffodil is planted early in autumn and its leaves are left alone after flowering.

Questions from the margin

Read on

Planting almanac for this region

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