Middle New Zealand: coastal mildness, inland frost and what to plant when

Frost and rain

Middle New Zealand takes in Waikato, the Bay of Plenty, Taranaki, Wellington and the warm coast of the north South Island around Nelson. It is a zone of contrasts. The coasts are mild and sunny, with few frosts, and Wellington and Taranaki are windy. The inland plains and valleys, from Hamilton and Rotorua to the Wairarapa, have frosts every winter. For a gardener the first question is how far you are from the sea, and the second is how much wind you have.

Frost

NIWA’s regional climatologies give the average number of frosts in each month. An air frost is a reading below 0 °C in a screen 1.3 m above the ground, and a ground frost is -1 °C or lower at the surface of clipped grass. Inland, both are common. Hamilton averages about 21 air frosts a year, in months from April to October and most from June to August, with 63 ground frosts. Taupo averages about 35 air frosts and Rotorua about 20. Masterton in the Wairarapa has about 41, with air frosts from April to November.

The coast is gentler. NIWA notes that frosts are infrequent close to the sea in the Bay of Plenty, and Tauranga has about four air frosts a year and 55 ground frosts. New Plymouth has about one air frost and nine ground frosts. Wellington’s Kelburn has no air frosts, and about 11 ground frosts. In Nelson frosts are quite common in the cooler months, though less so than elsewhere in the South Island, with air frosts from April to October.

Frost forms on calm, clear nights in flat ground and in valleys where cold air cannot drain away. In an inland garden, cover tender plants with fleece before dusk, and leave frost-damaged growth in place until spring.

Rain and watering

Rain falls all year, with a winter maximum. NIWA finds about 31 percent of Waikato’s rain in June to August and about 22 percent in summer, and about 30 percent and 22 percent in the Bay of Plenty. In Wellington winter has the highest share, from 28 to 34 percent. Most of the Wellington region receives 1,000 to 1,200 mm a year, the hills more than 2,000 mm, and some Wairarapa plains less than 800 mm. Around Nelson and the Waimea Plain it is about 1,000 mm, spread fairly evenly through the year, with February and March the driest months.

Summer can still be dry. In Taranaki NIWA notes moisture deficiency during most summer months, and in Nelson dry spells of more than two weeks are quite common. Mulch, and water deeply once a week in a dry summer. In the house the cool, wet winter slows growth, so water houseplants less often from June to August.

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 beans and basil for a late picking, and sow carrots, beetroot and lettuce for autumn. Water deeply and mulch, particularly in Taranaki and Nelson. The general list is on January, south of the equator.

February

Late summer, with Nelson at its driest. Sow the winter brassicas in trays, sow sweet peas, and divide agapanthus after it flowers. The general list is on February, south of the equator.

March

Autumn begins. Sow broad beans, spinach and onions, and plant spring bulbs while the soil is warm. Frosts are still weeks away. The general list is on March, south of the equator.

April

The first air frosts become possible at the inland sites. Sow lettuce, spinach, onions and carrots, and plant strawberries. Bring tender pots under cover. The general list is on April, south of the equator.

May

Frost arrives inland. Plant garlic, hardy hedging and perennials, and mulch against the cold. Cover tender plants on clear, still evenings. The general list is on May, south of the equator.

June

Winter, wet and frosty. Plant garlic and dormant blackcurrants, prepare planting holes for roses and apples, and sow broad beans. Hamilton averages about six air frosts this month. 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. Keep fleece within reach inland. The general list is on July, south of the equator.

August

Late winter, with the frost easing. Chit seed potatoes, and plant lavender, rosemary and thyme. The general list is on August, south of the equator.

September

Spring, with frost still possible inland. Plant potatoes, and sow tomatoes, capsicums and beans under cover. Sow in short rows a fortnight apart. The general list is on September, south of the equator.

October

The last air frosts are rare but possible, with Masterton averaging two in October. Plant out tomatoes and capsicums where the frost is over, and sow beans, cucumbers and courgettes. Keep fleece handy. The general list is on October, south of the equator.

November

Early summer. Sow beans, courgettes, cucumbers and basil, and stake the tomatoes. Masterton still averages 0.6 air frost days in November. The general list is on November, south of the equator.

December

Water deeply, mulch, and pick beans and courgettes often. Sow broccoli, cabbage and cauliflower in trays for autumn. Dry spells can begin. The general list is on December, south of the equator.

Plants that belong here

• New Zealand broadleaf takes salt-laden coastal wind and suits the exposed coasts of Wellington and Taranaki.
• Hydrangea wants dappled light and soil that stays damp all summer, so mulch it through dry spells.
• Rose is planted bare root and dormant in winter.
• Apple is planted bare root in the dormant season and the entry gives a minimum of -25 °C.
• Potato is planted deep once the frosts are ending and earthed up as it grows.
• Garlic needs the cold months, so plant it in May or June.
• Tomato is killed by frost, so plant it out after the last frost.
• Beans are tender and wait for warm soil.
• Rosemary comes from dry coastal hills and does better with less water than with more, which suits a dry Nelson summer.

Questions from the margin

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