Gardening in the cool zone

In cool districts the calendar is not a suggestion. The growing season opens late and closes early, frost is reliable rather than occasional, and a tomato planted a fortnight too early is a dead tomato. In exchange, the zone grows deciduous trees, cool-season vegetables and cold-hardy shrubs better than anywhere else in the country.

What defines the cool zone

Winters are genuinely cold, with regular frost through several months and snow in alpine and highland districts. Summers are short, and while daytime maximums can be high, nights cool off sharply. The season between the last spring frost and the first autumn frost may be only five or six months, and in the highest districts less.

Latitude is not the deciding factor here; elevation is. Armidale in NSW sits above 900 metres and gardens as a cool-zone town at a latitude that is otherwise subtropical. Ballarat, Orange, Canberra and much of inland Tasmania behave the same way. If you are in hill country anywhere in the south-east, assume you are one zone colder than the nearest coastal city and plan accordingly.

The year in a cool garden

Autumn, March to May. March and April are the planting months, so that roots settle before the ground gets cold. Plant shrubs and small trees, and plant spring-flowering bulbs. Rake leaves into a separate heap for leaf mould rather than mixing them through the main compost. Lift and store tender summer bulbs before the first hard frost. Clear vegetable beds and sow a green manure crop. Mulch everything you plant, both to hold moisture and to insulate roots.

Winter, June to August. The bare-rooted season, and the best value planting of the year: deciduous trees, fruit trees and roses all go in now, whenever the ground is workable rather than frozen or saturated. Prune deciduous trees once leaves have fallen, and prune apples and pears while fully dormant. Protect frost-tender plants with hessian screens rather than plastic sheeting, which traps moisture against foliage and makes damage worse. Stay off frozen or saturated lawns. Sow tomatoes and capsicum indoors in August; in this zone they need the head start.

Spring, September to November. Wait for frost to finish before planting anything tender — and be honest about when that is, because late frosts in valleys are common well into October. Sow peas, broad beans and hardy greens direct in September. Feed lawns once growth genuinely restarts. Divide crowded perennials as they push new growth. Renew mulch to full depth in October. Stake tall perennials and climbing vegetables in November, before they need it. Keep something ready to cover tender seedlings overnight.

Summer, December to February. The short productive peak. Keep vegetable beds evenly moist; a check in growth during a five-month season costs you the crop rather than delaying it. Harvest berries and summer vegetables as they ripen. Prune spring-flowering shrubs once flowering has finished. Take semi-hardwood cuttings from shrubs in December and softwood cuttings in February.

Working with frost instead of against it

Frost is not uniform across a block. Cold air flows downhill and pools where it cannot escape — against a solid fence, a dense hedge or a retaining wall at the bottom of a slope. Those pockets can be two or three degrees colder than ground twenty metres uphill, which is the difference between damage and none. Walk the garden at 8am after a clear cold night and note where the white frost sits longest. That map is worth more than any published frost date.

Where a frost pocket is unavoidable, two things help. Removing a solid barrier at the low point lets cold air drain away rather than banking up. And planting the tender material higher on the slope, or against a north-facing masonry wall that releases stored heat overnight, moves it out of the worst of it.

Do not rush to tidy frost damage. Blackened growth on a shrub in July is insulating the live tissue beneath it, and cutting it away in mid-winter exposes what you were trying to protect. Wait until the last frosts have passed, then cut back to healthy wood.

The last frost date question

Every cool-zone gardener wants a date, and no honest guide will give one, because it varies street by street and year by year in ways no regional average captures. A garden on a north slope may be safe three weeks before one on the valley floor a kilometre away. Keep your own record: note the date of the last frost you actually observe, for two or three years, and use your own numbers. Until you have them, plant tender crops a fortnight later than you think you should.

Plants that suit cool conditions

Deciduous trees perform outstandingly here. Acer palmatum, at 5 m by 4 m, is the classic courtyard tree for this zone, and it needs the cool sheltered conditions the zone provides — its leaf margins scorch in hot dry wind, so a position out of afternoon sun matters more than soil quality. Parthenocissus tricuspidata gives strong autumn colour on walls, though it should be kept off painted render and timber cladding.

Among shrubs, Grevillea lanigera at 1 m by 1.5 m is the most frost-reliable of the commonly grown grevilleas and comes from cold country originally. Leptospermum scoparium is wiry and thoroughly cold-hardy with a heavy spring flowering, but it is short-lived; plan on replacing it after ten to fifteen years rather than treating it as permanent structure. Prostanthera ovalifolia suits dappled shade with reliable moisture. Camellia sasanqua and Buxus microphylla both do well in sheltered positions.

For grasses and strappy-leaf plants, Poa labillardierei is the natural choice: a soft blue-green tussock at 1 m by 1 m for cool damp ground, cut back to about 150 mm every second winter to renew the clump. Carex appressa handles ground that stays wet all winter. Dianella revoluta and Lomandra confertifolia cover the drier positions. Lobelia pedunculata and Ajuga reptans are the useful low covers for cool shade with consistent moisture.

Clematis aristata is the cool-climate native climber, and it wants the classic clematis arrangement: shaded roots and a lit top. Plant it behind a shrub rather than in open sun, and mulch the root zone well.

Vegetables in the cool zone

Cool-season crops are the strength: brassicas, peas, broad beans, carrots, silverbeet, potatoes and lettuce all do well, and many are frost tolerant enough to stand through winter. Sow autumn and winter vegetables in February; anything sown later will not size up before the light goes.

Warm-season crops need help. Start tomatoes, capsicum and cucurbits indoors in August so they go out as substantial seedlings the moment frost is finished. Choose a warm position, ideally with a north-facing wall behind it. Capsicum in particular needs warm nights and is often better grown in a pot against a wall than in an open bed. Sweet corn is marginal in the coldest districts because the season is simply too short.

Cold soil in early spring is a bigger constraint than air temperature. Soil that is still at 10 °C will not germinate beans no matter how mild the afternoon feels; the seed sits and rots. Wait, or warm the bed by clearing mulch off it for a fortnight before sowing.

Common mistakes in this zone

Planting tender crops on the first warm week of spring. The single most common cool-zone loss, repeated annually.

Cutting back frost-damaged growth in mid-winter, which exposes the live tissue underneath to the coldest weeks still to come.

Covering plants with plastic sheeting for frost protection. It traps moisture against the foliage and generally makes damage worse; use hessian or shadecloth, held clear of the leaves.

Walking on saturated or frozen lawn and beds. Winter compaction in cold heavy soil takes years to undo.

Sowing into cold soil. Air temperature recovers weeks before soil temperature does.