Gardening in the arid and semi-arid zone

In the dry interior, a garden is a water-management project with plants in it. Rainfall is not just low, it is unreliable: a year's total may arrive in two events months apart. Evaporation exceeds rainfall for most of the year. Every decision — where beds go, how deep the mulch is, which species go in, what shape the ground is — comes back to whether water reaches the root zone and how long it stays there.

What defines the arid and semi-arid zone

Summers are long and hot, with extended runs above 35 °C and low humidity that drives high evaporation. Winter days are mild and bright, but clear still nights radiate heat away rapidly and frost is regular. This combination — genuine summer heat and genuine winter frost in the same garden — narrows the plant list considerably, because a species must tolerate both.

Soils across the zone are often alkaline, sometimes saline, and frequently hard-setting: a surface that seals over after rain and sheds the next fall rather than absorbing it. Where the soil is deep red sand, water infiltrates readily but drains past the root zone quickly. Either way, the problem is retention.

The year in an arid garden

Autumn, March to May. The planting season, and effectively the only one. Daytime maximums drop, soil is still warm, and anything planted now has six months to establish roots before the next summer. Build a shallow basin around each new plant to catch what rain does fall. Mulch immediately, keeping mulch clear of the stem. Sow leafy vegetables and root crops for the cool season.

Winter, June to August. Prune deciduous trees while dormant. Do not cut back frost-damaged growth yet — the dead material insulates what is below it, and removing it in June exposes live tissue to the coldest weeks. Plant bare-rooted deciduous trees in July. Fork compost through vegetable beds. Check windbreak plantings for gaps, because a gap in a windbreak funnels wind and does more damage than no windbreak at all.

Spring, September to November. Plant summer vegetables after the last frost. Renew mulch to full depth before temperatures climb. Water new plantings deeply twice a week to push roots downward. October is the last sensible month for new perennial planting; anything later meets summer with too little root behind it. Set up shade over young trees and vegetable beds, and check ties and stakes before the hot northerly winds.

Summer, December to February. Maintenance only. Do not dig, because disturbing soil in peak heat dries the profile out. Do not prune, because cut surfaces scald in direct summer sun. Water established plants deeply and infrequently, early in the morning; midday watering largely evaporates before it reaches the roots. Deep-water any tree planted in the last two years right out to the drip line, not at the trunk.

Shaping the ground to hold water

The most effective work in an arid garden happens before any plant goes in. Water that runs off a hard surface and disappears down the driveway is water you have lost. Water directed into a shallow depression around a planting is water that soaks in.

Build a basin around each tree and large shrub: a saucer roughly the diameter of the eventual canopy, 75 to 100 mm deep, with a low rim. On slopes, run shallow swales across the contour rather than down it, so runoff is slowed and spread rather than concentrated. Test the shape by running a hose and watching where the water actually goes; ground rarely behaves the way it looks like it should.

Then cover everything. Bare soil in this zone bakes to a hard crust that sheds water and radiates heat into the root zone. A settled 75 mm of coarse mulch — chunky bark, gravel, or a coarse composted product — cuts evaporation from the surface substantially and keeps soil temperature down. Fine mulches mat together and become water-repellent, which is worse than nothing. Keep mulch 50 to 100 mm clear of trunks and stems. See mulching properly.

Watering that actually works

Deep and infrequent beats shallow and often, everywhere in Australia, but here the difference decides whether a plant lives. A light daily sprinkle wets the top 50 mm, evaporates within hours, and trains roots to sit in the hottest, driest layer of soil. A long slow soak that wets the profile to 300 mm sends roots down to where moisture persists.

Use drippers or a slow-running hose rather than sprinklers, which lose a large share to evaporation and wind drift in dry air. Water early in the morning. Check emitters regularly, because hard water in this zone blocks drippers with mineral deposits far faster than in the south. And remember that "low water" species still need regular water for their first two summers; drought tolerance is a property of an established root system.

Plants that suit arid conditions

The saltbushes are the backbone. Atriplex nummularia, old man saltbush, reaches 2 m by 3 m and tolerates saline and alkaline soils that defeat almost everything else, making it the default large screening shrub inland. Rhagodia spinescens holds a neat silver mound at 1 m by 2 m on hot exposed banks with no supplementary water once established.

Eremophila glabra, at 1 m by 1.5 m, flowers for a long season and asks for nothing but sharp drainage — it dies quickly in wet clay, so it belongs on the sandy or raised parts of the garden. Alyogyne huegelii gives large mauve flowers on a fast 2 m shrub, but it is frost-tender and brittle in wind, so it needs a sheltered position rather than an exposed boundary.

For small trees, Hakea laurina (5 m by 4 m) and Eucalyptus caesia (8 m by 4 m) both suit dry sandy ground. Neither tolerates summer irrigation over the root zone, which means they should not be planted in a lawn or in an irrigated bed. Lagerstroemia indica works in the semi-arid margins where some water is available and gives useful deciduous shade.

At ground level, Chrysocephalum apiculatum and Myoporum parvifolium are the two most reliable spreading covers for hot verges. Brachyscome multifida and creeping thyme suit gravel edges. Dianella revoluta and Lomandra longifolia carry the strappy-leaf mass planting; both are frost tolerant and low water.

Vegetables in the dry inland

The cool season is the productive one. Sow carrots, beetroot, lettuce, brassicas, peas and broad beans from March onwards, and grow them through the mild winter. Summer crops are possible but need shade cloth over the bed through the worst of the heat, deep mulching, and consistent watering; tomatoes and capsicum planted in September will crop before the harshest weeks if the bed is well prepared.

Raised beds help less here than in the wet tropics, because raising a bed increases evaporation from the sides. In-ground beds with a generous compost content and a thick mulch layer hold moisture better. Add organic matter every season: it is the only practical way to lift the water-holding capacity of a sandy or hard-setting soil.

Common mistakes in this zone

Planting in spring. It gives three months of root growth before a summer that will test the plant to destruction. Autumn gives six.

Watering little and often. This is the single most common cause of shallow-rooted, heat-vulnerable plants.

Using fine mulch. It mats, turns water-repellent, and sheds the rain you were trying to capture.

Removing frost-damaged growth in mid-winter, which exposes live tissue at the worst possible moment. Wait until the last frosts have passed.

Planting free-draining dry-country species into an irrigated lawn. Hakea and Eucalyptus caesia in particular resent constant summer moisture over the roots and decline without any obvious cause.