Gardening in the tropical zone

The tropical north does not have four seasons. It has two, and they are so different from each other that a single garden effectively operates under two separate sets of rules. Almost every mistake made in tropical gardens comes from applying temperate-zone timing — planting in spring, pruning in winter, watering little and often — to a climate that does not work that way.

What defines the tropical zone

Temperatures are high and remarkably even. Darwin's average maximum varies by only a few degrees between the hottest and coolest months, and minimums rarely drop below the high teens. Frost does not occur anywhere in this zone, which removes the single biggest constraint that governs southern gardens and replaces it with two others: water excess and water absence, in sequence.

The wet season runs from about November to April, though the exact opening varies by weeks from year to year. It brings heat, very high humidity, and rainfall that arrives in intense falls rather than steady drizzle. The dry season, May to October, is warm, bright, low in humidity and almost rainless. Between them sits the build-up, roughly October to November, when humidity climbs ahead of the rain and conditions are at their most punishing for both plants and people.

The year in a tropical garden

Early dry, April to June. This is the planting season. Soil still holds moisture from the wet, temperatures are pleasant, and anything planted now has five or six months to establish roots before the next wet arrives. Plant shrubs, small trees, palms and perennials now. Sow the main vegetable crops — tomatoes, capsicum, beans, cucurbits — because the cooler dry is the tropical vegetable season, not summer.

Mid dry, July to August. Growth slows. This is the time for structural pruning, because cut surfaces dry and seal quickly instead of staying wet. Hedges cut now regrow tidily. Deep-water established beds once a week rather than lightly every day; frequent shallow watering in the dry keeps roots in the top 100 mm where they are most vulnerable.

Late dry and build-up, September to November. The hardest months. Anything planted since April needs consistent deep watering. Renew mulch to full depth before the first storms. Prune anything overhanging paths, roof lines or power service lines before the wind season. Resist the urge to plant on the first storm; a single downpour followed by three dry weeks kills more new plants than the whole dry season does.

Wet season, December to March. Maintenance rather than establishment. Check that beds drain within a day of heavy falls. Weed weekly, because wet-season weeds go from seedling to seed-set in a fortnight. Keep stakes and ties sound, since squalls do most of the physical damage to young plants. Leave major pruning alone: cut surfaces stay wet, healing is slow, and soft regrowth is immediately battered.

Drainage is the whole game

More tropical garden plants are lost to saturated soil than to anything else. A bed that holds standing water for more than about 24 hours after heavy rain will kill the roots of most shrubs and small trees, regardless of how well suited the species is to heat and humidity. Roots need air as well as water, and saturated soil has none.

Test your beds during the wet, not during the dry. Dig a hole 300 mm deep and 300 mm across, fill it with water, let it drain, then fill it again and time the second draining. If the second fill takes more than four hours to disappear, you have a drainage problem to solve before you plant. The solution is almost always to raise the bed — 200 to 300 mm of additional soil above the surrounding grade — rather than to dig deeper, because a deep hole in heavy soil simply becomes a sump. There is more detail in watering and drainage.

Pots are a specific risk here. A pot standing in a saucer during the wet is a pot with permanently waterlogged roots. Lift containers onto feet so water runs away freely, and use a coarse open potting mix rather than a fine water-holding one.

Plants that suit tropical conditions

The dependable structure plants for this zone tolerate both extremes. Melaleuca linariifolia, listed at 8 m by 5 m with moderate water, handles wet feet in the wet season and dry ground in the dry, which is exactly the combination the zone demands. Callistemon salignus behaves similarly at a similar size and copes with compacted ground. Syzygium australe, at 7 m by 4 m, is the standard evergreen screen: it holds cover to the ground and clips into a tall hedge without thinning out.

For lower planting, Lomandra longifolia is the single most adaptable strappy-leaf plant in the country and works in every zone including this one, at 1 m by 1 m and low water once established. Dianella caerulea does the same job at 0.6 m in part shade. Themeda triandra, kangaroo grass, is a warm-season grass that greens up with the wet and browns off in the dry, which suits the rhythm of the zone rather than fighting it.

Among climbers, Pandorea jasminoides at 5 m by 3 m and Hibbertia scandens at 3 m by 2 m are both reliable, the first for pergolas and the second for sandy coastal ground. Passionfruit grows fast and fruits heavily here, but it is short-lived; plan on replacing the vine roughly every five years rather than trying to rejuvenate an exhausted one.

Backhousia citriodora, at 6 m by 4 m, is a dense rainforest tree grown for its strongly scented foliage. It is slow for the first few years and will not tolerate dry roots, so it belongs in a bed you actually irrigate through the dry rather than in an outer boundary planting.

Vegetables in the tropics

The tropical vegetable calendar is inverted relative to the south. The main growing season for tomatoes, capsicum, cucurbits, beans and sweet corn is the dry, roughly May to August, when humidity is low and night temperatures are comfortable. Attempting these crops through the wet produces soft, collapsing plants in saturated soil.

Through the wet, stick to what suits the conditions: leafy crops in raised beds with excellent drainage, grown under some shelter from the heaviest rain. Raised beds are close to essential; 200 mm of workable soil above grade is enough for most vegetables and makes the difference between a crop and a rot. Space plants more generously than a southern guide suggests, because air movement through the planting is what keeps foliage dry.

Wind and exposure

Coastal tropical gardens face salt-laden onshore wind for much of the dry and squall-force gusts through the wet. Establish shelter first and ornament second. A permeable windbreak that slows air is far more effective than a solid barrier that deflects it and creates turbulence downwind — see planting a windbreak for spacing and layering.

Stake new plantings at the time of planting, not after the first storm bends them. Use two low stakes and a broad flexible tie rather than one high stake and wire, and check ties at the start of each wet so nothing is cutting into bark.

Common mistakes in this zone

Planting in the wet because it seems like the obvious time. It is the worst time: soil is saturated, roots cannot breathe, and the plant has no chance to establish before conditions get harder.

Watering daily through the dry. Shallow frequent watering keeps roots near the surface, where they cook. One deep soak a week produces a plant that can look after itself.

Mulching once a year. Organic mulch breaks down two to three times faster here than in southern gardens. Check depth every few months and top it up to a settled 50 to 75 mm, keeping it clear of stems.

Treating humidity as a watering problem. Yellowing, collapsing foliage in February is usually about drainage and air movement, not about how much water the plant is getting.