Watering and drainage

Two opposite problems sit on this page, and most gardens have both in different corners. In one place water leaves too fast, or never gets past the surface. In another it arrives and does not leave, and roots suffocate. Neither is solved by adjusting how often you turn the tap on.

Deep and infrequent, not little and often

This is the single most useful watering principle in Australian conditions, and it is the one most consistently ignored. A light daily sprinkle wets the top 50 mm of soil. Most of that evaporates within hours on a hot day, and the part that does not trains roots to stay in the hottest, driest, most volatile layer of the profile. A plant watered that way for a year has a shallow root system and no reserve at all when the weather turns.

A long slow soak that wets the profile to 300 mm does the opposite. Roots follow moisture downward into soil that stays cooler and holds water for far longer, and the plant becomes progressively less dependent on you. The rule of thumb is two deep waterings a week for new plantings through summer, tapering after the second year, rather than a short burst every day.

Check what you are actually delivering rather than assuming. Water a bed for the length of time you normally would, wait twenty minutes, then push a trowel or a long screwdriver in and see how far down the soil is damp. Most people are surprised, and most are watering for a third of the time they need to.

When to water

Early morning is the best time in every zone. Evaporation is at its lowest, wind is usually light, and foliage that does get wet dries during the day. Evening watering is second best and carries a real cost in humid districts, where foliage that stays wet all night is the main avoidable cause of leaf problems. Midday watering in an arid summer loses a large share of what you apply before it reaches the root zone.

Water at the base of plants rather than over the top of them. Overhead sprinklers lose water to evaporation and wind drift, wet foliage unnecessarily, and deliver unevenly. They are appropriate for lawns and for very little else.

Establishment versus maintenance

Every plant on this site, including the ones described as low water, needs regular water for its first two summers. Drought tolerance is a property of an established root system, not of a plant in a 150 mm pot. The listing tells you what a plant will need once it has settled, and settling takes roughly two years for a shrub and longer for a tree.

Then taper deliberately. This step is what most gardens skip. Low-water species in temperate, arid and cool zones should come off supplementary water entirely after the second summer; continuing to irrigate them keeps warm soil wet around the root crown, and that is how most established natives are actually lost. Grevillea, Eremophila, Correa, Hakea and Banksia are all vulnerable to this.

Water trees at the drip line — the outer edge of the canopy — rather than at the trunk. That is where the active feeding roots are, and watering at the trunk keeps the collar wet without reaching them.

Grouping by water need

The most effective water-saving change available to a garden costs nothing: put plants with similar needs together. A bed containing a Correa that wants no summer water, a camellia that wants steady moisture and a row of annuals that wants three waterings a week cannot be watered correctly. Whatever the controller is set to, two of the three are wrong, and the low-water plants usually fail first and least visibly.

Sort the garden into zones before you sort it by colour. Low-water plantings go on the outer, sunnier, drier parts of the block and come off irrigation permanently. Moderate and regular-water plantings go close to the house where they are easy to reach and easy to water separately. Vegetable beds get their own line. This one decision usually cuts a garden's water use more than any equipment change.

Delivery: hoses, drippers and sprinklers

A hose on a slow run is perfectly adequate for a small garden and delivers water exactly where you want it. It requires you to be there, which is its only real disadvantage.

Drip line is the most efficient option for beds and vegetable gardens. Water goes into the soil at the root zone with almost no evaporation loss and no foliage wetting. Run it under mulch, not on top, so the mulch shades the wet soil. Check emitters seasonally: in hard-water districts, particularly inland, mineral deposits block drippers steadily and the first symptom is usually a dead plant rather than a visible leak.

Sprinklers suit lawns and nothing else in a domestic garden. If you use them, run them in the early morning and check coverage by putting a few straight-sided containers out and measuring what actually lands in each. Coverage is almost never as even as it looks.

Whatever the method, mulch is the multiplier. A settled 50 to 75 mm layer cuts evaporation from the soil surface substantially, and an unmulched bed can lose most of a light watering before it does any good. See soil and mulch.

Water that will not soak in

If water beads on the surface, runs off, or sits and then disappears leaving dry soil 50 mm down, the soil has become water repellent. This is common in sandy soils in southern Australia after a dry summer. Break the surface with a fork before watering, then water in several short cycles rather than one long burst so each cycle softens the surface for the next. Keep the ground mulched afterwards so it never dries to that point again, and build organic matter into the top 150 mm over successive seasons.

Drainage: the opposite problem

Roots need air as well as water. Soil that stays saturated has none, and root death follows within days for species that are not adapted to it. More garden plants are lost to waterlogging than to drought, and the loss is harder to diagnose because the symptoms — wilting, yellowing, collapse — look exactly like drought and are usually met with more water.

Test drainage properly. Dig a hole 300 mm deep and 300 mm across, fill it, let it drain away, then fill it again and time the second draining. Two to four hours is good. More than four hours is a constraint you must design around. More than twelve hours means the bed cannot be planted at grade with anything that is not specifically wet-tolerant.

Fixing a bed that stays wet

Raise it. This is the reliable answer and usually the only one that lasts. Building the bed 200 to 300 mm above the surrounding grade puts the root zone above the saturated layer. It works regardless of what the subsoil is doing, which digging does not.

Do not dig a deeper hole. A deep hole backfilled with improved soil in heavy clay becomes a sump: water flows into it from the surrounding ground and has nowhere to go. The plant sits in a bucket. This is one of the most common and most counterproductive pieces of garden advice in circulation.

Shape the ground. Fall away from the house and away from beds that stay wet. A gentle crossfall of even 1 in 100 moves surface water. Where a low point cannot be avoided, plant it with species that suit it — Carex appressa, Callistemon citrinus, Melaleuca linariifolia, Ficinia nodosa — rather than fighting it.

Break a compacted pan. If drainage is poor because heavy machinery compacted the ground during construction, forking the profile to full depth every 200 mm and levering to crack it will help. Do this when the soil is just moist, never when it is wet.

Check what is upslope. A bed that stays wet is often receiving water from somewhere — a downpipe discharging at the surface, a neighbouring paved area, a driveway. Following the water back to its source is frequently a faster fix than anything done in the bed itself. Downpipes discharging into garden beds are a common and easily corrected cause.

Slopes and runoff

On sloping ground, water that runs off is water lost, and it takes soil with it. The remedies are shallow and simple. Run swales — shallow depressions — across the contour rather than down it, so runoff is slowed and spread. Build a shallow basin around each tree and large shrub, roughly the diameter of the eventual canopy and 75 to 100 mm deep, so water is held where it can soak in. Plant groundcovers that root as they spread to bind the surface.

Test the shape by running a hose at the top of the slope and watching where the water actually goes. Ground rarely behaves the way it looks like it should, and five minutes of observation saves a season of guessing.