Most gardens that go wrong don't fail loudly. They fail quietly over a winter or two: a patio that starts to dip at one corner, a border that never fills out, a fence post that snaps in a February gale. The cause is usually something decided in the first week, when the focus was on how the garden would look rather than how it would behave.

The mistakes below turn up in gardens everywhere, and most can be avoided with measuring, patience and a willingness to spend money where it's buried rather than where it's seen.

Drainage: the mistake that takes a year to show itself

If water has nowhere to go, it finds the worst possible route — towards the house, under a fence, or into the middle of a new lawn. Typical symptoms: a soggy patch that never dries, moss creeping across paving, plants rotting at the crown over winter.

Before you design anything, spend twenty minutes working out how water moves across the plot.

1. Dig a hole about 30cm square and 30cm deep where the garden is wettest, then fill it with water. If it hasn't drained overnight, you have a drainage problem rather than a soil problem.
2. Watch the garden during heavy rain. Note where puddles sit, which way they flow and where water eventually leaves the site.
3. Check the obvious first: gutters, downpipes, gullies, water butts. A surprising number of "drainage problems" are a blocked downpipe emptying against a wall.
4. Look at neighbouring levels. If the ground slopes towards you from two or three sides, you're sitting at the bottom of a bowl.

Fixes range from a French drain or soakaway on permeable subsoil to a raised bed, or a shallow channel that makes a feature of the water rather than pretending it isn't there. Where water collects close to a building, a retaining wall or a boundary, take professional advice — redirecting water near foundations is not a weekend job.

Ignoring levels and falls

Everything else sits on your levels. Get them wrong and nothing can be fixed without taking the whole job apart.

Two rules cover most of it: keep external ground at least 150mm below the damp-proof course, and make sure paving falls away from the building rather than towards it. A fall of roughly 1:80 — about 12mm per metre — is a sensible working target for a patio: enough to shed water, not so much that it feels like a slope.

Other traps worth checking before you order materials:

• Single awkward steps of 50mm or 70mm that people trip over. Group changes into proper risers, or keep the area flush.
• Beds raised so high that soil sits against a timber fence panel, speeding up rot.
• A shed or greenhouse propped on a slope without a level base. Frames twist, doors stop closing, glazing cracks.
• Paths sitting below lawn level, so soil and clippings wash onto them all winter.

An hour with a laser level, or a long spirit level and a straight edge, will save you a summer of annoyance.

Plants chosen for the picture, not the plot

Plant labels describe a plant at its best in ideal conditions. Your border is wet clay in a north-facing corner, or free-draining sand in full sun with wind coming off open fields.

Match plants to what you actually have. Lavender and rosemary sulk and die in heavy, wet soil; they want sun and sharp drainage. Hostas scorch in a hot, dry border. Anything destined for an exposed or coastal site needs to tolerate wind and salt. On heavy clay, improving the soil with organic matter and choosing plants that cope with winter wet will beat fighting nature every time.

Then plant for the size a plant will reach, not the size it is today. A shrub bought in a two-litre pot can spread three metres. Read the ultimate height and spread, add a third, and leave room for it.

Cheap materials that fail in British weather

Economise on pots, cushions and annual seed. Don't economise on anything buried, load-bearing or permanently wet — that's what fails first and costs most to replace.

• Decking: thin, low-grade boards cup, split and turn green. Choose a suitable board thickness, properly pressure-treated timber and stainless or galvanised fixings.
• Timber edging and sleepers: untreated softwood in contact with soil has a short life. Steel or concrete edging costs more and quietly outlasts it.
• Weed membrane: the thin stuff tears as you walk on it. Buy a heavier woven grade, and remember it slows weeds rather than stopping them — you still need a mulch on top.
• Sub-base: skipping compacted MOT Type 1 under paving is the most common cause of a sunken patio. Build it in layers and compact each one properly.
• Topsoil: cheap bagged soil can be mostly sand and rubble. Buy from a supplier who can tell you where it came from and what's in it.

Spend on what you'll never see again — sub-base, drainage and fixings — before you spend on what you will.

Designing a garden you'll actually maintain

Generous planting schemes look wonderful on paper and overwhelming by August. Be honest about your time. Fewer, larger borders with a decent mulch and a mix of shrubs and perennials are far more forgiving than a dozen tiny beds of annuals that need watering every evening.

The practical details get forgotten while everyone argues about paving:

• Paths wide enough for a wheelbarrow — around a metre.
• A tap near the main planting areas, and somewhere to store the hose.
• Space for bins, a shed and a washing line that isn't in the middle of the seating area.

Working with the weather, not against it

Timing matters as much as technique. Bare-root and rootballed plants establish best in autumn, when the soil is still warm and rain does the watering for you. Turf laid in a heatwave needs watering twice a day to survive; laid in September it needs almost nothing. Mortar, concrete and some adhesives shouldn't be used in frost, and timber shouldn't be painted or sealed when rain is forecast inside the drying time. Never dig or walk on saturated clay — you'll compact it into something closer to brick, and it will take years to recover.

A five-minute check before you start

1. Watch the garden in the rain.
2. Measure your levels against the house, the boundaries and each other.
3. Test the soil: dig a spade's depth, squeeze a handful, and see how the hole drains.
4. Write down what the space has to do — storage, drying, sitting, playing, growing.
5. Decide what you're realistically willing to maintain in a wet February.

Get those right and everything else gets easier. Get them wrong and no amount of clever planting will hide it.

Photo: Aleksander Dumała / Pexels