How do you clean a pool before closing it for winter?

Almost everything written about winterising a pool comes from climates where the water freezes. In Marrakech that factor is practically never in play, and carrying those tips over means treating a problem you don't actually have.

What a Marrakech pool is genuinely exposed to between November and March:

• stillness. Water that stops circulating, or barely circulates, in a pool nobody looks at for five months. It is the number-one factor;
• late-summer deposit. The pool goes into winter carrying the build-up from the windy season: fine sand on the floor, a marked waterline, a greasy film on the walls;
• plant matter. Leaves, twigs, fallen fruit, which decompose on the floor and produce a stubborn residue;
• rain episodes, rare but sometimes heavy, which fill, overflow and load the pool with soil;
• temperature swings, which stay wide between day and night and put strain on materials, particularly in a pool left empty.

None of these five points is solved by a product poured in on the last day. They are solved by the condition the pool is left in — which is exactly the subject of this article.

The question comes back every autumn, and there is no single answer. There are two situations, each with its own constraint.

Keeping the water in. This is the more common choice on a pool in good condition. The water keeps counter-pressure on the walls and the floor slab, which removes the structural risk. In return, it needs a minimum of upkeep: periodic circulation, occasional checks, leaves cleared out.

A pool left full and completely unattended for five months turns green and cloudy by spring — the situation described in what turns pool water green — and it will have to be drained anyway.

Leaving it empty. This makes sense for a pool that needs work, repair, or won't be used at all. But it carries a real cost:

• the structural risk remains as long as the pool is empty. Waterlogged ground after a rain episode exerts a thrust that nothing offsets any more — which is why the timing of a drain is decided with the pool specialist;
• the finish deteriorates. Five months of sun and temperature swings chalk a paint coating, harden and shrink a liner, hairline-crack a render, crumble a cement joint;
• the pool fills up regardless: sand, leaves, soil, and occasionally a small animal that has fallen in.

The rule is simple: a pool should not sit empty longer than necessary. If draining is decided on, the cleaning and any repairs follow on directly, and refilling comes after.

If only one mechanical intervention is scheduled in the year, this is the one. Three reasons why.

The wind has dropped. The chergui — Marrakech's hot, sand-laden wind — blows mainly from July to September. A clean done in August holds for only a few days, because the deposit re-forms straight away. Done in October, the result holds for several months.

The material is still fresh. The end-of-season deposit hasn't had time to dry out and harden. It comes off with a scraper and a wet vacuum, whereas the same deposit tackled in March forms an adherent crust that has to be scraped off — and the difference in working time is considerable.

The ground is stable. This is ahead of the period when rain makes draining risky, and past the heat that makes working in a dry pool very taxing.

A pool cleaned in October goes into winter clean, and comes out in spring needing only a light refresh — which considerably lightens the pre-season preparation.

The list fits on one page, and it applies equally whether the pool is kept full or drained.

In the pool

• leaves and plant debris, skimmed then vacuumed. A leaf left on the floor for five months leaves a tannin stain on a light-coloured finish, and that stain doesn't always come out;
• sand and floor deposit, which compacts with moisture and turns into a crust;
• the greasy film and the waterline, treated while the mark is still recent. A waterline left one season longer sets hard;
• skimmer baskets, emptied and washed.

Around the pool

• the deck and coping, cleared of the sand built up in the joints — otherwise the first rain washes it straight into the pool;
• planters and pots set along the deck, moved back or emptied of their saucers;
• deck drains and outlets, cleared before the rains;
• furniture and accessories put away, never left in the empty pool where they mark the finish.

One special case: if the deck comes out of building work in the autumn — new paving, joints redone —, cleaning it falls under end-of-works deck and terrace cleaning, which does not include the pool itself.

It is often presented as the answer to winter. It is part of the answer, provided you know what it actually does.

What it brings

• it blocks most of what falls in: leaves, sand, dust, debris. That's a real gain in a tree-lined garden or during a windy spell;
• an opaque cover limits light, and so limits algae growth in a pool left full;
• it forms a safety barrier for children and animals, provided it is designed for that purpose.

What it makes worse

• a loosely fitted cover traps rainwater and dust in a pocket. That pocket becomes open standing water, in other words a highly productive mosquito breeding site — the mechanism is explained in our article on mosquitoes and standing water around gardens and pools;
• a cover placed over a dirty pool seals the dirt in: nothing is visible for five months, and spring reveals a pool in far worse condition than expected;
• a cover that rubs against coping or the waterline ends up marking the finish.

The rule that follows: never cover a dirty pool. Clean first, then cover. In that order, never the other way round.

This is the blind spot of closing up for winter, and it's where most spring surprises actually come from.

What is regularly found there in October:

• standing water on the floor of the plant room, kept topped up by a small leak or by condensation. It attracts insects, keeps constant humidity around the equipment, and becomes a breeding site in its own right;
• leaves and sand built up around the filtration unit, blocking ventilation;
• a filter lid, pre-filter or basket clogged and left untouched for five months;
• signs of rodents — a warm, dark, rarely visited plant room is an ideal shelter.

What the owner can do: sweep the floor and the surrounding area, clear out the debris, and write down what they see to pass on to the pool specialist.

What belongs to the pool specialist: shutting down the installation, draining the filter, dismantling the pump, purging pipework, fitting floats. All of that must be carried out by someone who knows the installation.

For anyone still hesitating over an October visit, here is what turns up every spring on pools closed without preparation.

• Floor deposit turned to crust. Silt, decomposed leaves and sand have dried into an adherent layer. The working time in spring is well above what an October clean would have taken, and the risk of scratching the surface while scraping it off is real.
• A hardened waterline. Limescale and grease have had five extra months to bond. On a fragile finish, part of the mark becomes permanent.
• Organic staining left by leaves that sat in the same spot.
• A degraded finish, on pools left empty: paint chalking, a shrunken liner, crumbling joints.
• A green, cloudy pool, on pools left full with no upkeep at all, with draining becoming unavoidable.

None of these situations is dramatic on its own. Together, they turn an October half-day into two days of work in March, plus repairs. That is the real reason to act before winter sets in.

The split of roles, to be written down before ordering anything.

The owner or their staff: clearing leaves, freeing the deck and drains, putting the furniture away, covering a clean pool, keeping an eye on the level and noting anything odd in the plant room.

The pool specialist: the drain if one is decided on, the mechanical cleaning of the drained pool — floor, walls, steps, waterline, coping —, refilling, treating and balancing the water, shutting down or starting the filtration, purging pipework, fitting floats, repairs to the finish or to fittings sealed into the structure, and a seasonal maintenance contract if you want one.

If building work is planned over winter, the pool clean is scheduled at the end of the works rather than before: a clean done too early would have to be redone.