Spring is here, temperatures are rising, and your pool has been sitting idle for months. Whether it was fully covered, partially protected, or simply left to its own devices through the winter, opening a pool for the season is more than just pulling off a cover and jumping in. The water chemistry has shifted, debris has accumulated, and equipment that’s been dormant needs restarting carefully.

Done properly, spring pool opening takes a weekend and results in clear, balanced, swim-ready water. Done hastily or incorrectly, you’ll spend weeks fighting algae, cloudy water, and equipment problems that could have been avoided with a bit of upfront attention.

This guide walks you through every step of cleaning and opening your pool after winter, from removing the cover to balancing chemistry and getting the filtration system running smoothly again.

Key Takeaways

Open your pool before water temperatures consistently reach 20°C (68°F), because waiting longer encourages algae growth that’s harder to eliminate.
Remove and clean the winter cover carefully to prevent debris from entering the pool, then store it properly for next season.
Inspect all equipment like pump, filter, skimmers, or lines before turning anything on. Address any damage or wear now, not mid-season.
Balance water chemistry in the correct order: pH and alkalinity first, then calcium hardness, then shock treatment.
Brush and vacuum thoroughly before shocking. Loosening debris first makes shock treatment far more effective.
Run the filter continuously for 24 to 48 hours after shocking to fully circulate chemicals and clear the water.
In Portugal, mild winters mean most pools don’t fully winterise, but spring pollen and debris still require thorough cleaning at opening.

In most of Portugal, outdoor pools are usable from March or April through October or November, depending on your location and whether you have heating. The climate is mild enough that full winterisation like draining lines, adding antifreeze, or completely shutting down equipment, isn’t usually necessary the way it is in colder northern European or North American climates.

However, spring opening still matters. Even in Portugal’s relatively gentle winters, pools accumulate debris, water chemistry drifts significantly without regular maintenance, and algae spores are waiting for the first warm days to establish themselves. Spring pollen which is particularly heavy in April and May coats surfaces and clouds water quickly if not addressed.

The process below assumes a pool that’s been covered or minimally maintained through winter, which is typical for most Portuguese residential pools.

When to Open Your Pool

Timing matters more than most people realise. The rule is simple: open your pool before water temperatures consistently stay above 20°C (68°F) during the day. This is the threshold where algae growth accelerates dramatically.

In Portugal, this typically means:

Southern regions (Algarve, Alentejo): Late March to early April
Lisbon and coastal areas: Mid to late April
Northern Portugal (Porto, Braga): Late April to early May

If you wait until you’re ready to swim, around mid-May or June, you’re opening a pool that’s already begun growing algae. Green, cloudy water is far harder to fix than preventing it in the first place. Start the opening process a few weeks before you plan to use the pool, and you’ll save yourself considerable effort.

Step 1: Remove and Clean the Winter Cover

If your pool has been covered, start here. If it’s been uncovered all winter, skip to Step 2.

Before removing the cover, clear off as much debris as possible. Use a pool brush, leaf net, or blower to remove leaves, twigs, and dirt. If standing water has accumulated on top of the cover (common with solid covers), pump it off using a submersible cover pump. Do not try to remove a cover with significant water on it — it’s far too heavy and you risk spilling dirty water into the pool.

When the cover is as clear and dry as possible, carefully remove it with at least two people. Work slowly from one end to the other, folding as you go to prevent debris from falling into the pool. Even with care, some dirt will end up in the water, but don’t worry, you’ll be vacuuming soon enough.

Once the cover is off, lay it flat on a clean surface (lawn, driveway), rinse it thoroughly with a garden hose, and scrub any stubborn dirt or algae with a soft brush and mild detergent. Let it dry completely in the sun as this prevents mould and mildew growth during storage.

When dry, sprinkle a light layer of talcum powder or baking soda over the surface to absorb any residual moisture, then fold it loosely and store it in a cool, dry place. Do not store it while damp, as mould will ruin it over the summer.

Step 2: Inspect the Pool and Equipment

With the cover removed, take a slow walk around the pool and look for any damage or issues that developed over winter.

Check the pool structure: Look for cracks in tiles or grout, chips in the plaster or liner, and any visible staining. Address these before filling the pool further or starting equipment.

Inspect the deck and coping: Check for loose pavers, cracked concrete, or any trip hazards. Repair them now, before the pool is in use.

Examine equipment: Look over the pump, filter housing, heater (if you have one), and all visible plumbing for cracks, leaks, or signs of damage. Check that all drain plugs were reinstalled if they were removed for winter. If you used winterising plugs in your skimmer or return lines, remove them now.

Test underwater lights: If your pool has lights, check that they’re still sealed and functioning. Water intrusion into lights is a safety hazard.

If anything looks questionable, address it now or call a professional. Starting up a pool with damaged equipment can turn a small problem into an expensive one.

Step 3: Remove Debris from the Pool

Use a leaf rake or skimmer net to remove as much floating and settled debris as possible, such as leaves, twigs, insects, pine needles, and any other organic matter that found its way over winter.

For debris that’s sunk to the bottom, you can either vacuum it now (if your equipment is operational) or wait until after you’ve refilled and started the filtration system. For very dirty pools, it’s often easier to do an initial rough clean by hand or with a net, then vacuum properly once the system is running.

Empty the skimmer baskets and pump baskets. These often accumulate leaves and debris even when the pool isn’t in use.

Step 4: Refill the Pool to the Correct Level

If the water level dropped significantly over winter (evaporation, or intentional lowering), use a garden hose to refill the pool. The correct level is halfway up the skimmer opening – high enough for the skimmer to draw water properly, but not so high that debris bypasses it.

Filling can take several hours depending on pool size and water pressure. While the pool is filling, move on to the next step.

Step 5: Reconnect and Start the Filtration System

Before turning anything on, double-check that all drain plugs are reinstalled, all winterising plugs have been removed, and all valves are in the correct positions.

Prime the pump: If your pump lost its prime over winter, you’ll need to fill it with water before starting. Open the pump lid, fill the housing with water from a hose or bucket, and replace the lid securely.

Open the air relief valve on top of your filter. Turn on the pump. Water should begin flowing through the system. Watch the pressure gauge on the filter. Once water starts spraying from the air relief valve, close it. The system is now primed and pressurised.

Check for leaks around the pump, filter, and all visible plumbing. Listen for unusual noises like grinding, screeching, or excessive vibration indicates a problem. If anything sounds wrong, turn the system off immediately and investigate.

If everything is running smoothly, let the pump run. You’ll be keeping it on continuously for the next 24–48 hours during the opening process.

Step 6: Brush the Walls, Floor, and Steps

Before adding any chemicals, give the entire pool a thorough brushing. Start at the waterline (where oils, pollen, and calcium scale accumulate most) and work your way down the walls to the floor. Brush the steps, ladders, corners, and any other surfaces where algae or biofilm might be clinging.

This step dislodges debris and algae spores, putting them into suspension where your shock treatment can reach them. Brushing after shocking is far less effective, so do it first.

Step 7: Vacuum the Pool

Once you’ve brushed thoroughly, vacuum the pool floor to remove the sediment, dirt, and organic material that settled over winter. Use a manual vacuum or robotic cleaner, whichever you have available.

For very dirty pools with heavy silt or algae, consider vacuuming to waste (if your system allows it) rather than through the filter. This prevents clogging your filter with material it would struggle to capture.

Step 8: Test and Balance Water Chemistry

With the pool brushed, vacuumed, and the filtration system running, it’s time to address water chemistry. This must be done in the correct order. Certain adjustments affect others, and doing them out of sequence wastes chemicals and delays results.

Use a reliable test kit or test strips to check the following parameters:

pH: Target 7.2–7.4

pH measures how acidic or alkaline your water is. If pH is too low (acidic), it corrodes surfaces and equipment and irritates skin and eyes. If too high (alkaline), chlorine becomes far less effective and calcium scale forms more easily.

To raise pH: Add soda ash (sodium carbonate).
To lower pH: Add muriatic acid or dry acid (sodium bisulfate).

Always adjust pH first. It affects how every other chemical behaves.

Total Alkalinity: Target 80–120 ppm

Alkalinity acts as a buffer for pH, preventing wild swings. If alkalinity is too low, pH will fluctuate constantly. If too high, pH becomes difficult to adjust.

To raise alkalinity: Add sodium bicarbonate (baking soda).
To lower alkalinity: Add muriatic acid in small increments.

Adjust alkalinity before or alongside pH. The two are closely linked.

Calcium Hardness: Target 200–400 ppm

Calcium hardness prevents water from becoming corrosive (too low) or forming scale deposits (too high). In many parts of Portugal, tap water is moderately hard, so calcium levels are often acceptable without adjustment. Test it, and if necessary:

To raise calcium hardness: Add calcium chloride.
To lower calcium hardness: Partially drain and refill with softer water (this is rare).

Cyanuric Acid (Stabiliser): Target 30–50 ppm

Cyanuric acid protects chlorine from UV degradation. Without it, sunlight destroys chlorine within hours. With too much, chlorine becomes sluggish and ineffective.

If your pool uses stabilised chlorine (tablets or granules with cyanuric acid included), this level will build naturally. If you use liquid chlorine or saltwater generation, you’ll need to add a stabilizer manually.

To raise cyanuric acid: Add stabiliser (cyanuric acid powder) slowly through the skimmer with the pump running. It dissolves slowly so run the pump continuously for several days and avoid backwashing during this period.

Step 9: Shock the Pool

Once pH and alkalinity are balanced, it’s time to shock. Shocking adds a concentrated dose of chlorine (or a non-chlorine oxidiser) that kills bacteria, algae spores, and organic contaminants that build up over winter.

How much to use:

For clean, clear water: 1 kg (roughly 2 lbs) of granular chlorine shock per 40,000 litres (10,000 gallons).
For cloudy or slightly green water: 2–3 kg per 40,000 litres.
For heavily contaminated or green water: 5 kg per 40,000 litres.

How to add it:

Dissolve the shock in a bucket of water first (especially for vinyl-lined pools) and pour it slowly around the perimeter of the pool with the pump running. Never dump shock directly onto the pool floor as it can bleach liners and damage surfaces.

Shock in the evening or late afternoon. Sunlight degrades chlorine quickly, so adding it when UV exposure is low ensures it has time to work overnight.

Step 10: Run the Filter Continuously for 24 to 48 Hours

After shocking, keep the pump and filter running non-stop for at least 24 hours, ideally 48. This circulates the shock treatment throughout the pool and filters out the dead organic matter and suspended particles.

Check the skimmer and pump baskets regularly and empty them as needed. Check the filter pressure gauge. If it rises significantly above the normal baseline, backwash (sand or DE filters) or clean (cartridge filters) as needed.

Step 11: Add Algaecide and Clarifier (Optional but Recommended)

Once chlorine levels drop back below 3 ppm (usually 24 hours after shocking), you can add additional treatments:

Algaecide: A preventative dose helps suppress algae growth during the critical first few weeks of spring when pollen and warm temperatures create ideal conditions. Use a non-foaming algaecide to avoid surface foam.

Clarifier: Helps the filter capture fine particles like pollen, dust, and microscopic debris that would otherwise make water look hazy. The clarifier is particularly useful in Portugal during spring pollen season.

Follow product instructions for dosing. Both products work slowly so allow 24 hours of circulation before evaluating results.

Step 12: Retest Water Chemistry and Make Final Adjustments

After 48 hours of continuous filtration, test your water again. Check chlorine, pH, alkalinity, and calcium hardness. Make any final adjustments needed to bring everything into the ideal ranges.

Once your water is balanced, clear, and chlorine levels are between 1–3 ppm, the pool is ready to use.

Step 13: Reinstall Pool Accessories

Add back ladders, handrails, diving boards, pool toys, or any other accessories you removed for winter. Rinse and clean them first. There’s no point introducing dirt and grime back into freshly cleaned water.

Ongoing Maintenance After Opening

Opening your pool is just the beginning. To keep it clean and clear all season:

Skim daily.
Test water chemistry weekly (more often during the first month).
Brush and vacuum weekly.
Clean the filter monthly.
Shock every 1 to 2 weeks, or after heavy use or storms.

Consistency is what keeps a pool clean. A pool that’s skimmed every day and tested weekly almost never has problems. A pool that’s ignored for two weeks at a time will constantly need rescuing.

Natural Pools: A Fundamentally Different Approach

Everything above applies to conventional chlorinated pools. But if you find the chemical routine like shocking, balancing, testing, and adjusting more burdensome than enjoyable, it’s worth asking whether the design of your pool is working with you or against you.

Natural pools eliminate chemical treatment entirely. The biological filtration system with aquatic plants and beneficial microorganisms in a planted regeneration zone, continuously processes organic matter and maintains water clarity. There’s no shocking, no weekly chemistry testing, no chlorine routine.

Spring opening for a natural pool is simpler: remove any debris, check water flow between swimming and regeneration zones, inspect plant health, and restart the pump. That’s it. The ecosystem does the ongoing work.

In Portugal, where the climate supports year-round plant growth and spring pollen is particularly heavy, natural pools have a distinct advantage. The biological system processes pollen as organic matter rather than requiring chemical intervention to clarify the water.

Oásis Biosistema designs and builds natural pools across Portugal. If you’re planning a new pool or reconsidering your current setup, our team can show you what’s possible without the chemical routine.

Conclusion

Opening a pool after winter is straightforward but requires care and patience. Remove the cover properly, inspect all equipment, clean thoroughly before adding chemicals, balance water chemistry in the correct order, shock the pool, and run the filter continuously until the water is clear.

Done properly, the process takes a weekend and results in a pool that’s ready to use and easy to maintain all season. Rush it, skip steps, or add chemicals out of order, and you’ll spend weeks fighting problems that could have been avoided.

Take your time, follow the steps, and you’ll be swimming in clear water by the time Portugal’s warm weather truly arrives.

FAQ

How to get a pool clean after winter?

After winter, remove debris, vacuum the pool, and shock the water to eliminate bacteria. Clean filters and balance chemicals before use. Brushing walls and skimming surfaces helps clear dirt and algae, restoring water clarity and swim readiness.

Why do people put baking soda in their swimming pools?

Baking soda raises water alkalinity, stabilising pH and preventing corrosion or irritation. It helps maintain balanced chemistry without drastic changes. Used in moderation, it improves water quality and clarity, making the pool safer and more comfortable for swimmers.

How to shock a pool after winter?

To shock a pool after winter, balance pH first, then add chlorine shock according to instructions. Run the filter for several hours and brush surfaces. Shocking eliminates bacteria and algae, helping restore clean, safe water for swimming.

How do you clean a pool that hasn’t been cleaned in years?

A neglected pool requires draining, scrubbing, and deep cleaning. Remove debris, pressure wash surfaces, and shock the pool before refilling. Replace or clean filters and balance chemicals. Professional help may be needed for severe algae or structural issues.

How to Clean a Pool After Winter: Complete Spring Opening Guide