Maintaining a clean and balanced swimming pool throughout the off-season is essential to avoid costly cleanups and repairs come spring. If you’re preparing to close your pool for the winter, you may find yourself asking a common and important question: should I put algaecide in my pool before closing? The short answer is yes—but the why, when, and how are just as critical as the answer itself. In this in-depth article, we’ll walk you through the role of algaecide during the pool closing process, the benefits it provides, best practices for application, and why skipping this step can lead to big problems later.
Understanding How Pool Algaecide Works
Before diving into whether or not algaecide should be used before closing your pool, let’s explore what algaecide actually does.
Algaecide is a chemical designed to kill and prevent the growth of algae in swimming pools. Algae are microscopic plant-like organisms that can thrive in stagnant or poorly maintained pool water, especially in warm, moist environments. Once algae begin to grow, they can discolor the water, create slippery surfaces, clog filters, and—in some cases—contribute to bacterial proliferation.
There are several types of pool algaecides available, each tailored to specific kinds of algae:
- Copper-based algaecides offer long-lasting protection and are effective against green, black, and mustard algae.
- Quaternary ammonium (quat) algaecides are popular due to their affordability and moderate effectiveness, primarily targeting green algae.
- Polyquaternary (polyquat) algaecides are more potent than quats and are often used as a follow-up treatment or in preventative maintenance.
- Bromine or chlorine-based algaecides, while not always labeled as such, also serve an anti-algae function by maintaining high sanitizer levels during periods of low pool usage.
In general, algaecide should be used in conjunction with proper water balancing and sanitizer maintenance, not as a substitute for chlorine.
Why Algaecide is Crucial Before Closing Your Pool
Now that we’ve reviewed how algaecide functions, let’s address its role in the winterizing process.
1. Preventing Algae Growth During Winter Hibernation
During the off-season, your pool is vulnerable to contaminants, especially algae spores. Once the filtration system is off and the water is stagnant, algae have an opportunity to flourish. If left unchecked, even a small amount of algae can bloom into a large-scale infestation over the months.
By adding algaecide before closing your pool, you’re effectively creating a hostile environment for algae, helping to prevent their multiplication. This doesn’t eliminate the need for a strong chlorine level—that should still be addressed before covering—but it provides an extra layer of protection.
2. Reducing Spring Opening Maintenance
One of the biggest benefits of adding algaecide before closing is that it significantly reduces the amount of time and money spent on opening your pool again in the spring. A pool that remains free of algae during the winter will start the new season with clarity and balance, minimizing the need for aggressive shock treatments, filter cleanings, and brushing.
3. Protecting Against Mustard and Black Algae
While green algae are the most commonly seen, mustard (yellow) and black algae are also common in closed pools due to their resilience and ability to thrive in low-light environments. These can be difficult to eradicate completely in the spring, even with good prep. A copper-based or polyquat algaecide, added before closing, is particularly effective against these types.
Timing and Quantity: When and How Much Algaecide to Use
Applying algaecide at the right time—and in the right amount—is key. Here’s what you need to know to get it right.
1. The Ideal Time to Add Algaecide
Pool closing involves several sequential steps: shocking the pool, balancing pH and alkalinity, lowering the water level, adding winterizing chemicals, and securing the cover. Algaecide should be added just before closing the pool, typically after shocking and when all other water balances have been confirmed and corrected.
This gives the algaecide time to circulate fully and maximize its effectiveness before the system is shut down and the pool is covered. Most manufacturers recommend adding algaecide 24 to 48 hours after shocking the pool, ensuring chlorine doesn’t neutralize the active ingredients right away.
2. Determining the Correct Dosage
Dosage varies depending on the brand of algaecide and the size of your pool. Always follow the label for dosage instructions. However, general guidelines are:
- For a typical 20,000-gallon pool:
- Copper-based algaecide — about 2 to 3 quarts
- Quat or polyquat algaecide — approximately 1 to 2 quarts
It’s important to avoid overdosing. Excessive amounts of copper-based algaecide can cause staining, especially if the pH or chelation system is inadequate.
Preparations Before Adding Algaecide
While algaecide is a helpful component, it’s not magic. Using it effectively requires thoughtful prep work to be fully beneficial.
1. Shocking the Pool
Shocking the pool before closing is essential to destroy organic contaminants and prevent algae from taking hold. A chlorine-based shock—such as calcium hypochlorite or sodium dichlor—should be applied a few days before adding algaecide. Make sure the water temperature is still above 65°F (18°C) for shocks to be most effective.
2. Balancing the Water Chemistry
Water balance must be spot-on before using algaecide. Important levels to test include:
| Parameter | Ideal Closing Range |
|---|---|
| Chlorine | 1–3 ppm (after shock, allow to fall to 0 ppm before closing) |
| pH | 7.2–7.6 |
| Total Alkalinity | 80–120 ppm |
| Calcium Hardness | 150–250 ppm (vinyl), 200–300 ppm (concrete) |
| Cyanuric Acid (Stabilizer) | 30–50 ppm |
Well-balanced water ensures that algaecide—especially copper-based—is more effective and less likely to cause staining or scaling.
3. Cleaning the Filter and Draining Excess Water
Before the pool closing begins, clean the filter thoroughly. A clean filter prevents debris accumulation and helps reduce the likelihood of algae spore harboring in media or cartridges. Also, lower the water level appropriately before adding the algaecide, typically below the skimmer opening—but this may depend on your pool type and climate zone.
Choosing the Right Algaecide For Winterizing
Not all algaecides are created equal—and not all are appropriate for winter closure.
Copper-Based vs. Quat-Based Algaecides
Copper-based algaecides are ideal for winterizing because of their long residual effect. The copper ions remain active longer than those in quat or polyquat formulas, offering extended protection over the off-season.
However, these algaecides carry the risk of staining if not stabilized or if used in incorrectly balanced water. Therefore, it’s crucial to know your water profile before using any copper-based algaecide.
Choosing Polyquat-Based Algaecides
Polyquat-based algaecides are safer for certain pool surfaces (like vinyl) and do not typically cause staining. Their residual lifespan is shorter than copper-based ones, but their stability across a range of chemistries makes them a versatile closing option.
Dual-Action Winterizing Chemical Kits
Many pool chemical companies offer winter pool kits. These typically combine a winter algaecide, some form of a phosphate remover (to starve algae of food sources), and a chlorine or non-chlorine shock. These kits are designed to make closing a pool easy and effective. They can serve as a convenient alternative to figuring out precise chemical needs individually.
Myths About Algaecide in Winter Pools
Inexperienced pool owners often fall for several misconceptions about using algaecide before closing. Here, we uncover and debunk these myths.
Myth 1: Algaecide Can Replace Chlorine
While algaecides are an important part of algae control, they’re not a substitute for chlorine. They complement the pool sanitizer. In fact, chlorine helps keep pathogens at bay, while algaecide provides an auxiliary strike against algae.
Myth 2: You Can Add Algaecide Any Time
Timing matters. Algaecide should be added after shocking but before the pool is covered. Adding it too soon or in conjunction with high chlorine levels can neutralize the algaecide’s active agents and render it useless.
Myth 3: More Is Better
Excessive algaecide use, especially copper-based, leads to staining, water discoloration, or scale buildup. Always follow the label instructions and calculate dosage based on the pool’s volume.
Alternatives: Is Algaecide Always Necessary?
While the benefits are clear, some pool owners may wonder if there are alternative ways to avoid algae without using algaecide. Let’s explore those options and assess their viability.
Using Extra Chlorine or Winterizing Sticks
It’s common to hear that adding extra chlorine before closing is sufficient. While maintaining a proper chlorine residual is absolutely key, it may not offer 100% protection against algae—especially stubborn types like mustard algae. Winterizing chlorine tablets or sticks that dissolve slowly can help maintain a chlorine level but may not act as effectively as algaecide does specifically.
Applying Enzymes or Clarifiers
Some closing kits contain enzymes or clarifiers. These are intended to break down organic matter and clear water by coagulating particles. These are beneficial for water clarity and filter efficiency but are not substitutes for algaecide since they don’t kill algae or inhibit its growth.
Using a Pool Cover Without Algaecide
A tight-fitting winter cover is crucial for keeping debris and pests out of your pool. However, it won’t stop algae growth inside the water. Algae can thrive even under a cover if spores are already present and nutrients are allowed to build up.
Conclusion: Incorporating Algaecide Into Your Closing Routine
So, should you put algaecide in your pool before closing? The answer, based on pool chemistry and seasonal best practices, is a resounding yes.
Incorporating algaecide into your pool closing routine is a strategic investment in future savings—both in time and money. It helps prevent algae blooms that could stain your pool surface, clog filters, and render the pool unusable for weeks in the spring.
The right algaecide, applied at the correct time, and in the correct dosage, as part of a complete closing protocol, will provide long-term protection. By understanding your pool’s chemistry, assessing your environment, and following best practices, you can ensure a smooth, clear, and algae-free reopening next season.
Final Steps Before Closing
To summarize your next steps before pool closing:
- Balance the pool water as recommended.
- Shock the pool with a compatible chlorine compound.
- Allow chlorine levels to drop naturally or through neutralizers.
- Add algaecide, according to the manufacturer’s instructions.
- Clean and winterize the filtration system.
- Add winter chemical kit if desired.
- Install a snug winter cover with cover clips and water bags.
By taking each of these steps seriously, you ensure your pool remains protected all season long. When you open your pool next spring, the effort you invest now will shine through with remarkably easier maintenance and sparkling water.
By consistently prioritizing proactive care, you extend the life of your pool, protect your investment, and make future seasons more enjoyable from day one.
What is algaecide and why is it used in pools?
Algaecide is a chemical product designed to kill and prevent the growth of algae in swimming pools. Algae can enter a pool through the air, water, or contaminated equipment, and thrive in warm, stagnant environments—especially when the pool is not properly maintained. Using algaecide helps keep the pool water clear, prevents unsightly stains, and reduces the likelihood of slippery surfaces that can pose safety hazards.
When applied correctly, algaecide works alongside sanitizers like chlorine to maintain a balanced and clean pool environment. It disrupts the life cycle of algae, preventing them from multiplying and becoming a larger problem. There are various types of algaecides (such as copper-based, quaternary ammonium compounds, or polyquats), each with specific functions and strengths against different algae types, so choosing the right one is important for effectiveness.
Why is closing a pool properly so important?
Closing a pool properly is crucial to protecting it during the off-season, particularly through the winter months when it is not in use. Without proper winterization, water can freeze in pipes and equipment, leading to serious structural and mechanical damage. Algae and bacteria can also grow during the closed period, especially in warmer climates, leading to difficult cleanup tasks in the spring.
An effective pool closing involves balancing the water chemistry, cleaning the pool thoroughly, adding appropriate winterizing chemicals like algaecide and chlorine, and securing a pool cover. By following best practices when closing your pool, you help preserve water quality, protect your equipment, and minimize maintenance when the season resumes.
Should I add algaecide before closing my pool for the winter?
Yes, it is generally recommended to add algaecide before closing your pool for the season. Algaecide helps eliminate any existing algae and prevents new algae growth during the time the pool sits idle. This is especially important because during the off-season, the pool’s filtration system is typically not running, which can allow algae and bacteria to proliferate unchecked.
When adding algaecide during pool closure, it’s best to use a quality winterizing algaecide that works well in cold temperatures and is compatible with your sanitization system. It should be added along with other winterizing chemicals such as chlorine or non-chlorine shock, depending on your current water balance and method of sanitization.
How much algaecide should I use when closing my pool?
The correct amount of algaecide to use depends on your pool’s volume and the specific brand you are using. Typically, most manufacturers suggest a dosage of 1 quart of algaecide per 10,000 gallons of water, but it’s essential to check the product label for precise instructions. Using the right dosage is crucial for effectiveness—using too little may not be sufficient to prevent algae growth, while too much can cause foaming or other chemical issues when you reopen the pool.
It’s also beneficial to distribute the algaecide evenly throughout the pool by adding it in several different spots or circulating the water briefly after application. Many pool professionals suggest adding algaecide after adjusting the pH and shocking the pool, ensuring that the chemical works efficiently and reaches all parts of the water to protect against algae development.
Will algaecide stay effective throughout the winter season?
Algaecide can remain effective for a considerable period during the winter season, especially winter-grade formulas designed to work in colder conditions. These specialized products are formulated to maintain potency over months of inactivity when the pool is covered and unprotected from sunlight and debris. While its effectiveness may diminish over time, it significantly reduces the chances of algae developing and helps maintain water clarity during dormancy.
For long-term effectiveness, it’s important to use a stable algaecide that is compatible with your pool type (chlorine, bromine, saltwater, etc.) and follow the instructions carefully. Some pool owners also choose to combine algaecide with a metal sequestrant to prevent staining from minerals like iron or copper, particularly if their algaecide contains copper.
Can I skip algaecide if I shock the pool before closing?
While shocking the pool is a vital step that kills existing algae and bacteria, it should not be relied on alone for long-term protection during the closed season. Chlorine shocks have a limited duration of activity and will not persist effectively over the course of several months. Algaecide, on the other hand, provides residual benefits that continue to work while the pool is closed, especially when used in combination with other winterizing chemicals.
Skipping algaecide can lead to increased chances of algae blooms when the pool cover is removed in spring, requiring extended clean-up and increased chemical usage. To adequately prepare a pool for closure, it’s best to shock first to destroy all existing contaminants, and then follow with algaecide to help prevent future algae development until the pool is reopened.
What happens if I don’t use algaecide when closing my pool?
Failure to use algaecide when closing your pool can lead to the development of algae and bacterial growth over the winter months. Without algaecide, especially if the water chemistry isn’t perfectly balanced or the pool cover fails to keep debris out, algae can grow under the cover and make reopening the pool more difficult. This can result in murky water, staining of the pool surface, and potentially damage to the circulation system due to the buildup of organic matter.
In addition to complicating the reopening process, skipping algaecide can lead to a higher demand for time, chemicals, and cleaning equipment in the spring. You may also risk voiding warranties or increasing long-term maintenance costs. Whether in warmer or colder climates, using algaecide when closing the pool is a best practice that protects both your investment and peace of mind.