The only thing required for green algae, mold, and mildew to grow on your outdoor surfaces are moisture, shade, and a food source. The most common areas are along the northern side of homes, under trees, and in other shaded areas where drainage is slow. But we have 5 easy ways to get rid of algae, mold, and mildew on your outdoor surfaces.
Besides being an unsightly mess, algae and other undesired growths are often slippery when wet. In addition, algae and/or mold can trigger allergic reactions in people and pets.
I have a problem with green algae on one side of my pool patio. It just so happens that this area is also where I have a dog run for my fur babies to access the backyard. I have noticed that when I don’t keep it under control, my dogs itch uncontrollably.
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How to Get Rid Of Algae and Mold On Outdoor Surfaces
You may notice either green algae or black mold and algae growing in any humid location. Patios, decks, house siding, and roof shingles are just a few of the places that this stuff can thrive.
Because every location is different, there is not a one size fits all solution. You need to take into consideration the surroundings and the possible downsides.
Is the area porous, soft, or susceptible to fading? Will pets need to travel through the treated areas? Are there plants nearby?
All of these questions should factor into your decision on which method to use. However, I have listed several options below. Something for everyone. That green algae will be yesterday’s news in no time!
Use a Pressure Washer
Of all the sure-fire ways to remove algae, mold, or mildew, a pressure washer is the most common and quickest solution. If you need to get rid of it like yesterday, this is the way to go.
For mild growth, you can usually get away without using a pressure washer. If you have a thick mat of algae or moss that has started to grow, you will most likely need to use a pressure washer for the initial clean-up.
The only downside to using a pressure washer is that can be expensive. You can either buy one, which is a good idea if you know you will need to repeat the process, or you can rent one.
You could also pay a professional to do the dirty work for you. They typically use the pressure washer, in combination with the bleach solution that I am sharing with you below, to handle algae and mold.
Use This Bleach Solution
Professional pressure washers use a prewash cleaning mix of bleach, water, and a surfactant. The surfactant prevents the solution from evaporating too quickly. This allows time for the bleach to kill the algae and other undesirables.
You can make your own mix by combining 1.5 cups of bleach and 1tsp of blue dawn dish detergent to a 1-gallon container. Fill the remainder of the container with water.
I use a 1-gallon lawn spray because it allows precise application. I can get every spot that I need without getting any overspray on my plants and lawn.
The bleach solution will kill algae almost immediately. It also brightens surfaces and makes concrete and driveways look refreshed.
If you are concerned about bleach causing damage to your plants, you should know that bleach very quickly neutralizes to into salt, oxygen, and water.
Giving the plants and lawn a nice pre-soaking of water. Apply the treatment on a calm and overcast day, and allow it to rest for several hours before rinsing away. This will give it plenty of time to work and plenty of time to neutralize.
I’ve been using the bleach solution for years and have yet to experience any negative effects on plants or lawn.
Works wonderfully on concrete, stones, and pavers, and other hard surfaces.
Where to Avoid:
Avoid using bleach on stained concrete or on surfaces that you don’t want to lighten/brighten. Always do a spot test.
Use a Vinegar Solution
You can make a similar solution using vinegar – skip the water and go for full strength with a tsp of dish detergent.
It will work on lighter coatings of algae, but it is also much slower and often requires the additional scrubbing action from a good brush.
Vinegar can harm plants and will also require you to do a good presoak and to exercise caution with the application. If the rinse water will run off to landscaping, it will alter the ph for those zones and may result in damage.
Vinegar works best in areas where bleach may harm a colored concrete finish or on stained surfaces. It may also be used in areas where plants are not a concern.
Where to Avoid:
While vinegar can work well on siding and other non-porous areas, you should avoid using it on natural stones as it can cause erosion. It can also cause damage to any surface that contains cement.
Use This Copper Solution
Copper is another option for dealing with algae. This is why it is included in so many pool treatments for the same problem. It is also amazing at keeping slugs off plants.
Since copper is a common ingredient in plant fungicide sprays, it is also less likely to cause damage to plants and lawn when used properly.
However, it can kill fish. The solution and technique described below is not recommended near ponds or anywhere you have fish.
The additional jug is a must. Do not put the crystals directly into your sprayer. They will clog the sprayer, and the mix will be too strong.
Put 1.5 cups of crystals into the mixing jug and fill with water. Allow this to sit overnight. You will probably notice that all of the crystals won’t dissolve. That is normal. Just leave them there to be reused in the next batch.
This solution will be super concentrated. Do not apply it directly to anything. Always use the diluted version below.
To treat algae and mold areas, add 2 cups of the concentrated solution to your 1-gallon sprayer* and fill the rest of the way with water.
Spray on dry surfaces for the best results and allow to dry completely. The copper molecules will penetrate all porous surfaces and will be reactivated each time it rains.
The length of protection will vary. The porosity of the surface and weather will dictate its life span. You may need to reapply the solution several times throughout the year.
Copper sulfate solution really comes in handy with treating roof singles. It also works well on all porous surfaces including concrete and wood.
Where to Avoid
Avoid using it on any metal surfaces as copper can have a corrosive reaction with other metals. Be mindful of zinc screws and exposed aluminum that may be used in some construction areas.
Wet and Forget
A very popular brand product is Wet and Forget.* It can provide long-lasting results, but the elimination phase can be quite slow. It works best on light coatings that get some sunlight.
The biggest problem with this formula is the wait times. With a medium to heavy layer of algae, you may have to reapply several times and wait several months up to several years before the algae die off.
Some people prefer this product because it is supposed to be safe for animals. If that is a concern for you, it is certainly worth trying.
I recommend this product if/when you have a very light layer of green algae and for areas where one of the other options can’t be applied.
Where to Avoid
You may want to avoid this product in areas that have thick coatings. If you notice moss or lichen growing in the area, this product is likely to have a hard time wiping out the population.
Protecting Plants During Treatment
Since I am listing several chemical treatments to get rid of algae and/or mold and mildew, I feel it is important to stress the need for plant protection.
Each option listed above will present its own unique potential for damaging your plants. You will need to judge which option is safest.
However, it is incredibly important that you always do a pre-soaking on your plants and/or lawn when using any chemicals. The water will help to protect leaves and to saturate the soil in an attempt to dilute any runoff.
It is also important that you be mindful of the direction of your water runoff and act accordingly. Focus most of your pre-soaking on areas that will receive the brunt of runoff.
Always use a controlled application container, such as this lawn treatment sprayer.* And always do your treatments on calm days.
Any breeze can carry mists of chemicals to plants that you want to protect.
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How to Prevent Algae from Returning
You can always use any of the treatments above to keep algae, mold, and mildew in check. But you may be better off tackling the conditions that make it possible for them to thrive in the first place.
In areas where water is slow draining, consider adding drainage or changing the slope to allow water to run off naturally.
Some areas may benefit from adding water-loving plants. If an abundance of water is the problem and the location gets at least partial sunlight, a few well placed plants may be just the thing.
Improve Air Circulation
If possible, remove any structure or plant that is blocking the airflow. Although you can’t always move an obstacle, you would be amazed how much improvement you can get from relocating a trash barrel or trimming a wayward shrub.
Let the Sunlight In
Because algae, mold, and mildew like the dark, bringing sunlight to those areas will drastically reduce their growth. Sometimes removing a few branches from a shade tree is all that is needed.
Add Copper Flashing
As already mentioned, copper is a natural bacteria killer. You can use this to your benefit by installing copper roof flashing,* downspouts, or trim.
In the image above, you can see how the copper vents and finials have prevented the algae from growing beneath them. But the rest of the roof is covered with black streaks of algae.
You can place copper at any location where the rain runoff will pass over, and it will keep everything in its path free and clear.
Seal Concrete and Brick Pavers
After your patio, pavers, or deck is clean, a good preventative method is to seal everything with a good sealant. This inhibits algae and mold from getting a good foothold in porous surfaces.
While sealing a surface won’t completely eliminate algae growth, it will make it much easier to wash away.
The treatments listed above are the best options. As with any treatment, you have to weigh the pros and cons to find which solution is the best for your situation.
However, keep in mind that algae will always return if the conditions are right. You will likely need to repeat the process each year, and quite possibly several times per year.