Why is my Pool Deck turning green and black?

Why is my Pool Deck turning green and black?

March 17, 2023 Fred Hueston Comments Off

Why is my Pool Deck turning green and black?

Frederick M. Hueston

 What is that green and black stuff growing on my pool deck? The majority of these unsightly spots are the result of biological growth in the form of Algae, Molds, Lichens and  Mosses as well as plants such as ivy and grasses. The following guide will help you in identifying these biological growths as well as removing and controlling them.

Biological Growth Types


Algae is commonly green in color but can also be black, orange, red or yellow. It is easy to identify since it grows in mats, films and patches on the surface of stone, brick and Concrete.

Algae is a single cell plant but lacks roots, leaves, stems and vascular tissue.  Most algae, especially the green type needs moisture and sunlight to grow. However there are species that will grow in low light but damp moist areas are usually required.


The most common moss is green but can also be red. Like algae it requires sunlight and moisture to grow but in addition needs a mineral source often found in soils. Unlike algae it has root structure and a very small leaf structure. It will often be growing in the grout between the stone and masonry.


Mold is not a plant but a fungus. It does not require sunlight but does require moisture and an organic food source. Mold is generally black in color but can also be red, organe, brown or yellow. Mold grows easily on many types of limestone since they contain organic matter to feed the mold.


Lichens comprise a fungus living in a symbiotic relationship with an alga or cyanobacterium (or both in some instances). They can be red, yellow, red, green, white or yellow. They are identified by crusts or leaf like structures with defined borders, growing away from the surface of the stone or masonry.

Ivy, Grasses and Higher plants

These plants are easily identified by there root system and there large leaf structure. They are often green in color.

Removal of Biological Growth

Regardless of the type of biological growth removal requires the uses of biocide cleaning chemicals such as peroxide, bleach and other biocides. The following is a basic cleaning procedure that is recommended to remove these growths.

  1. Wear gloves and protective clothing. A mask is recommended since some mold and algae spores can emit spores which can be harmful to your respiratory system.
  2. Algae is best removed when it is dry since the spores are more likely to become airborne when wet. Allow the surface to dry or dry it with a fan. Once dry remove as much as possible by scraping of scrubbing with a stiff broom or brush.
  3. A pressure washer can be used to remove the remaining biological growth but there are some precautions and recommendations as follows:
  4. Do not use high pressure. Test the pressure in a non-conspicuous area to make sure you are not damaging the stone or masonry.
  5. Use hot water if possible. Hot water will also kill some of the biological growth as well sanitize the surface.
  6. Use a wide nozzle tip to minimize damage. Keep the tip of the wand at least 12 inches away from the surface.
  7. Bleach or Vinegar can be used to clean suborn growth. Mix the bleach or vinegar. Use one cup of bleach or vinegar with one gallon of water. test this mixture in a non-conspicuous area to test for discoloration. Some stone and masonry types can lighten up with bleach or vinegar.
  8. Apply the bleach or Vinegar mixture on the deck and allow it to sit for 15-20 minutes. Do not let it dry. If it dries, apply more of this mixture.
  9. After 15-20 minutes scrub the area with a nylon brush and rinse with plenty of clean water. Be care of nearby plants since this mixture can kill landscaping plants and grasses.

In lieu of bleach or vinegar you can use many of the commercial cleaners that are available at most home centers. Be sure to read the directions carefully.

Spot Removal

If you have just a few spots you can use 3% hydrogen peroxide. Simply spray the growth with the peroxide, allow it to sit for several minutes and then rinse with water.


To keep biological growth at bay you should clean the deck on a regular bases. There are also sealers and preventive biocides that are available which will hasten the growth. But you best defense is to keep the deck clean.