This is a common problem. It could either be mildew, or a reaction from tannin in the wood. Given that you have black residue on rocks, it may be mildew. If the rocks/stones are near the deck/pergola, it may just be stained water dripping on to them and then staining them.
Simply put, black stains on flagstones are caused by certain types of mold and mildew. These fungi find their way onto the stones either from their spores landing on the stones or from simply extending onto the stones from patches of mold or mildew right beside them—a pretty common occurrence for garden pavers.
Black spots appear when mould and algae have settled into the pores of the paving stones, but before you reach for one of the best patio cleaners on the market and get stuck into how to clean a patio, it's worth considering how to avoid the issue occurring in the first place, and it can help to have a good drainage ...
Pressure Treated Wood Turning Black
When pressure treated wood turns black, it means it has a fungal issue and is rotting. This fungus consists of small organisms that feed on the wood. This causes the pressure treated wood to soften and decay. After this happens, the wood begins to rot.
It is easy to spot the differences between mold and wood rot. Wood rot presents as decay, and mold presents as discoloration, usually black or white. Wood rot is a dangerous but avoidable condition often found in the wood of older houses.
Sanding and chiselling will leave unsightly marks on your paving, bleaching can work well but you have to be careful not to kill your plants and dilute your bleach to the correct level, some people recommend using this method only in the winter months to avoid damage to plants but you still need to be really careful ...
The patio black spots that grow on your garden stonework are actually spots of black lichen. The spores from the black lichen start life as tiny dust-like spores growing on the underside of leaves on trees and plants. These spores are carried, by the wind and the rain, onto your stonework in their millions.
The use of a high temperature, low pressure steam cleaner (such as Doff) will kill the black spot but unfortunately will not always remove the more stubborn black marks on the stone.
Mold requires water and moisture to fester and grow. So if you manage to rid the area of water, the mold won't be able to grow. Patio maintenance involves keeping the area dry and clean thoroughly. Sweeping with a broom rids any dirt and organic matter from seeping through the cracks and allowing the mold to feast.
What are the black spots on your patio? If you have dark areas of discolouration on the hard surfaces of your garden (patio, drive, paths etc) it could just be dirt so see if it can be removed with a scrub over. If it can't, and it looks like ink splatters, it's most likely to be black lichen, also known as black spot.
If you don't have detergent, a simple solution of washing up liquid with warm water should do the trick. Starting from the top corner, hold your pressure washer at a 30-degree angle and spray diagonally across your patio. Don't stand too close to the surface as the powerful jet of water could damage your patio slabs.
Once again, acetic acid will wreak havoc with your block paving. Porous stones like limestone and sandstone are more susceptible to damage than most, as the vinegar can seep into the stones themselves, though even sealed and non-porous stones can discolour and bleach.
Similar to cleaning pavers with muriatic acid, avoid an intense concentration of bleach, as it can damage the pavers.
Patio Magic contains a fungicide that will kill off the following: Lichens (mostly black spots, sometimes white, on patios, concrete and blocks). Mould. Algae.
Second, bleach isn't suitable for man-made concrete slabs – only natural paving stones. Bleach will fade artificial slabs over time. Lastly, you'll need to wear protective clothing to stop the bleach from getting on your skin.
Remove those ugly green and black mold stains on your deck with Wet & Forget Xtreme Reach™ Hose End. Spray your wood or composite deck with Wet & Forget no-rinse cleaner. Allow the surface to dry (4-5 hours). Let Mother Nature do the rest of the work for you!
Wet & Forget Outdoor will tackle all of these ugly stains on your concrete, with no elbow grease required. Here are the steps to get rid of these growths once and for all: Mix 1 part Wet & Forget with 5 parts water in a pump-up garden sprayer. Spray the affected concrete with the diluted Wet & Forget solution.
Cleaning a patio with bleach is easy.
Whether your patio is sealed concrete or cement, or even sealed concrete pavers, a bleach and water solution makes it easy to clean away built up soil, stains from leaves and pollen, mold, mildew and algae.
Wood that's exposed to the sun's UV rays over an extended period of time will darken and even turn gray. This weathering happens because the sun causes chemical changes to the tannins in the wood, kind of like oxidation.
Pre-staining the wood with black tea or coffee can produce more tannins and thus, darken the wood. An oxidation solution with apple cider vinegar can also produce a slightly darker result without the need for a pre-stain.
When you see the black mold cover wide areas of lumber, usually this happens because the wood got wet and stayed wet while it was being stored. If it's warm and humid, the growth of the mold and mildew can be rapid. Spotty outbreaks can sometimes be traced to food or liquids that nourish mildew or mold.
However, the color black isn't unique to Stachybotrys — people often confuse it with similar-colored molds like Cladosporium, Aspergillus, Alternaria, and Dreeschslera. In earlier stages, black mold may appear more greenish-grey or greenish-black in color with fuzzy white edges.