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I have flat stones which create a patio area in my back yard. In between the stone, the small gaps are filled with pebbles. The stones near the edge of my house are shaded through most of the day and stay damp longer after a rain. As a result they tend to get green algae growth on them.

I have been using chemicals, but as soon as I stop the algae come back, and the chemicals are rather expensive. I was wondering if it would be more cost efficient to buy a UV light that I could plug in and leave shining on the stones for a few hours a day (when its not raining). Would or could this work?

EDIT: I have a pressure washer and have pressure washed it off, but it takes more time then the chemicals, and shoots the pebbles in the cracks all over the place.

  • Since I am not sure it will work I wont create an answer for it it but I was thinking you might try salt. Just grab a handful every now and again and toss it on and around the rocks in question and that might do the trick. – Rob May 7 at 20:12
  • If it were at all possible I would 'blow out', sweep out all of those pebbles and put them elsewhere in your yard. Do you have a gas powered blower? I couldn't live without my blower and my line trimmer. – stormy May 7 at 20:57
  • Well the pebbles (pea gravel) were decoration. I am thinking about switching to white sand though, because even without pressure washing, the pebbles sometimes come out and hurt alot when you step on one. White sand may be worse with a pressure washer. I like to sit out on my patio bare feet sometimes. – dmoody256 May 7 at 21:03
  • Yay, another bare foot weirdo! I thought I was rare but I just learned that there is like a 'cult' or a 'club' of barefoot weirdos!! You need to check out Xero Shoes!!! Yummm. anyway, When the sand gets compacted it should not be a problem but now I am thinking you should just get that sand with concrete you just brush into the cracks. Be careful when pressure washing that you don't focus the water onto that concrete, sand in your joints. This will work best for you. Plants in cracks take some maintenance. And it sounds like you need to put your feet up and relax more than garden! – stormy May 7 at 21:08
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Use of UV for sterilizing water vinegar for killing algae

I am familiar with these UV bulbs and systems for ponds, aquariums and hot tubs. You've got a good idea! But I couldn't find anything about killing algae on surfaces without water.

If you find anyone who tells you that your idea is possible, would you please let us know?

If I were you, I would own a pressure washer. They have smallish easy to use home style for home owners pressure washers. Problem with algae is you wash it off and it will always come back. Chemicals would be tough on your plants as it was washed off. Pressure washing does the same thing without chemistry involved.

Vinegar might be viable to use. Just household vinegar diluted with water? Still not so great for lawns and plants. Just 'spray' from time to time? Allow to dry instead of washing off immediately. You can get an $11 hand pump sprayer (gardening section usually) that would be far easier than anything else AFTER you pressure wash.

Vinegar can make an area of soil unable to support ANY plants or soil organisms. This is a good choice to spray graveled walks, patios, driveways. When sprayed on soil and the cracks, that vinegar LOWERS the pH so low no plant will grow. When you decide to grow plants, one adds lime and brings the pH back up to levels plants and soil organisms can grow.

Better living through chemistry! If one understands chemistry! Grins...I hope this might be a good solution. Could you send a picture of your patio?

  • Thanks for the pressure washer suggestion, I forget to mention I originally was doing that to clean the algae off, but it was too time consuming so I switched to chemicals. I updated my question to include a note about trying pressure washing. Also the chemicals I use are supposedly eco-friendly, but not sure how much to trust that! – dmoody256 May 7 at 20:10
  • Chemistry is chemistry. Whenever we mix chemicals we need to know the end result. Eco-friendly? I have no idea what that means I know I do NOT trust that either! grins. For instance, somewhere in the link I sent they said if someone is using a medication for 'ick' a copper chelate? If they turned the UV light on that UV light would break that medication down to IODINE components and become instantly fatally toxic. I LIKE pressure washing. Love watching the gook come off leaving a surface looking brand new, how big is your patio? – stormy May 7 at 20:53
  • The rocks which grow algae take me about 2 hours to pressure wash, and the algae grows back in less than a month. The chemicals I spray every two weeks take about 15 mins, and it takes about two months for the chemicals to get the algae completely gone, but if I keep up with it, it doesn't comeback. Also the pressure washer doesn't completely remove the green, even when I have the tightest stream and less than an inch off the stone, there's still a green tint. My pressure washer is rated at 2000 psi. – dmoody256 May 7 at 21:01
  • Gees, I've never had problems with algae that pressure washers weren't able to take care of. 2000psi should be FINE. I remember wide swaths of razor edged water...6 to8 inches. We did patio furniture, siding, concrete and pavers. I am off on another idea, thinking vinegar. Or very acidic solutions. Salt? Where does the water go off of your patio? Onto the lawn? Plant beds? – stormy May 7 at 21:14
  • I added another link for you... – stormy May 7 at 21:16
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Update

I’ve bumped my original suggestion to the end of this answer as it seems that algae are immune to the effects of ammonium hydroxide.

So, a very cheap solution to your problem is the application of brand-free household bleach.

Household bleach is a dilute solution of sodium hypochlorite.

Sodium hypochlorite is toxic to humans and requires careful application including the use of personal protective equipment such as rubber gloves and eyewear, however I note the following benefits.

  • relatively inexpensive;
  • small amount of concentrate will treat a large area;
  • no detrimental effects to plants.

Continued use over a period of years could lead to a buildup of sodium salts in soil in concentrations greater than what can be synthesised by soil microbiology.

If this presents as a problem, application of gypsum powder may neutralise the sodium salts. Alternatively the addition of fresh compost, peat (or coir-peat = finely ground coconut husk fibre) or activated charcoal or all three will almost certainly balance the soil such that it is productive and nutritious to plants.


Original Answer

A very cheap solution to your problem is the application of brand-free household ammonia.

Household ammonia or ammonium hydroxide is a solution of NH3 in water.

Ammonium hydroxide is toxic to humans and requires careful application including the use of personal protective equipment such as rubber gloves and eyewear, however I note the following benefits.

  • relatively inexpensive;
  • small amount of concentrate will treat a large area;
  • no long term build up of chemical salts in surrounding soil;
  • no detrimental effects to plants.

Always have a Material Safety Data Sheet (or such similar document) on hand when using hazardous products such as ammonium hydroxide, as this document will explain what to do in case of emergency.

  • I looked up this ammonia mixture which I could pick up from home depot, and the SDS says it doesn't affect algae? – dmoody256 May 7 at 21:51
  • Ok interesting... I’ll do a little more research and update my answer. – andrewbuilder May 7 at 21:53
  • The problem with all the proposed chemical solutions is that they are all very soluble in water. Algae spores are everywhere in the environment and the first rainstorm will get rid of your attempt at killing them. (And if you want to walk around in bare feet on something treated with sodium hypochlorite or ammonium hydroxide, check first if your medical insurance policy covers treating skin conditions!) – alephzero May 8 at 1:44
  • Although bleach will certainly work, please be careful with it. It can not only discolor the stones, but it will also get into the groundwater and therefore it may kill more in your garden than only algae. – benn May 8 at 8:54
  • I considered bleach, but was worried about build up in the soil which would affect my garden (which is slightly downhill from my patio area). I would like to stay away from chemicals all together if possible (hence my question is about UV). – dmoody256 May 8 at 13:22

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