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Question DetailsAsked on 10/14/2017

Can I use copper sulfate to control mildew growth on my pavers?

Seems I read an article a while back that you could mix copper sulfate with water and apply to pool paver deck to help control mildew growth. If so, any idea how much to use per gallon of water?

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2 Answers


Yes - a tablespoon or two per gallon of warm water (dissolves more readily in warm water) is a common dosage, preferably applied by brush or hand-held spray bottle (like old windex bottle) rather than broadcasting because it can also yellow or kill vegetation alongside.

Leave in place for about a week, then wash off with copious water to avoid concentrating it in adjacent vegetation and killing it.

Another alternative is zinc powder or spray - like Lilly Miller Moss Out - they have one designed for moss/algae on pavers and lawns.

Generally - and test first to be sure - the copper sulfate can leave bluish staining so generally good for bluestone and slate and such, not for light-colored, concrete, or red pavers or light-colored sandstones or such. For those the zinc (which can leave a slight grayish staining) is usually less objectionable. Also - copper sulfate and zinc sulfate (comes both as a zinc powder and as zinc sulfate) can leave yellow/white deposits on some stones which are more acidic or are exposed to acidic lawn fertilizer, so generally the pure zinc powder is a safer bet on paving, Generally leaves little color change and washes off pretty well - though leaving it in place and not washing off (and putting a bit more on periodically) will also act as a preventative same asit does on roofs, whereas sulfates get reduced by moisture pretty quickly and, in my experience, are less effective for long-term treatment though does seem more effective initially, especially against algae as opposed to moss.

In a pool deck environment I would probably go with the copper sulfate just because that is a chemical already commonly being added to the pool water, whereas I don't know what the zinc would do - plus may tend to settle as a gray film in the bottom of the pool.

One other thought - the sulfate forms are salty tasting so pets will sometimes lick it (especially rabbits and cats) so beware of that possibility - not good for them at all. You would have as vet how dangerous - it is added to most pet foods as a source of copper so obviously not dangerous in small quantities, but I can't imagine licking up quantities off the ground is going to be good for them. From what I found on vet site on the web, zinc powder they would have to consume a pretty fair amount to be dangerous - the body regularly processes zinc in foods. It can also damage pads if they walk in it - especially when the application is wedt or pads are wet, plus of course if it damages the pads they then start licking them, ingesting the chemical.

Another alternative if worried about pets is using chlorine bleach or sodium hypochlorate (ends up as same thing mixed in water) - confine animal then apply and leave on the pavers for just a couple of hours (as opposed to the days needed for zinc/copper treatment to work), then wash off, diluting it well to avoid damaging adjacent vegetation. Can bleach your pavers though -to minimize differential bleaching lightly mist the surface with water first so you don't get concentrated bleach in just spots, scrub in with a broom for consistent application, thenn let sit. Bleach is somewhat harder on easily dissolved stones, and on mortar grout so not a first choice for algae/moss removal, though of course is the prime solution if going after bacterial sources of infection or cleaning off animal waste or such.

With all these treatments, will require a stiff wet brooming afterwards to remove the killed algae/moss - and if a heavy coating will need multiple applications.

And obviously, wash off away fromt he pool to minimize how much gets into the pool or on pool-edge tile.

Answered 3 years ago by LCD


couple of things came to mind, in rereading my as-posted answer:

1) if using normal household (laundry strength) bleach like Purex or such, normal concentration is half and half with water. Follow instructions on bottle about skin contact (really irritates some people, others like me hasno significant effect)- but definitely be aware it will bleach out your clothing and can ruin shoes (especially if leather or stitched-on sole), and wear your goggles or faceshield for eye protectiona against accidental splashes.

2) if you do use a sulfate product and get yellowing or a buildup of a yellowish or whitish powdery or foamy substance, it will come off easily with white vinegar (for small areas) or diluted muriatic acid (for larger areas) - but be aware if your pavers/stones are a carbonate like limestone or dolomite or travertine or marble or coquina (rock made of shells) - or if concrete - it will etch the more resitant ones like concrete, and can rapidly dissolve the pure lime ones like limestone and coquina - so use sparsely and scrub in and wash off quickly till you see how much set time it needsto remove the chemical residue wihtout damaging the stone. Generally (especially with muriatic acid) will remove or peel sealants, so that will have to be redone in all likelihood if stripping off sulfate residue.

3) if your stone turns bluish from copper sulfate, generally it will fade rapidly with time, as the azurite and similar minerals which can form in contact with the pavers is readily weathered and oxidized - so will generally turn to a pale bluish whitish soft residue which generally wears/brushes off easily after a few weeks weathering.

Answered 3 years ago by LCD

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