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I have cement base under my lawn therefore my lawn has just 7 inches of soil and then below that it is fully covered with cement. This is done to prevent water seepage in my basement so I can't drill holes in it to improve drainage because then my basement may suffer from water seepage. As there is cement base during heavy rains due to poor drainage I can see green algae at some places in patches. After few days when sun falls on it it turns to black and looks very ugly. I just pour handful of soil over it to cover it up. So, basically I am just hiding the problem instead of solving it.

Is there any solution to algae? May be pouring diluted vinegar but I read it can harm the grass as well. I need some solution that can kill the algae but not my lawn.

Any help is appreciated.

Edit:

So here is the picture I found from the internet. You can see the green colored substance in the picture. If I rub my fingers it feels Jelly like. This is exactly how it looks and once rain stops and sun falls on it it becomes completely black:

enter image description here

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    The only permanent solution to this is somehow get rain water to run off the top of the cement base to somewhere it can drain away. Otherwise, basically you are trying to grow a lawn on the mud at the bottom of a stagnant pond, and unsurprisingly that isn't working well. – alephzero Sep 12 '19 at 8:25
  • This is not meant to be facetious but as an actual suggestion. A concrete pad with turf on top of it is exactly how many football stadiums in the US are built. In those cases, though, the turf is an artificial product like FieldTurf. If you have the budget for it, you may want to consider that as an alternative to a lawn that really isn't working for you. – Jurp Sep 12 '19 at 12:20
  • Wouldn't artificial turf without drainage be susceptible to algal growth too? – Max Spencer Sep 12 '19 at 13:17
  • @Max Spencer, I don't see why it would be susceptible to algae, because there is no organic material in the turf. You would have to vacuum it periodically rather than mow it, though. – Jurp Sep 12 '19 at 16:28
  • @Jurp - rainwater itself can often carry enough nitrogen to feed an algal bloom. That said I'd limit fertilizing the lawn if you haven't already done so. Also as everyone says, you've got to get the water out of there. Do the roof gutters dump onto the lawn? Divert them. How about a channel drain through the worst of it? – That Idiot Sep 12 '19 at 16:40
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I assume you are talking about moss not algae... If so it isn't actually dead when it is dry, just dormant... I wouldn't worry about it, if it were me, a lawn should shade it out if it is healthy, infrequent deep watering will help the grass over the moss, because the grass has long roots and the moss in non-vascular.

if you are in the US and want to use a chemical herbicide. There are a bunch registered with names like: moss killer, moss super death spray, etc... normally they contain some sort of iron ion: ferrous ammonium sulfate or ferrous sulfate... they should be pretty safe for the environment once they are applied and watered in.

  • I believe OP is correct with respect to algae. I've seen it often in areas that regularly get saturated. It happens far too quickly for it to be moss. – That Idiot Sep 12 '19 at 16:38
  • I dont know if it changes the advice... maybe it would allow copper based things to be used, but generally, water less frequently, let the grass shade it out... – Grady Player Sep 12 '19 at 16:58
  • @GradyPlayer: Thanks. I think it is algae. Please see picture I posted in my question. – TCM Sep 13 '19 at 8:28
  • The photo makes it look like you've got soil compaction issues, which could exacerbate any moisture issues. Combined they could lead to anaerobic conditions which would prevent the grass from growing there - roots need oxygen. To remedy you'd want to aerate (core aerator if possible) and possibly even add some dolomite to improve soil structure and percolation. – That Idiot Sep 13 '19 at 11:12
  • @ThatIdiot the photo isn't the OP's lawn. It's something the OP found on the internet. – alephzero Sep 13 '19 at 22:29

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