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I’m wanting to lay grass in my garden. There has been an area of unlooked after ground which was riddled with roots, some of which were pretty deep. I’ve gone through and dug up as much as I can but I’m concerned they will come up again - this time through the grass.

What are my options for stopping this? My approach was going to be:

  • apply weed killer everywhere
  • lay plastic sheeting over the ground
  • apply top soil
  • apply grass

The concern is whether grass can thrive with a plastic layer below it? And how much depth should there be between the grass and plastic?

Thanks!

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    You solarise the ground with black plastic tarp. It works best in sunny locations You cover the area with plastic for at least one year. After that time you can remove the tarp. During this time the weeds have been exposed to the heat of the earth stimulating growth, but the growth dies out from lack of sunlight. It's an environmentally friendly way to kill off the weeds. Then you can apply your topsoil and grass seed. – GardenGems Jan 5 at 21:06
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Whether the roots will be a problem rather depends what the roots are from; if they are off a tree such as a cherry, you may well find little cherry trees trying to grow off it. In general, though, regular mowing will prevent this problem. However, if the roots are from pernicious weeds such as bindweed, thistle, dock, dandelion and so on, or from plants like bamboo, they will constantly regrow, and faster than the grass during growing season, so those roots need to come out. You presumably know what was growing there before you dug it over?

It is not a good idea to put a layer of plastic beneath any growing area; water will collect because plastic blocks drainage. You could, in theory, lay a geotextile membrane, which is porous, but growth from roots such as bamboo, japanese knotweed or tree saplings/suckers will easily punch through that layer so there's not much point in laying some.

On the subject of weedkiller, which type is important. In many countries, particularly in Europe, pre emergent weedkillers are not available; most other weedkillers such as glyphosate work 'through the green', meaning they kill through the green parts, which should be growing strongly when applied. This latter type of weedkiller will do nothing at all if there is no green growth present and will also not deter any growth of tree roots because they don't kill woody parts of large plants. Other weedkillers such as Paradise will kill off any plants present, but will prevent any and all growth for up to six months - including your grass.

Without knowing what the roots are from, it's hard to say definitively whether you will have a problem after laying grass. If you don't recall much growing there at all, but the area is surrounded by trees or large shrubs, then the roots from those are likely to have been what you found in the soil. Grass should still grow over those, though the presence of many trees might mean the grass does not get enough sunlight, so that's another consideration.

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  • Once you have the lawn growing and broadleaf weeds appear you can, if available in your area, apply '2,4-D'. ---- From wikipedia '2,4-Dichlorophenoxyacetic acid (usually called 2,4-D) is an organic compound with the chemical formula C8H6Cl2O3. It is a systemic herbicide which selectively kills most broadleaf weeds by causing uncontrolled growth in them, but leaves most grasses such as cereals, lawn turf, and grassland relatively unaffected.' – GardenGems Jan 5 at 19:50
  • That'd be Resolva lawn weedkiller in the UK... – Bamboo Jan 5 at 20:14
  • It goes by a few names in N. America like Killex and WeedOut. – GardenGems Jan 5 at 21:01

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