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How long does it take for black plastic sheeting to kill the grass for making new flower beds?

I've moved to a new house, there is a vegetable garden, grass and trees but not much in the way of flowers. I am going to make new flower beds either side of the fruit garden gate opposite the house.  I had some pond liner left over from years ago when I muddled the measurements and bought too much, so I put it on the area to kill the grass. Glyphosate is banned here in France. Does anyone know how long it will take to kill the grass enough for digging? 
Dordogne and Norfolk


  • tui34tui34 Béziers, Herault, FrancePosts: 2,581
    Hello @Busy-Lizzie  I have tried this method, but gave up because I was just blanching the weeds and the heat and humidity was just encouraging more.  I bought some "garden friendly" weed killer at LeClerc - takes a while but it works.  I also have a heated rod (electric) from Lidl that I use to burn the weeds that come up between the slabs.  Otherwise, I dig my veggie garden and pull the weeds out as I go.  I never spray the veggie garden.

    A good hoeing is worth two waterings.

  • B3B3 South East LondonPosts: 24,068
    I would just slice the turfs off in brick sized lumps. I mark off the 'bricks' with a half moon and slice them off with a wallpaper scraper. It's not that hard to do and no chemicals or patience involved😊
    In London. Keen but lazy.
  • ObelixxObelixx Vendée, Western FrancePosts: 28,567
    If there's couch grass in there it won't kill it and they'll just head sideways till a shoot finds some light.  We used plastic sheeting to kill off an area in our Belgian garden when we first started and it took a year and even then perishers like couch grass, thistle and bindweed survived.

     I would suggest one of 2 methods. 

    The quickest would be to use a strimmer to scalp all the grass to the ground then lay down cardboard covered by well-rotted garden compost or cheap compost (terreau) when they start with spring offers and plant into that.

    The second is to use clear plastic to cover the ground but you need hot sun which then burns or bakes the plants and seeds near the surface.   That will take a bit longer and means you have to wait for hot sunshine.  Then proceed as above with the cardboard and compost.
    "We don't stop playing because we grow old; we grow old because we stop playing." - George Bernard Shaw
  • NollieNollie Girona, Catalunya, Northen Spain.Posts: 6,548
    I laid a heavy black pond liner type material (actually an old pool cover) on the ground here and left it weighed down for two years over a grassy area I wanted to plant in eventually. Peeled it up and the ground was covered in etiolated but still viable white roots. Total waste of time! Still had to dig it over and weed by hand before prepping the area. Yes, there was couch grass and bindweed and the bindweed still comes back. So it does depend on the constituent grasses/weeds as @Obelixx says. I have heard of the clear plastic burn method, but never tried it, my terraces get very hot so it should work, but they are also surrounded by mature woodland and I fear I might start a raging inferno 🔥 
  • Busy-LizzieBusy-Lizzie Posts: 20,435
    Thank you. I haven't seen any couch grass or bindweed, but then it's winter. Bother. I'll look for garden friendly weed killer in Leclerc. The grass is quite thick and needs mowing but it's too wet for the garden tractor and we need a new lawnmower. Strimmer won't start, needs attention. Daughter has given me 5 pink shrub roses to plant.

    Dordogne and Norfolk
  • StephenSouthwestStephenSouthwest Southwest EnglandPosts: 427
    I really don't think that there is such a thing as a garden friendly weedkiller.
    If it was my space and I was planting shrub roses in there, I'd use a no dig method, and include some manure.
  • ObelixxObelixx Vendée, Western FrancePosts: 28,567
    I wouldn't use chemicals either @Busy-Lizzie.  If the strimmer's dead, maybe worth considering buying a lightweight electrical one?  Then a double layer of cardboard - all those packing boxes? - followed by compost, manure and topsoil and then plant straight in.
    "We don't stop playing because we grow old; we grow old because we stop playing." - George Bernard Shaw
  • LoxleyLoxley NottinghamPosts: 4,976
    I would lift the turf and put it out of the way with the black plastic over it, and it will convert to loam which you can re-use in the future. I would top up the level with a mix of soil dug up from elsewhere in the garden and compost / rotted horse manure. Bit more work, but immediate results. If you think it through, you could dump the turf you dig out in the same place you get your soil from.
  • B3B3 South East LondonPosts: 24,068
    I've never used weedkillers and I really don't understand the need to use them in a domestic garden in the absence of seriously pernicious weeds  such as jkw
    In London. Keen but lazy.
  • SkandiSkandi Northern DenmarkPosts: 1,576
    Black plastic will take around 3-4 months over winter to kill grass and around 1-2 months in summer, docks and dandelions will take a full 12 months to kill, but if you wait the 3-4 months and pull the plastic the docks and other taproot weeds are very easy to see and dig up.
    I use a huge amount of plastic (I have enough for 100m x 50m at the moment) it works very well but it works best in large pieces as creeping weeds like couch grass or nettles can only creep so far under it. even when it doesn't kill everything it makes removal much easier as it will have killed and helped the decomposition of most of the weeds.
    There are no "safe" weedkillers and all the vinegar based ones will do is kill the leaves, you would get exactly the same effect mowing it very short.
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