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Weed-Grass growing under decking

fishynelly99fishynelly99 Posts: 3
in Problem solving
Good morning all, First time poster here.

I'm looking for some advice!
First a short back story on the problem...
Last year just after summer I installed a new decking area. Before it went down, the area was thoroughly dug, turned and churned, and at least the top few inches were sieved clear.

I then covered it in weedkiller, extensively, twice.
It then had weed netting put down, the stronger plastic weave stuff, not the cloth/fabric lightweight one.
And on top of that went shingle stone, and a raised decking.

Queue a few months of mild winter, and somehow, a weed-like long-bladed grass is growing through the gaps of the decking. I don't know what it is, it's not in 'patches' just a few shoots, but all over the decking area.

I'm going to give a try with the idea of using white vinegar, but if anyone has any suggestions that might make sure it NEVER comes back, I would appreciate it!

Posts

  • wild edgeswild edges Posts: 10,497
    Plastic weave doesn't make the best weed barrier as tough weeds can still force their way through the weave. It's also possible that weeds can grow happily in the gravel on top and push roots down though it. Always better to use smooth black polythene under structures and avoid gravel on top. Soil always builds up under decking gradually and it gets trapped in gravel rather than washing away. It would also have been better environmentally not to drench the ground in weed killer chemicals if you were going to weed barrier it anyway.

    Can you lift some decking boards and see how the weeds are growing? Better to rectify the situation under the decking if you can rather than have to keep treating for weeds.
    If you can keep your head, while those around you are losing theirs, you may not have grasped the seriousness of the situation.
  • BobTheGardenerBobTheGardener Posts: 11,385
    edited March 2019
    It will be couch grass which will be growing happily in your shingle stone above the weed matting (it doesn't need much in the way of nutrients), so the weedkiller (which type did you use?) is below the roots of the grass and therefore completely ineffective against this particular weed.  You need to remove some of the decking planks or crawl underneath and spray all growth with a glyphosate based weedkiller (note that is a suspected carcinogenic and/or bee-killing substance and likely to be banned soon.)  The glyphosate will be absorbed by the grass leaves and translocated to the roots over a few weeks which will kill them.  You would probably have been better just fixing down the weed suppressing matting directly onto the soil without adding the shingle on top.  Couch grass is infamous for re-infesting areas though, so make sure there is none growing outside your decking area or it will creep back under in time.
    A trowel in the hand is worth a thousand lost under a bush.
  • B3B3 Posts: 27,505
    It sounds a bit unhealthy to be spraying poisons in an enclosed space. Perhaps you should look for another way.
    In London. Keen but lazy.
  • BobTheGardenerBobTheGardener Posts: 11,385
    edited March 2019
    I'm spending today digging out the roots of couch grass, ground elder and an invasive euphorbia from one of my bordersd, B3.  It's the only way to be rid of them completely and safely (both for myself and the environment.)  Not so easy to do that under decking though. ;)
    A trowel in the hand is worth a thousand lost under a bush.
  • fishynelly99fishynelly99 Posts: 3
    Sadly, it would involve an awful lot of work to get under the decking (namely removing 80% of it), not to mention that it involves a shed as well.

    It does look like it might be couch grass, I had been struggling to identify it, so thanks for that.

    From what I've read on weed killers, most of them don't work well against grass, but I have read several suggestions on using Vinegar, which is much more friendly to the environment. I hope to raid a nearby supermarket later to try this out :)

    The weed killer was used as the garden was completely 'reset' last year. It was a complete removal of unkept raised beds with numerous bad plants in, and a plethora of weeds, old lumps of concrete patio/pathway, 10 different types of grass, and about a 12 inch slope taken out.

    All replaced, with a new decking area built on weed netting, plastic grids, shingle stone and a wooden support beams, and a new shed. The weed netting/grids/shingle stone is to improve rain soakaway, and these are all permeable materials for rainwater, which concrete and polythene weed barriers are not.

    The garden isn't exactly huge, and up until last year had a damp problem due to the proximity of large trees, which ironically were cut down just after I completed the decking by a neighbour.

  • josusa47josusa47 Posts: 3,530
    I bet none of the people who sold you the decking, membrane, gravel or weedkiller warned you this could happen?  They promote all kinds of hardware and chemicals as being low maintenance, it's a load of b%%%%%%ks.
  • fishynelly99fishynelly99 Posts: 3
    I actually designed and built it myself. Given the choices of this, concrete, or direct ground placement, this was the best option.
    A lot of research went into it, and it was about 4 months of casual/weekend/spare time work to prepare the ground, and about 4 weekends to build the base, shed, and decking.
  • wild edgeswild edges Posts: 10,497
    If a weed barrier is permiable for rain water then it's permiable for weeds. Concrete and polythene are fine for drainage as long as they are laid with some forethought as to where the water will go. Hopefully you'll be having some plants in pots on top though?
    If you can keep your head, while those around you are losing theirs, you may not have grasped the seriousness of the situation.
  • raisingirlraisingirl Posts: 7,093
    Just on the glyphosate vs vinegar question: acetic acid is a herbicide. Household vinegar is usually a fairly weak solution of acetic acid. Glyphosate is thought to be potentially carcinogenic but in tests it is LESS toxic to animals than acetic acid (less likely to kill you immediately if you drink a glass of it). Vinegar is less effective as a herbicide, therefore to get the same effect you will have to use more of it and anyway, it's effectiveness against a perennial weed such as couch grass is doubtful. I don't know whether vinegar or glyphosate is more harmful to bees and pollinators but to your soil and it's ecosystem, by the time you've actually had an impact on the couch grass, the vinegar is likely to do more harm.

    Either way, couch grass is one of those weeds that WILL come back, unless you can eradicate it entirely from your own and your neighbours' borders. So you either need to accept you are going to have to kill all the invertebrates and soil bacteria in the vicinity every year in perpetuity, or bite the bullet and take up enough of the decking to rake out the shingle and lay some heavy duty black polythene instead. 
    Gardening on the edge of Exmoor, in Devon

    “It's still magic even if you know how it's done.” 
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