In favour of membrane:
1) weeds can't grow up from the soil below it.
2) the shingle stays cleaner, because there is a physical barrier keeping the soil down, and the shingle up.
3) if it's not laid properly, it looks appalling
4) if it's not maintained, and weeds are allowed to grow, it's almost impossible to weed it.
Before we go through those four points, what do we mean by membrane? People usually mean landscaping fabric, a woven plastic-based material which allows water through, and which doesn't rot for many years. You can also use any sort of plastic sheeting, those bulk builder bags cut up into flat sections, even old compost or bark bags, cut open and laid flat. If using a jigsaw of little pieces rather than one continuous roll, it is vital to overlap them by at least 6", otherwise weeds will creep through the gaps.
The correct way to install it is to decide on the route of the path first, clear away the turf (if it's currently grass), and remove any perennial weeds, either by digging them out, or by weedkilling them. Perennial weeds are deep-rooted ones which come back year after year, and if you don't get them out before you lay the membrane, they will continue to grow, so either a) you'll get strange lumpy humps in your shingle path (if you weren't sufficiently generous with the shingle) or b) they grow sideways and pop up at the sides of the path, making it impossible to get at their roots, so you can never satisfactorily weed them out. Without using weedkiller.
Then decide on an edging to your path - if you use any sort of loose material such as shingle, slate chips, etc, then without an edge to keep it in place, the material will creep away over the edges. This is merely annoying if it's adjacent to a flower bed, but lethal if there is lawn next to it, as the mower will pick up the bits and fling them out at high speed, while making horrible expensive-sounding noises.
Install the edging, digging out some of the soil if necessary to give you a decent depth of shingle inside the edging. Lay the membrane, ensuring that it goes right under the edging and out the other side. Then fill - generously - with your loose material. Don't skimp! Use lots!
When it comes to which membrane to use, I always tell people that the most expensive membrane will not keep out "all" of the weeds: and the cheapest possible membrane (such as black bin liners - yes, I've seen it done!) will keep out "most" of the weeds. And this is the whole point of membrane: it drastically reduces the amount of weeding you have to do, as in point 1).
However, no matter what membrane you use, seeds will still float gently down from above, so there will always be some sort of weeding issue.
Here's an illustration of a badly laid membrane, to illustrate point 3) :
If I'd been responsible for this, I would have taken the membrane right under the fence line, and pinned it down a good 6" into the other side.
You can buy custom-made metal pegs for membrane, they are like gigantic staples: or you can buy garden wire, cut it into 15" lengths (errrr, 37-40cm for you youngsters) then bend it so you have two 6" legs (125cm approx) and a straight bit across the top. Then just push them through the membrane into the ground. Simple! Then spread the shingle (or whatever) on top.
As it is, the membrane was just a bit too narrow, and the edges kept being scuffed up, because there wasn't any shingle on them to hold them down. After a while, the edges of the membrane fold back as you can see in the photo, debris blows underneath them, and then it's very hard to get them to disappear again.
If your paths are doing this, the only thing to do is to pull back the offending edge a bit further, scrape out all the debris and muck underneath it, including the inevitable pile of shingle which has worked its way in there: lay the membrane down flat against the earth, pinning it down if possible, then scrape the shingle over it from the middle of the path outwards. Then go and get some more shingle! There is not much worse than a skimpy layer of shingle over a highly visible membrane....
The other drawback of neglecting this type of membrane is that weeds can grow through it, which leads to point 4): when this happens, it can be very hard work to get them out: plus, it often damages the membrane because the roots have pushed down and can make quite big holes for themselves. Plus, as though that weren't enough, the action of pulling out big clumps of, for example, grass, will "lift" the membrane, rucking it up, and will pull a lot of soil and debris up to the surface.
I had one example a couple of years ago where a garden owner had installed a "native mix" hedge, planting through membrane, and covering it with a very deep chipped bark mulch. Alas, with no maintenance by the owner, masses of grasses grew all in among the saplings, choking them and looking rather untidy. The owner tried and failed to dislodge any of the clumpy grass: I tried, failed, and told them the best way would be to remove the membrane. They were not happy! "But we spent hours on a freezing cold winter day cutting holes in the membrane and planting through them!"
I was not about to suggest weedkiller, partly because I don't like using it unless necessary, but mostly because of the risk of overspray killing the saplings.
So the only answer was to remove the membrane, which I finally did. It took me nearly as long to get the stuff out as it did them to lay it in the first place, as I had to scrape off the bark chips between the huge clumps of grass, thistles, and other weeds; carefully cut from sapling to sapling, ease out the mucky, weed-stuffed membrane, then do the weeding, and then replace the bark chips.
It looked fabulous afterwards, though, and although some weeds have reappeared, nearly a year later, it was a quick and easy job - relatively - to weed it this spring.
Now, if you've been paying attention, you'll note that we've covered points 1, 3 and 4, and have skipped neatly over 2.
Point 2 was in favour of membrane: the shingle stays cleaner, because there is a physical barrier keeping the soil down, and the shingle up.
Well....... most of the time, it does!
This is the bottom layer of some really good, deep shingle, which looked perfect on top.
But when I had to dig a planting hole in it, I scraped back the top layer, to find that below was a mass of mud and shingle.
"Yuck!" I thought, "what happened to the membrane?"
I kept scraping, and there it was. You can just see the white membrane being uncovered.
This picture illustrates my final point about shingle/gravel/slate/loose material surfaces: debris is constantly falling from the skies. Rain is dirty (otherwise we wouldn't bother with window cleaners), birds drop seeds which rot, leaving organic matter behind: sometimes weeds die of their own accord, and their bodies contribute to this mass of organic matter. And if you use weedkiller on your shingle instead of pulling up the weeds, the dead ones rot away and fall down between the shingle, adding to it even more.
The good news is that worms live in this sort of environment! I know, don't pull that face at me, it's true, I frequently find real live worms squirming about in deep shingle. They can push themselves through the matrix of spaces, and it's lovely and damp down there because rain and dew work their way down and sit in all these little spaces: and on hot days, the heat draws moisture up out of the soil below, which then condenses on the lumps of shingle, thus keeping them moist. Lovely for worms, and the birds can't see them, so they can go about their business unmolested.
So there you have it, the full story behind the simple question of whether shingle paths need membranes or not.
What do you think?
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