How to fill a patchy Forsythia hedge?

No. You need woven weed suppressing fabric. I've tried barrier membranes and they tear and then that's that

 

Thank you both for the further info. When you say:

'Laurel can be cut at the same time as privet (probably won't be necessary to do it as often) but when the plants are first growing it would be best to cut them by hand so you can be choosy on where you want them to bush out.'

How would a novice like me want to prune it? I know it's obvious to you!

This is the Lansdscape Fabric I have (sorry can't post as a working link yet due to forum restrictions, so please google description):

Wickes Heavy Duty Landscape Fabric 12M X 1M
Product Code: 160175

It says it's woven, is it likely to be OK?

 

I can

http://www.wickes.co.uk/Wickes-Heavy-Duty-Landscape-Fabric-12m-x-1m/p/160175

But that ain't the right type (comments against that product aren't good). I think it is intended to be used like Terram under paths etc.

You need Mypex or Zebel or similar (but not Yuzet as it is not UV protected)

Here's an example:

http://www.ebay.co.uk/itm/ZEBEL-1m-...ound-Cover-Fabric-Membrane-Roll-/121204993094

 

Thank you both for your further info and for reposting my link. I'll try to obtain the better grade fabric you describe. I may well take you up on your offer re pruning advice! The only question that I didn't ask, what sort of planting density (depending obviously on plant size) would you suggest for fairly quick dense cover, without squeezing the life out of them?

 

I'd describe it differently: Rather than a a "better grade", its a different type of material. It is designed to prevent being ripped / tearing, whereas Terram is designed to be laid, not puncture, and to then stay put.

I have used Terram under a hedge and it was a disaster. After a year or two it ripped, weeds came though, and it now impregnated with weeds and looks a complete mess.

Sorry, don't know about planting density for a Forsythia hedge. I always think that plant proximity is far too close - I like to consider the plants when mature, rather than what would be appropriate when planted. For example: I think that growing a foot apart is very close for a mature hedge plant.

I have planted tall hedge plants, to try to get a hedge established more quickly, and I have planted small ones, and after 3 years the small ones have caught up ... so I now favour planting small-ish plants, quicker to establish, and a huge saving pricewise (comparing a 2M bare root hedging plant with a 40/60cm one)