Bin liners

I don't think they'd last very well to be honest - I would think they would tear from any sticks/stones in the soil or tool impacts). Also, they won't be UV stabilized, so any bits that stick up won't survive.

Do you know any pond-owners? If they have pond liner and are thinking about replacing it in the pond, that is very tough stuff (we are using our old one as a winter cover for the "veg" bed in the back garden). You could also try the plastic sheeting that you can get on a roll from Garden centres. Both of these will need fitting. Old ground sheets if you know any campers?

Do you actually need a liner?

You're probably better off with pond liner. Bin liner wouldn't last long and would rip easily. If you're lining the bottom as well as the sides don't forget to stab lots of holes in to it or you'll end up with a bog garden

Thanks Mike
I was told I would need liner so the wood doesn’t rot. Is that right?

Compost bags are quite hard wearing,Ive got some bags out there that are years old holding old compost.

any wood in contact with dampness (and air) will rot eventually.

Putting a waterproof membrane in the equation may not help matters (as any water that gets in there won't get out) - so if you do use one, it's important that it's not ripped, but then you run into drainage issues unless you have drain holes at the bottom (IIRC your beds are not on the ground).

I have no definite experience as my raised beds are only a year or two old. What I have done is used tanalised wood (modern tanalisation does not have any compounds currently known to be toxic in the process), and painted them with wood preservative (paying particular attention to the end grain).

If you do go down the route of putting liners in, I think you need to make sure they are robust and have some drainage arrangement in the bottom (probably as simple as a hole in the liner that lines up with a hole in the wood, with some flatish crocks inside to stop all the compost being washed out).

My bean frame is made of entirely untreated 2x1 and it has been put up each year and taken down again (with the bottom 6" in the soil) and left outside at all times - it has done 3 years and I expect it to do this year and possibly next.

I have also just made a raised bed out of untreated wood, and we'll see how long that lasts. Not long I fear.

I think in your circumstances, I would (on balance) put a liner in, but it would need to be a tough one, and only effective if pretty much non-porous. I'd get it as one piece and accept there will be some folding up at the ends.

A possible alternative would be yacht varnish. But that would take several coats and would be quite a job.

You could try and give the inside of the bed a good soaking with linseed oil to help repel the water before lining with plastic. The cheaper pond liner might be better as it is not so stiff.

The best thing to do (imo)is to treat the wood but if that is not practical or economical use the bin liners, and if your raised bed is on the ground I wouldn't worry about drainage, my raised bed is built on top of an old greenhouse concrete floor and I have no problems with water build up it drains away no bother and I live it a very wet area

I cut strips off a damp proof membrane sheet and fixed to the insides, put nothing in the bottom, no idea if correct thing to do or not

I think any type of lining is just an encouragement for creepy crawlies to make a home there.

I got a lovely big wooden planter last year, and I lined it with empty large dog food bags. Reduce, reuse and recycle, as they say lol

Was watching a craftsy video the other night. They coated planters with Veg Oil. Good excuse for fish and Chips on Friday.