Raised beds - to treat or not to treat wood?

Shoxt3r

Hi there,

So I've started putting together a raised bed for some vegetables for the new season, including digging down and filling the area with compost. I'm now at the next stage where I've cut my pressure treated wood to length.

However, while I was in the garden centre I picked up some wood treatment (standard fence panel stuff) and have read that plastic sheeting should be used to line any raised bed to stop the preservative leaching into the soil - see RHS' guide on building raised beds.

Is this true and should I grab myself some plastic sheeting, or will the preservative be fine for ground contact?

Cheers,

Andrew

Shoxt3r

Sorry for double-post but one concern about putting down plastic sheeting is that the plants will no longer get the goodness from the compost laid down - I guess the earth/topsoil above would be enough though?

Obelixx

The plastic sheeting is just for lining the inside walls of the wooden frames, not the whole bed.  It stops water in the soil from rotting the wood from the inside so your raised beds will last longer.   We did this for raised beds made form old railway sleepers and 18 years later they're still going strong.   We did not treat the tops or outer façades.

If your wood is pressure treated you shouldn't need any extra protection unless you want to paint them for decorative purposes.  You do not want wood preservative products in contact with plants, especially if you intend them for eating.

Last edited: 08 February 2017 13:23:47

Shoxt3r

Ah thanks for clarifying - are there any particular paints that would be safe to use to avoid the problem of it being contacted by plants? I guess the wood will naturally age and discolour over time anyway so wouldn't be too worried about painting them...

Will look into getting some plastic sheeting to protect the wood though - any thoughts on the type/thickness to go for?

Hostafan1

I lined some raised wooden planters with polythene once, but I changed it for porous " weed membrane" as the polythene held all the moisture in and the wood dried out and cracked. Depends how thick the wood is , but it's another consideration.

Obelixx

You'd have to look on the tin but the safest would be water based.   We just used rolls of black plastic sheeting from a builders' yard but you can also find it in good DIYs and garden centres.  Maybe a difference in price so look around.   Thing is, once painted or stained you have to maintain it so maybe just let the wood age and fade naturally if maintenance time is going to be at a premium.

Ours was thicker and stronger than the average bin liner and folded over at the top for neatness then staple gunned to the timbers at what would become soil level.   That way it isn't too unsightly.

Fairygirl

Mine are all lined with heavy duty plastic and finished like obelixx's. They also have an edging round the top, but that's because they're ornamental rather than functional. They're made from heavy duty fencing timber and painted with a proprietary water based paint - again for aesthetics, but if you're using them purely for growing veg it wouldn't be necessary.

They should be open to the ground underneath - only the sides need lining. 

I don't think mine will dry out - we get plenty of rain to keep them moist 

Shoxt3r

Hi all,

Thanks for the replies - seems the best way would be just to leave the wood to weather itself rather than using preservative - not too worried about the look of it (i.e. using paint instead), it's more to protect the wood from the elements really.

The other thing is that the stakes I've cut which will be about 30cm deep will likely rot through fairly shortly - any suggestions on how to protect them without affecting the soil?

Thanks also for the suggestions on the plastic sheeting - will nip down to the local HB over the weekend!

Cheers,

Andrew

Obelixx

We found that stakes lasted quite a few years without treatment and were easy to replace.