Welcome to GOY. No they do not need a bottom. If you are putting them on grass it might be a good idea to put a three or four pages layer of newspaper to kill off the weeds and stop them coming up through the first year.When the paper breaks down the soil below will be available for your plants to root in to. Some people put a liner between the soil and the wood as a damp course as it were. Good luck with your project.

Hello Bernia, I started my raised veg beds last year, I have popped photo's up of my efforts. I decided to flip the grass over with a spade & dump loads of compost & top-soil straight on top. If for any reason you are unable to do this then as already suggested an excellent method is to layer newspapers down first & then dump as much compost/topsoil on top.