Question
Thanks for your detailed answer! As a follow-up:

- do you think I should run the plastic all around the insides of the box too, to help protect the wood? (I wasn't thinking about a weed barrier, good idea!)

- do you recommend wood for the sides, or some other material

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Followup To
Question -
Hi - I plan on building raised beds for a vegetable garden. I will probably use redwood 2x12 planks, 3' x 6' box. My questions:

- What would be a good depth? I thought I saw someone mention why 12" is better than 24", but I don't remember why they said that. Do you have an opinion?

- I thought I would line the wood box with heavy plastic to help keep it from rotting. I never see plans mention this, so I wonder if there is a down side to this.

Thanks!


Answer -
Robert,
I recommend a depth of 20 to 24 inches deep of a combo of fresh potting mix(go to Lowes or Home Depot,they have huge bags and will save on cost),aged compost(if you don't have any ready you can buy it now at most garden centers),regular topsoil or garden soil that you may need to have hauled for you,but just about a pickup load.Also get the largest bags of vermiculite you can find(Lowes or Home Depot).It is white and looks like little pieces of styrofoam. It should be already added to your fresh potting mix but a few more bags wouldn't hurt.Also a bag of peat moss wouldn't hurt. The peat moss and vermiculite will arerate your soil and get your plants roots established more rapidly.  

And YES, you do need a weed barrier. I have used black plastic for 20 years and it does the job,is easy to work with(not on a windy day.LOL!)and is super cheap. I buy mine at WalMart in the paint department.It is normally used for drop clothes I guess. I DO NOT recommend the cheap landscape fabric that you find in garden centers. It may block weeds and let water through but it also holds on to roots of invasive weeds and becomes impossible to get out.If you watch these HGTV shows they skip a lot of important steps or just barely cover them. That can really mess up a beginning gardener or homeowner.Believe me,I have seen some horrible flower beds and landscaping where people have thrown in expensive plants without any thought to weeds entering them. And believe it or not even some reputable(HA!)landscape nurseries have done the same thing. That is how I got started,fixing other's mistakes.
A little prevention goes a long way!
Make sure your weed barrier is in place all the way under your planks and then cut the excess away. You will also need some metal garden staples to hold down your weed barrier and make it as tight as possible. Take a screwdriver and punch some holes(not a lot)in your plastic just for drainage purposes. Then add your soil mixture,water it down well to where it is evenly damp,but not wet. Add plants and then water them just a bit.Don't forget a layer of mulch. Again I would recommend Lowes or Home Depot because they have such larger bags of mulch too.
Your 3'X 6' sounds like a good size because you can reach the plants without getting into the bed.
Also,make sure you have a water hose close to water your new plants if you don't get much rainfall. Just stick your finger into the soil around the plants base to test the moisture content.
Most of all remember to have fun gardening! Good luck to you!

Answer
Robert,
Treated wood would last longer than raw lumber but it is a bit more expensive. Yes you should run the plastic up the inside surface of your boards to prevent water damage to the wood and to prevent weeds from coming up the sides of the bed.The mulch will hold it up but just in case,you should use a few medal garden staples to hold it down in the corners. Good luck and have fun!