What is the best wood to use a raised garden?

Some years ago and elderly friend built raised beds because he was in a sheelchair. He (with some neighborly help) built table-like frames, put small mesh (about 1/2") screen on the bottom and built 18" walls around it, which were the "beds". Fille with soil and planted in there. Gave him under the "bed" storage for things that could weather rain, etc. The screening on the bottom let water drain out, etc. These were quite successful for him. The 12 to 18 inch pre-packaged "raised" beds just don't seem high enough to be helpful if you have back problems or some other disability. I believe he used 4/4's for the legs of the frames. He filled his whole backyard with these beds and raised vegetables as well as flowers. Hope this idea is helpful.

I wiuod hesitate to use pallet wood - railroad ties or phone poles not knowing how it was CHEMICALLY treated.

Commercially produced pressure treated lumber that’s available now is fine.

Here are some links regarding raised garden beds that I hope might be helpful.

Here are some links regarding the construction of raised bed garden. Perhaps one might help you

https://growingagreenerworld.lpages.co/raisedbedguide/

installing raised vegetable beds, re suggestions for building sides.

https://piedmontmastergardeners.org/article/raised-beds/

http://web.extension.illinois.edu/dmp/eb284/entry_9875/

http://extension.oregonstate.edu/question-of-the-week/raised-bed-lumber-pressure-treated-safe

http://www.hort.vt.edu/HUMAN/pub426020d.html

Comprehensive raised bed gardening article is informative for all gardeners (to include seniors)

https://www.extension.umn.edu/garden/yard-garden/landscaping/raised-bed-gardens/

https://www.hometalk.com/diy/outdoor/garden/s-17-amazing-garden-features-we-ve-been-saving-for-summer-13944901

Some pallet wood contains pesticides and so does pressure treated wood... cedar is a good option.... however....

My husband and I are both in our 60s... and we did not want to be weeding on our knees.... we have successfully used the galvanized horse troughs/stock tanks for our raised gardens.... they come in 4 ft to 8 ft lengths are normally 2 ft high and 2 ft wide....the 4ft x 2ft x 2ft size costs about $85-$90 (Tractor Supply, Lowe's) the 4 ft length can be carried by 2 people when empty and moved around if needed (if we want to move one, we just tip the dirt out onto a tarp, then move the container). We live in Phoenix, AZ so we put a shade cloth structure over ours... a T shape like a clothesline post.... using 2x4 lumber and shade cloth... the leg of the post is bolted thru the end of the tank..... for things like cucumbers and melon, beans, etc... they can all be trained up a trellis.. to save space

For longetivity, use cedar or cypress wood that has been sealed with weather-resistant stain-sealer. Cypress gets harder with age. ☺️

Apparently the new treated wood is now safe for gardens. The old wood had arsenic in it so was not good but the new one is free of arsenic. You can line the wood with a heavy plastic before putting in your soil.

If you can get your hands on 4 mil clear plastic sheet this would work very well. If you lay it in the bottom too you can punch some holes in it to let out the excess water. Small gardens0 tend to dry out quickly so a bit of plastic on the bottom and sides would help to retain the moisture. Not sure what is in the black garbage bags but expect there are some major chemicals.