Gilbert Fritz : I will take stab at the second part . Because the Insulation you use will be exposed to high levels of moisture from both sides of your raised beds
you will need a solid insulation with very low moisture absorption characteristics . The only material I can think of would be a dense closed cell foam panel !
I would expect surrounding your bed with panels 11/2 inches thick and placed down into the soil 3' would add a least 1 month onto both ends of your growing
season ! More so if you install a cold frame or clear plastic row covers as needed ! Sorry this is not more timely ! hope you find it helpful Big AL
Success has a Thousand Fathers , Failure is an Orphan
As ever, it depends. I would think that it might not be good to keep the bed insulated year round, but that fal/winter/spring it might be helpful in extending your season and helping things overwinter that might suffer from cold.
Were I going to do it, I would just go with strawbales. Effective insulation that is also useful for other purposes around the garden and won't introduce any kind of toxic gick, as opposed to conventional insulation panels, which might not be quite so good around your food plants and certainly do not have other beneficial applications in your garden.
Why is the word "abbreviation" so long? And this ad is so short?
Tomatoes! Ha! Anyone can grow that. Amaze your neighbors, grow your own shirt!