As to the garbage can idea---if you have it exposed to the sun at all, it's going to heat up. (Think how a car sitting the sun will heat up in the winter.) You don't want that. I'm familiar with zone 6 weather, having lived & gardened there for many years, so I have some idea of what you're dealing with...that is, you need to keep the soil cold when you get those weather warm-ups in mid-winter. i.e., the idea behind the insulation is to keep the heat out and the cold in the pots, to moderate the inevitable wide swings in temperature. (Think 'ice chest'.) If the pots are not in the sun, then the insulation board on top of the pots is prolly going to be sufficient. If the pots get any am't of sun OR are close to a bldg., then you need to think about keep the whole shebang cold. Naturally, you would place the insulation barrier around the pots after it's turned cold, really cold. About the wood tops idea...an inch of wood has an R-value of less than 1.5...compared to R-5 for an inch of XPS (the pink stuff) and R-8 for ISO (the foil faced yellow stuff). So, although wood is an insulator, there are better choices. Of course, you could glue XPS to the wood if you want to. Oh yeh, the pink stuff, XPS, will hold up to the elements much better than the foil faced ISO--it's yellow underneath the foil and is much more brittle than XPS. Also, the R-value of ISO degrades to where it's only marginally better than XPS.

It's OK to prune it to a more desirable height now. An added benefit could be multiple branches sprouting from the cut points where you pruned it back... especially now during the height of the growing season (assuming you're in the U.S.A./northern hemisphere) *...and you'd have cuttings to root, too!

Stair runner.

I am Trying To Close My Summer and Start My Winter As The Most Challenging. After Putting My Summer Fishing on Hold Due to Traffic and not Feeling 100% a Few Years now. I feel Age could be A Factor. But it has not affected My Working Firewood, Metalworking, Woodworking and Walking As Relaxing. Mainly due to no Real Traffic is involved vs Fun Troubleshooting Ideas. Attached is my major Firewood Harvest with my Trailor Over-Loaded not showing.