I tested temperatures on my porch (just moved in recently) during this cold event. The porch is an open air screen porch that is in-set and the outer wall is screen. It easily stays 5-7 degrees warmer than under open sky since the space is mostly contained in a very well heated building, despite it being open air. Water outside was freezing by 7:00pm the night before, but a small bowl of water I put out on the porch didn't even get a glaze of ice by morning despite lows well into the teens. This difference between the porch and outside may be less midwinter. Point being, open air porches on/in heated buildings can stay quite a bit warmer than outside under open sky.
As far as a large tub of mulch on the floor of the porch, in zones 7-8, this might be a good way to moderate temperature fluctuations as well as minimize root exposure to temperatures below 25F. Rockm, how many days per year would you say you have on average with calendar days highs below 32F? 3-4?
My point is, as far as zone 7-8 is concerned and considering porches/balconies tend to be slightly warmer than ambient, a large tub of mulch may actually be enough protection during a normal winter. Maybe not though, will take some trial and error.
My mulch tubs outside at the house are going to go flat on the ground near a building to take advantage of heat from the ground. I plan to take temperature measurements and I will post my results here.
Have had a few decent frosts, pot is on the ground frozen and it is just starting to slowly show Fall color. Elm tough!
I must admit it takes loosing a few and closely following some plants here to gauge it properly over a few years.
