I'm experimenting with some 2-year grafts this year, to see if I can pull off potted Japanese maples here in my 5b/6a urban location. I'm looking for tricks those of you in the more northern JM zones use to keep your potted maples happy over the winter. I've potted them, given them a good mulch layer, and I'm experimenting with putting bubble wrap on the inside of the pot, in a layer between the pot and bark/soil mixture...I'm hoping that helps protect the pot from freezing damage, but it would be a bonus if it helped with the maple. Once they drop their leaves I'm planning on giving each pot a good insulating wrap with larger bubble wrap, then storing them in our unheated/detached garage. Any advice? What about watering during the winter? What do all of you do with yours? Anyone have photos of their winter set-up?
Last winter was the first time I over-wintered my pots outside (of course, one of the coldest in recent memory), and with the exception of one maple - a touchy one to begin with - and a couple small conifers, all came through fantastically. We have a walk-out basement, with a concrete patio; we also have the luxury (?) of backing onto a wooded creek, so we have an ample supply of leaves. I raked a good layer of leaves onto the patio under the deck, up against the house. Put shelves (from those resin shelving units, the ones with holes/cut-outs in the shelves) down on top of this, then my pots on the shelves. With the exception of some of my larger pots, all of my pots are the fabric root-control smart pots, so I felt that having them off the cold concrete - which is superb at conducting the cold - was very important. The larger plastic/resin pots went directly onto the layer of leaves. I then packed leaves in between, around, and on top of the pots for further insulation, and hung a sheet up at the north end of the deck to help block wind. When we had the really cold stretches, I also covered the pots with some old sheets. This helped to keep some extra heat in, but also allowed the pots to breathe, not over-heat, and also still allowed light through (plastic drop cloths or tarps won't do these things). I plan to do pretty much the same thing this year, with a few slight modifications - last year's set-up was done very last minute for the most part, as we had a very unexpected, hard freeze and we had been out of town in the days leading up to it. I am in zone 6b, but last year we might as well have been 5a, as cold as we got. For you, the biggest factor will be protection from the wind - that will be vital to the small maples. Also, another thing you can try - instead of/in addition to the bubble wrap - is double-potting. Put a layer of mulch in the bottom of a larger pot, put your smaller pot inside, then fill in the gap around the two with mulch, and mulch well on top of the soil line. This will also greatly help with insulating the root zone but still allow for proper drainage (from snow, rain, etc). I have a few pots I'm going to have to do this for this year. And for a few small/newly grafted conifers I have, while I plan to leave them outside, if it gets really cold, they will be going into the garage.
Thanks for the tips and ideas-- I wouldn't have thought about some of those details. I ended up putting them in a narrow alley formed by our garage and our neighbor's fence, bubble-wrapping them, then blocking the wind from them by stacking hay bales. I also had to add in chicken-wire cages and tree wrap to keep the rabbits away. We'll see how they do!
The results of my 5b winter potting experiment are in. Two died: Ruby Stars and Shidava Gold. Shidava Gold didn't surprise me, but I thought Ruby Stars might make it. One made it through with almost no dieback: Shindeshojo One made it through with some dieback: Kuro hime What seemed to make the biggest difference was the size of the pot (larger pots did better), but also wrapping the tree. I don't think I'll do this again-- leave them outdoors, that is-- but it was an interesting thing to try here in 5b. This next winter I'll be storing them IN the garage (rather than outdoors, next to it) and I'll be using only larger planters.