Hi, I'm in zone 7b. I have a couple recently purchased Japanese Maples that are in 15 gallon containers. Are the pots large enough to stay outside all winter or should I bury them in the ground? Some winters we drop down to 1 degrees on the coldest days. Thanks
I'm in 7b also and have three JM's in the 5' to 8' tall range in large containers that have survived the Long Island winters. An Autumn Moon and two Sango Kaku. I recently bought a Twombley's Red Sentinel which will be left out in its container this coming winter. The rest of my containered JM's in smaller pots I move into my unheated garage whether they need it or not. Oh yeah, I have a small Tamukeyama which was planted last fall in-ground and apparently half-died this winter. I dug it up and planted that one in a large rectangular wood planter that is too big to move.
I'm also in 7b. This winter it got down to ~5 deg F for a few weeks and I moved all of my potted trees into the garage until it was back over ~20 degrees. Didn't have any damage. Nowadays I bury them half in the ground anyway, to make sure they don't fall over in thunderstorms. To answer your question - I would definitely bury them if you can't bring into unheated garage. Better safe than sorry.
Based on my experience with keeping semi-hardy citrus trees alive in our 7b climate, as long as the leaves are dry and fluffy you’d be adding valuable insulation, but then you have to concern yourself with keeping them from blowing away. As soon as they get wet and/or packed down, the air pockets that provide the insulation disappear. I used to bag up leaves in black plastic garbage bags when they were dry and then pile the bags around the pots to maximize insulation value. In spring I set those bags aside in the sun to rot down for leaf mould top dressing. But it’s not just the cold itself that will cause winter kill. Burlap (never plastic) staked around but not touching the drip line, at the very least on the sides that get the winter prevailing winds, helps massively with winter kill. Very tender potted trees can be burlapped on all sides and the void in the centre filled with dry leaves as well. But seriously, I’d only ever be concerned about my potted maples at 5F or lower unless you have a lot of wind. You lose a whole zone by putting a plant in a pot (so the trees are growing in 6b rather than 7b), but if the soil mass is large enough and the tree is otherwise healthy, there’s no reason to move them indoors above that temp. And in fact moving them back and forth could cause them to break dormancy before they should. Control the cold wind, yes. But my JMs on my roof deck in Vancouver (in cedar boxes approximately 8 cu ft) have thrived over several years of winter temps close to or worse than that. I do burlap them on the north and west sides for wind but otherwise they’re on their own.
I am in what used to be zone 5 and have several Japanese maples in large ceramic pots. They do great with some help from us of course. They servived the polar plunge of about 5 years ago where we had over a month below 0 F and over 72 hours at -20F. We also have not lost a pot either. So far no losses in 12 years. Oak trees in our area are trouble free disease and pest free. We use oak leaves because they do not absorb water or break down like maple leaves. We use heavy burlap that is rectangular in shape (I think it's 48" wide, could be narrower for shorter pots; length depends on pot diameter). Pile oak leaves on the burlap and sew together length wise with twine. You end up with a snake like shape with open ends. Stuff more leaves and then wrap the snake around the pot with one end sewn closed. End with the open end and stuff more leaves if needed. Over Lapp and you end up with a burlap donut with a pot in the donut holes. Sew the open end to the outer donut wall to close up the snake. If the pot is tall you may need to do another donut on top of the first burlap donut. You should not see the pot. The top rim needs to be covered by the burlap donut and covers the outer 20% of the pot soil on the surface. The remaining soil surface area should be covered with 1.5-2" of pine bark mulch. By doing this you not only insulate the pot from cracking (none of mine are frost proof pots and they never crack) but you are also protecting the root zone from sharp swings in temperature. See, most times it's not the cold that kills potted maples it's the constant swings in temperature that kills. Warming up from daytime sun then plummeting temperature at night. The burlap and leaves insulate against these temperature swings keeping the root zone a more consistent temperature and you take advantage of ground temperature keeping thing more consistent by keeping roots warmer as cold front comes through and cooler when we get a crazy 80F day in February! Snow also helps insulate and snow melt and rain help keep roots from drying out. It takes a little work on the front end, but then they are trouble free during the dormant season. Burlap or Dewitt frost cloth can be used as a wind break if exposed to heavy winds. New trees from West coast growers should have wind break for first few seasons as they get acclimated to a new winter climate. My small bonsia JM and others go in our garage in large plastic storage containers that are slightly taller than pot wall (no lids on storage bins). Dry sphagnum peat moss is poured in between pot walls to insulate between pots. Pine bark mulch double ground is placed on top about 1.5" and watered by hand every few weeks. This method is not 100% like my method for outdoor storage. In 8 years we have about 91% success rate with about 50 bonsai, used to be 55 total bonsai.
