Summer-Only Ground Growing JM?

DrTolhur

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I've got a few very small (~0.25" trunks) Japanese maples that are currently in pots. My plan was to plant them in the ground come spring to let them grow up some more before training for bonsai. However, I'm in zone 5 (middle of lower Michigan), and I'm a little concerned about their ability to long-term handle winters. I have an idea that I'd like to get some feedback on:
I could plant the trees in colanders in the ground, let them grow for the warm season, then pull up the colander in the fall to over-winter them in my unheated garage. This would allow the roots to grow freely and vigorously in the ground, but then also allow me to protect them in the winter without disturbing the main rootball.

(Note: the colander thing obviously isn't my idea, but I don't think I've seen this application of it mentioned before. Please direct me if there's some existing source out there.)

Questions:
1) Is my concern for them unprotected in winter reasonable?
2) Do you think my idea seems reasonable/would it work?
3) If I do that, would there be a problem pruning the roots outside the colander just as/after going dormant? Maybe I'd want to pull up and trim the roots a few weeks before going dormant to allow it to recover.
4) Would it be better to fill the colander with the native ground soil, potting soil, or bonsai soil?
 

Shibui

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I just checked online and it seems that JM are OK in zone 5 and some varieties can even manage down to zones 4 and 3. Your JM are no less hardy just because they are small or because they are bonsai.
I do not have any direct experience with cold winters but I've seen lots of talk about heavy mulch to protect trees through winter. Maybe someone will advise on techniques to provide additional protection for trees outdoors.

There have been other threads dealing with lifting colanders but I don't remember any that had longer term results. From a hort point of view there is no problem planting the colanders in the ground and lifting in fall. The small roots that grow through the mesh can be trimmed after leaf drop without any problem to the trees. I can't see any advantage to lifting before leaf drop.

Annual lift and store may slow growth slightly but that's probably less of a worry than possible winter damage or more likely spring damage from late freeze.

I'll look forward to opinions on soil, potting soil or bonsai mix in the colanders. There may be differing experiences depending on the local soil. Digging a hole in clay soil and planting in potting soil can cause problems as the hole tends to act like a pond and collect water. Better to have either similar soil inside and out or raise the soil level above natural ground to avoid those problems where natural soils don't drain well.
I've also seen examples where roots do not move between very different soil types so it is generally better to mix native soil with the potting soil around the roots when planting trees as landscape. Not sure how that applies to colander planted pre-bonsai however there is no real problem bare rooting maples when the time comes to move to pots.
 

DrTolhur

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All of my looking into Japanese maples shows most cold-hardy to zone 5 and some only listed as zone 6. The problem is that zone 5 goes to -20ºF, but my location sometimes goes below that. And with the addition of some varieties only showing zone 6 makes me feel like Japanese maples are kind of iffy in zone 5. I'm not big into the "try it and see what works" method when it knowingly risks killing trees that I've paid a fair bit of money for, so I'm definitely willing to take slower growth over maybe losing trees.
 

AcerAddict

Shohin
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If you're that worried about them not surviving the winter, then don't plant them in the ground. You'll have to deal with a mentally stressful few months every year when temps can potentially dip dangerously low. I'd put them in grow boxes, nursery pots, whatever, and just keep them in the garage when it get super cold outside. Soil for little 1/4" trees isn't super critical just yet as long as it drains well. Personally, I wouldn't spend the money on bonsai soil for seedlings that tiny, but others might. I'm sure others will share their opinions and input here.
 

DrTolhur

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I don’t follow what the stressful few months would be. I’d plant in May, leave in the ground during the growing season, and pull up after leaf drop. There would be no risk of death-by-cold that way.
 

Forsoothe!

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Midland is zone 5 and no problem for JM, especially little ones. Big ones have a problem with the bark splitting in spring when the sap is drawn up on a warm day, on the sunny side, and freezing overnight. It is always very damaging and often the beginning of the end. The trunks of JM should be shaded on the sunny side by some feature in the landscape, -a building, shrubs, as an under-story tree, whatever.

Maximum growth is achieved when planted in the ground. Pots sunk in the ground are effectively just as good. The best cycle is: pot in spring or early summer in a pot the size you want the roots; leave in place for two summers. The first summer it will grow nicely, the second summer will have normal in-ground growth, the pot will fill with roots and many roots will escape through drain holes. The pot is dug in the following spring and roots trimed to remove large anchor roots and keep fine, hairy feeder roots, repotted in the size pot you want the finished roots to be in. Repeat, ad infinitum until finished size is obtained. The first year in the ground will have better growth than not in the ground. The second year will have very good growth. If left in the ground a third year the pot will be filled with large, anchor roots and the feeder roots will be outside the pot and too far way. Correcting that will take time and be just as difficult as fixing a field grown tree. Using a pot the size you want as a finished size from beginning to end is important so as to keep the feeder roots close to the crown of the tree, as potted. The two-summer cycle promotes that end. The 3rd year will screw-up everything. Trimming the top wood every fall also contributes to growing the tree you want to finish with, especially as opposed to just growing something that has to have many years of re-programing the wood to be in artistic proportion after growing it too big.
 

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