"Should I begin hardening the seedlings by bringing them outside for a portion of each day, with the aim of getting them into the outer shed until spring arrives?"

That sounds like the best option.

A properly cold hardened seedling palmatum grown in a container will be ok down to a threshold of 14F. You are most likely right to suspect your seedlings are not fully prepared for winter. The difficult question is to gauge how low they can tolerate in their present state. How soft is the most recent top growth? When did they last send out any new shoots? Does it look like the remaining 40% leaves will drop any time soon?

If you can find out what the low temps in the shed are it would help to make a decision. How big is the difference from the greenhouse?

Without knowing the full facts it is hard to give advice, but if I was playing it safe I would expose them to 30F lows at first, then 25F after a week or two, and so on. Depending on what temps are forecast, you might have to shuffle them between the greenhouse and the shed for a while.

Good luck.

You state the current greenhouse conditions do not drop below 40F - how warm does it get? 40-45F is an appropriate temperature to trigger dormancy - much above that could impede it.

Ideally, the colder shed would be a better location as it more closely replicates outdoor growing conditions, however it may very well prove to be too cold for a containerized seedling JM. Containerized plants are far more vulnerable to cold damage than their counterparts planted in the ground - they do not have the benefit of that large insulating soil mass. Roots are generally more intolerant of cold than is top growth and the temperature at which root damage can occur is only about 24-26F.

Depending on the temps the regular greenhouse, I would be inclined to leave them be where they are. If that place really heats up, I might try the shuffle back and forth for awhile with the caveat that nights - or when the temps drop into the mid to low 20's - the seedlings are in the warmer greenhouse.

"Roots are generally more intolerant of cold than is top growth and the temperature at which root damage can occur is only about 24-26F."

I thought the accepted threshold for Acer palmatum root damage was -10�C, which is 14�F ? This is what the literature says and is also what I have observed myself.

The degree symbol doesn't seem to work here. The above message should read: "I thought the accepted threshold for Acer palmatum root damage was -10C, (which is 14F) ? This is what the literature says and is also what I have observed myself."

BTW, I am talking about properly hardened off plants with regards to the 14F, the OP wouldn't be advised to risk 14F with maples straight from the greenhouse.

14* is generally considered the industry standard as the lowest temperature mature roots of A palmatum can tolerate. This means that the entire organism would be expected to succumb to temperatures that low.

I agree wholeheartedly with Pam re her assertion that root damage can/does occur in A palmatum at temperatures as high as 24-26*, and I might even consider a couple of degrees warmer than that as a favorable lower limit, just as an added margin for safety.

Even though our trees might survive root exposure to lows in the neighborhood of 14*, they sure won't like it. Roots don't all die at the same temperature, they die incrementally as the temperatures drop. While it varies by species, and genetically by individual plant within that species, all roots on the same plant don't have the same tolerance to cold.

These numbers may not be accurate, but it gives you the feel for what I'm talking about. We say that maples are X hardy because they tolerate low air temperatures, but what about root temps? Most temperate trees, even in zone 4, never see actual 6" soil temperatures much below 27*, with 25* being unusual. This is true, even in bare field agricultural conditions. Half of Michigan's upper peninsula, under cover of forest canopy, never even sees frost in the ground.

We know that the finest hair roots - the most succulent and the ones that do the lion's share of the work are the first to die. In many woody plants, these roots begin to die as soil temperatures drop below 28*. As temperatures drop further, larger and larger roots succumb to killing low temperatures. The point is - that many trees that SURVIVE are left with only the largest roots to support them in spring because much of the rootage has frozen. These trees are slow to flush in the spring because they need to utilize stored energy to regenerate lost rootage before they can move sufficient water and the nutrients dissolved in water to support either growth or the flush of foliage that makes the food that allows the tree to grow (this, in the case of deciduous material).

So, while some trees might survive at extremely low temperatures unprotected, we KNOW it is better for the tree, especially from an energy management perspective, if we give them protection that ensures actual root temperatures don't drop low enough to kill even the finest roots. For most temperate trees, that means we should strive whenever possible to keep low root temperatures in the upper 20s at their lowest, and below 42* to inhibit the onset of growth until spring, when we can get them into good light w/o worrying about frost/freeze.

So, what the tree might tolerate and what's best for it are two distinctly different perspectives.

Al

Firstly, thanks to everyone who's posted; the info is in depth and much appreciated!

In regards to mafle's post, the shed is totally at the mercy of the elements and our lows are around 20 deg. F, sometimes lower. The trees themselves haven't had any new growth for some time, though a few bud tips appeared a couple months back but appear dormant at the moment. The tops of the trees have yet to lignify completely and are still fairly soft. The leaves on the lower portions of the trees have withered and curled but have not dropped. The most mature tree has several well developed leaves which show no signs of wear really, whereas the 2 smaller seedlings have fall coloring and more leaf loss to date.

From the postings so far, it seems like the greenhouse is slightly too warm to really put the trees into their dormancy period, whereas the shed is probably too cold for them to survive in as they are not winterized and are in containers.

Seems to me that the best option would be to get them mulched and put them outside in a windbreak near the house during the afternoons when our highs are around 30 deg. F and bring them inside to the greenhouse when the temps begin to dip later in the day. Does this sound amenable, or would anyone propose a different solution? Should I gradually bring them up to speed, such as a couple hours a day outside gradually working up to full afternoons outside before bringing them back in? Thanks!

Al,
I was interested to hear your opinion on the subject, thanks for sharing; apologies to the OP if I am going off-topic somewhat.

Where I live the winter lows over the last decade have usually been in the 20F to 25F range. I leave my container maples outside with no protection and while I do not expect the fine feeder roots to persist through the winter, the main storage roots seem to be ok come the spring. Any root death that does occur I like to characterise as "natural root pruning" and will typically be nearest the edge of the container as this area will be the coldest.

I like to think that any lignified roots will survive down to the quoted 14F temperatures (with some natural variance) and any fine or fleshy (or new) roots will perish at much higher temperatures.

This year is going to be a real test here as it has been much colder than usual for the UK, and it is still only December. I have a max/min thermometer probe positioned among my palmatum seedlings and it has already measured one night of 15F and one of 16F, plus many below 25F. On a couple of occasions it has stayed below 25F for 24 hours or more at a time, and the pots have often remained frozen solid for a week or more at a time.

My first season seedling palmatums are all very small, typically 3-4 inches, but tough-looking, gnarly and weatherbeaten. I am expecting them (nearly) all to survive at present, as long as we don't get any temps significantly below 14F. If they were large and fleshy and soft I wouldn't be anywhere near as confident. I do not try to grow these plants to their genetic potential in terms of maximum growth rates, but I do try to grow them near to their genetic potential in terms of hardiness - I prefer small and tough over large and floppy anyday. I will let you know how they do in the spring. Wish me luck.

I still think the OP should try and find somewhere in the 30F range to move his seedlings to, with the option of later moving them down to 25F if they seem ok.

Season's greetings to everyone,
maf

thornslack,
From the sound of your description of the state of the maples I would agree with your plan - outside when it is 30F or above, inside the greenhouse when it goes lower. They sound a little soft and I would be rather hesitant to suggest anything much below 30F unless, or until, they have dropped all the leaves and the tip wood has hardened somewhat.

All the best,
maf