Growing Concerns 243: Protecting a cold Japanese maple

I'm ready to give up trying to grow a Japanese maple, but we want one so badly. We're in zone 5 but near the bottom of a hill, and I suspect the cold air draining down to us makes us just too cold. After every winter, branches of the Japanese maples we've tried are dead, and eventually part of the trunk looks dead and the bark lifts off it. Is there some way we can protect a Japanese maple over winter if we're in a cold pocket, or are there hardier types so we can buy replacements that are likely to survive? - F.H. -

Some winters are especially tough. 1995-96 was terrible. So was 2013-2014. Record cold killed or seriously injured Japanese maples, even old specimens that were never before touched by the cold. On properties where we garden, some died, others lost the whole windward side or took such damage to the cambium layer below the bark that they are still limping along, losing limbs every year. They may yet die.

The Minnesota Landscape Arboretum located in Chaska, Minnesota in zone 4 collaborated with local zone 3/4 horticulturist and grower Bill Swanson to study Japanese maple cold hardiness.  We spoke to the expert involved, who delivered some "good news".                                                                                            

The study showed that Acer palmatum, the most well known Japanese maple species in all its red leaf, fern leaf, and weeping varieties, won't survive without protection in zone 4's average low of 30 degrees below zero -- that's a step below zone 5's average low of -20, and may be more like your hill bottom in a bad winter. A. palmatum in zone 4 will have dieback every season, and long-term survival is out of the question. At the Minnesota Landscape Arboretum, this species is grown in containers and taken into shelters for protection from winter's worst. Swanson's had luck only with weeping forms that grow so low as to be a groundcover -- A. palmatum' Crimson Queen' and 'Red Dragon' are two. Those can survive when snow covers them, providing extra insulation.

Hug the ground for more hardiness

So you might try a weeping Japanese maple that has not been trained to a high standard -- that is, its straight trunk gives way to weeping branches three feet or less above ground. Since we don't get reliable deep snow, you'll have to provide substitute cover. After leaf fall in autumn, make a chicken wire cage a foot taller and wider than the tree, then stuff the cage with dry oak leaves or straw to cocoon the plant in and under airy insulation.

Divert cold air

If you aren't at the very bottom of a hill, you can also provide extra protection by building a wall or growing a dense, evergreen hedge uphill of the tree to divert cold air around it.

If your hill was a scoop of ice cream, cold air would be chocolate sauce poured down the slope. The shoulders of the hill end up above the pooled sauce, the base is bathed in it, but bumps on the slope that part the flow may remain untouched. Your wall or hedge can be such a bump if you make it in a "v" with its narrow end pointing to the top of the hill.

Varieties and species with hardier genes

After the 1995-1996 devastation, experts working for botanical gardens polled growers and public garden managers to locate exceptional survivors. This search turned up a red-leaf Acer palmatum seedling that stood out in an Ohio field of its siblings, taking 25 degrees below zero without any damage. That's not hardy enough for zone 4, but is great for us in zone 5. Check availability from Girard Nursery in Geneva, Ohio, 440-466-2881 , as A. palmatum 'Robinson's Red.'

In zone 4, they grow with success three less common but equally beautiful Japanese maples species. Acer pseudosieboldianum survived without damage to 42 degrees below zero, Acer sieboldianum came through -38 degrees without harm, and A. shirasawanum has weathered -32 degrees -- those aren't wind-chill, but straight thermometer readings. These are all small trees suited to the same semi-shaded woodland sites A. palmatum prefers, with graceful horizontal branching and fabulous fall color -- red in A. pseudosieboldianum orange to yellow-orange in A. sieboldianum and A. shirasawanum. A. pseudosieboldianum also has marcescent foliage -- it holds its leaves through winter, like an oak, which can provide additional interest.

            We'll try Acer shirasawanum, which has rounded, almost fan-shaped leaves. The foliage and its sculpted, layered profile cause it to be mistaken for A. japonicum, the "full moon" Japanese maple. A. shirasawanum 'Aureum' is smaller than the species and has golden foliage to spice up the summer scene.

            A. pseudosieboldianum, A. sieboldianum, and A. shirasawanum are available from the following mail order nurseries.

            Forest Farm, Williams, OR 97544

            Greer Gardens, Eugene, OR 97401

About that bark that splits and peels

That is probably another problem. A tree can be hardy in a given winter or location but suffer cambium damage below the bark from big, sudden temperature changes in late fall and late winter. Shading the trunk on the exposed side can solve this. More on protecting a Japanese maple here.