Hardiness of Japanese and
Korean Maples
ZONE HARDINESS and CLIMATE ZONES
1.
Hardiness - Japanese Maples are supposed to be hardy to Zone 5, -20 F
according to J.D. Vertrees. (I question that, but I do recommend the
book!) It depends on many factors including the age of the plant.
2.
Variety - Some Maple varieties are hardier than others. Japonicas are
supposed to be cold hardy, as are Pseudosieboldianum (Korean Maple) and
is rated as hardy to Zone 4 (-40), but it is hard to find and has (as I
know it) only one named cultivar. It is a columnar form. I
only
have a few of those. (The same caveat applies, ALL young plants need
winter protection.)
3.
Is the plant in the ground or in a container? A container grown plant
may be hardy in a Zone 6 or 7 but not in a Zone 5-4. Root Stock - Since
almost all named Japanese Maples are grafted, the top (depending on
variety) may be hardier but the Root Stock is almost always an Acer
palmatum seedling. Root Stock plants are seed grown and so hardiness of
the under stock would vary. Mulching would therefore be a good idea.
4.
Site conditions - Zone microclimates play distinct havoc with stated
hardiness statistics. This would include slope exposure, frost
drainage, fluctuation of daily temperatures. In Denver, it might get to
10 below and climb on a sunny day to 70. The shaded side stays frozen
and the south facing bark will rapidly thaw. This causes splits in the
trunk and often the death of the tree. This applies to the Pacific
Northwest as well, explaining the need of shade cloth or protective
lath.
5. Length
of your cold snap, snow cover etc. The worst combination would be very
cold temperatures with no snow cover. Equally of concern would be
wildly varying weather conditions. One year I had three major arctic
expresses with warming periods between. I was in a Zone 6 zone and it
went to 7 below with no snow cover and bright sun. All the worst
combinations. The Nursery was toast. So would be most potted bonsai.
6.
Suggested mitigations: Mulch, provide wind protection, pile snow up
against the trunk and perhaps use anti-desiccant sprays, etc All the
above factors heavily as to how much damage is done by these temps.
Read snow is really GOOD! The best news is that we usually do not have
a prolonged period of such ultra-cold temps. This is a major factor in
the survival of the trees and how much damage they may receive..
Further wet weather may yet exacerbate the winter damaged root
conditions; so protect the plants from further excessive moisture until
new growth has begun.
'7.
Oxygen plus' might be one mitigating commercial chemical that might
help recover some root-damaged plants. Having a dried out hose
available to a frost free spigot is another helpful tool. Spraying the
frozen trees (lightly) will do much to mitigate damaging thawing – much
as they do in the citrus fields. Bring the hose back in and allow it to
drain, should you need it again.8. Always brush snow
carefully off your trees as the accumulations might worsen, freeze and
accumulate more. … breaking limbs.
JAPANESE
MAPLE DISEASES and PRUNING
Disease
by Pseudomonas syringae tends to be favored by wet, cool conditions.
The optimum temperatures for this disease tends to be in the winter
months when temperatures are below freezing. Considering this cold
winter I will focus on this pathogen. P. syringae is a very common
winter-born bacterium.
Do
you sometimes observe black stem canker in Japanese maples? Many
mistake this common maple disease for a fungal infection. The pathogen
actually is Pseudomonas syringae, a bacterium that should be better
understand in order to manage Japanese maples. Chemical application
control is possible but costly. A better tactic is to understand the
causation of the problem.
1.
Pseudomonas is a cold weather disease. The most critical factor is to
keep young plants from freezing, be they in nursery containers or in
bonsai pots.
2. The
bacteria are seed borne, and can be dispersed between plants via rain
splashing on soil. Mulching with bark or gravel will reduce the risk.
That said, insects and windborne activity can spread it as well.
3. Age of the plant is a factor, leaving tender young maples exposed to
below freezing weather is risky.
4.
Young plants should be moved under protective cover in early October.
Pseudomonas bacterium needs water on the plant structure to be
infective. Keeping the plants dry, especially at night is imperative.
Cold winter dew can cause damage, so can a humid cold greenhouse. 5.
Proper ventilation is a must, and one should take care to water all
such greenhouse plants only in the morning, preferably on a sunny day
-- so the foliage can dry off before dusk.
6.
Pruning or making any such wounds during the winter months should be
avoided. When pruning or grafting, the tools should be properly
sanitized with alcohol, TSP or even mouthwash. All open wounds provide
entry points for this bacterium. If pruned -- or the problem is
sighted, Spray with copper spray or lime sulfur. Doing this on a sunny
day where the spray can dry off is imperative.
Non-the-less,
normal winter freezing and thawing can create cracks in the stem
structure of many plants and provide entry points. Copper spray is a
control but nothing more. It will not "cure" the plant. Typical
symptoms include winter-spring tip blight; blackened stems that are
quite different from Verticillium Wilt -- more of a summer disease,
where sudden wilting occurs followed by the withering of the leaves and
the sudden die back of shoots.
Prevention
requires intelligent purchasing. When you buy these plants from a
Nursery avoid such stock. Be critical of the cultural needs of these
plants and if you see such plants stump pruned, walk away ... but
mention your concern to the Nursery.
Once purchased and planted in
the garden these plants will provide infection sources for the rest of
your garden. Handling of the soil, stems or going from one plant to
another can spread it. Planted in the ground, one's footprints can
spread it. Pruning should be avoided. If your gardener suggests winter
pruning, find another.
Maples
are affected by a number of fungal diseases as well. Several are
susceptible to Verticillium, which can cause significant damage. This
fungus will be present in the soil as well as on plant tissues. Poor
pruning techniques contribute to its spread. Death of maples can also
be caused by Phytophthora root rot and Ganoderma root decay.
Planting
too low in ground cloth or putting drip emitters next to the trunk
contributes to this. Typically one finds dead tissue with bluish-black
stains in the tissue beneath. Ones shoes can also spread these spores.In
late summer and autumn Maples, esp. Vine Maples are commonly disfigured
by leaf spotting, caused by Rhytisma fungi and mildew caused by
Uncinula and although unattractive these fungi may cause the tree to
be, they do not effect the trees' long-term health. Overhead sprinkling
contributes to this problem.
I will add a 2009 edit. For years I have suffered from a "Alder
allergy" which affected me for over a month every Spring.
After
stopping my spraying of Copper I have had no such reactions. I was not
suffering from allergies -- I was suffering from chemical overdose of
what I was spraying for decades.
To
put reference to a serious lung disease that my Aunt died of: I will
add that "Anthracnosis" (not a plant disease) is human based and caused
by the the accumulation of carbon particles in the lung. The common
culprit being the gulping down of carbon, be it from tobacco, diesel
fumes or the burning of late summer plant debris. All is dirt
to
your lungs and to your body and it never gets rid of it. It
stays
in your lungs.
Recenty I got a note from
a grower of Maples and he too had a problem with P. syrynae.
All I could do was repeat some essentials and perhaps add a few new
thoughts.
I really do not have much to offer except to re-iterate that some
winter/spring conditions exacerbate the problem. Here in Sequim, we
tend to get these thaw, freeze, thaw cycles and you are stuck with.
Drippings of bacteria filled raindrops from overhead trees or lath can
contribute to this problem. Most bonsai owners keep there plants under
lath. I move all of mine to an unheated overhang (with good) --- with
minimal winter watering.
Spray the plants especially the maples with copper spray the moment the
leaves come off. Clean and dump off the leaves before healing them in.
Funny thing I use (lavender stem clippings.) Not viable for you of
course, but perhaps using a reverse suction leaf blower to clean the
leaves off the containers would work. This would also slow down
anthracnose , phyllosticta leaf spot fungi. Rotating and letting the
ground cloth bare and heat able by summer sun also helps. Then put out
new stock in the fall..
All mulches can harbor bacteria so keep that in mind. Clean groundcloth
is best – and spraying it before putting on new product might be a good
idea. I think back on the Santa Cruz garden center I once managed and
shudder... near all our bins were filled with sawdust and plants
plopped on top. I shudder to think of the pathogen and insect reservoir
we created. Our bare root was in coarse sand (no longer ins sawdust– my
insisted change, and slowly we replaced the all the sawdust bins with
gravel beds. (It was a subsidiary of a lumber yard so that was a hard
sell.) I do wonder if the redwood sawdust might have had some
inhibitors that fir bark or hemlock sawdust did not. Still it was yucky
and after a few years became boggy and sour. Gravel won out! Much was
under wood lath which would have been counterproductive. Plastic mesh
shade cloth would be much cleaner..
Even grasses can be a reservoir for molds, bacteria and fungi – so have
a clean 6 ft perimeter around your growing area.
As I indicated keeping your plants cool and dry during the winter would
help. I am not a grower, but having them in clear plastic greenhouses
might contribute to your problem. (heating up during the day and then
freezing.... big problem. Using a white plastic cover would be better.
Ventilate during dry cool days.
Then again, as mentioned cleanliness and sterilization of pruning tools
is a must due issue and become second nature. Another good source for information is this site on large leaf maple diseases.
