Overwintering advice needed in terrible conditions.

[Azz]

So first of all I want to make sure I am understanding what kills the plants over winter. I have been researching and trying to learn for this year.

Unlike what many people think (me included at first) it's not the cold that kills the plant and not the fact roots are frozen, but it kills it by not being cold enough to have dormancy, or drying out from wind and sun whilst the roots are frozen and unable to supply water to the branches. Am I correct with this?

If this is so, how could I go about protecting my trees on my apartment balcony? My balcony is very open and exposed on 3 sides on the 3rd floor, It has railings that allow wind through easily, has a concrete floor, no roof (besides the balcony above) and is only 2m x 2m with maybe only 50cm of wall space with a weird L shape in the middle next to the little window and balcony door. It's absolutely terrible, plan to move next year and just gotta get through the coming winter.

In the winter we only have about 4 hours of sunlight a day in my area with MAYBE 30 mins of direct sunlight a day...if we are lucky enough for a clear day.. usually it's dark and overcast. Temperature can range from 6 Celcius to -30 Celcius, some weeks are warmer, some are cold, some have snow others not. It's is very random.

Any ideas?

I was thinking of maybe making a leaning cold frame/greenhouse out of wood with a window/panel of plexiglass on a hinge that I can open and close depending on temperature, allowing light and insulation in when it's very dark and cold or able to open it up and let in cool air on the sunnier days. Even pulling the plants out if it gets warmer on certain days/weeks.

Something like this photo but with mostly wood and only the single plexiglass window.



As for species, I only have yews, pines and conifers currently.

Any thoughts and advice would be great

 

[RoadManDenDron]

my understanding is that trees prepare for the cold as daylight hours decrease and days get colder. (Autumn)

I had a record breaking cold of -10c last winter so not experienced your coldest but all my pines and conifers did fine with no protection at all.

Wait for the cold to really start so the trees build their 'anti freeze' then mulch the pots together to protect the roots

my major lesson in winter care is its not about minimising the cold on the trees its about keeping them cold enough as false springs and warm spells seem to do more harm if the trees start to grow too early and late frosts catch them out

 

[rockm]

You are wrong that freezing doesn't kill roots. It does. That depends on many factors, however.

Important things to remember--Roots can freeze in more than one way. Lowest temps roots can withstand depends on species. Know your climate zone.

Worth reading. Worth watching.

Roots on many trees can withstand pretty low temps (in general, conifers tend to be able to withstand lower temps than deciduous), if they have the prep time and resources. However, if temps get low enough (species dependent) roots can be killed.

Generally, it works like this--as temperatures get to 25 F (-3C) and lower, the water in the soil in the bonsai pot freezes first, then water on the surface of roots and between root tissue cells freeze (intercellular freezing), then water INSIDE the cells of root tissue freezes (intracellular freezing). It is that intracellullar freezing which is the biggest cause of root death. Water expands when it freezes. Expanding freezing water inside roots ruptures cell walls, killing the cell.

Those stages can take a while, depending on the minimum air temps and exposure length. It is the final intracellular freezing you're looking to avoid.

Unfortunately, a small greenhouse or other enclosed space won't provide much protection, or even complicate overwintering. The smaller the enclosure, the more temperature fluctuations you will get--too warm, followed by too cold--which can set trees up for disaster. Greenhouses have to be pretty large (like 20' by 30' (etc.) to provide reliable protection-as it is their mass hold temperatures steady.

You situation is complicated by your balcony being surrounded by air on three sides. Overwintering is easier when there is access to the ground--which is relatively warmer than the air and sustains those relatively higher temperatures because of the soil mass. On a balcony, the most massive thing that "stores" heat it the building holding the balcony up. You might want to think about how to use that--mulching plants up against the exterior wall for instance, would tap a bit of the ambient heat from the building...

 

[Japonicus]

Best to have trees that are rated 1 grow zone lower than you live in.
Transpiration is accelerated in cold frames if left closed during the day.
The clear or opaque material one might use in a cold frame provides greenhouse
like conditions, warming the trees inside. Consider tropical indoor bonsai.

 

[Tieball]

I’m curious. What do the trees that you want to protect look like? Photos?

Do you have a lot of snow that accumulates and stays on your balcony during the winter months? Snow is an excellent insulator in my climate.