mountainbamboonut wrote:
However the average lowest temperature over a span of a decade or so seem more critical in determining which plants will die or not. Even hotter than normal temperatures will not kill any bamboos that I own. But putting them in the ground in an area with extreme lows for a given variety will result in top kill or the death of the planting altogether. So I do find that Ag Zones are very critical, maybe more than anything else.
In my area heat is usually also not that much of a problem but there will be people with hot and dry areas that struggle to keep boo like fargesias alive. If you want a common ground for discussion obviously you also have to include climates dissimilar to yours!
Average lows are definitely not the best metric for determining plant survival. All it can take is a few hours of extreme cold to topkill or completely erase a plant.
When you have an average value it is not clear how extreme outliers can be. It makes a huge difference if most of the year you have moderate lows and then once in a while it gets extremely cold or if it is a bit colder more often but you never see those extremes. The average lows of both will look similar and don't tell that story.
For instance where I am from we generally have mild winters with almost no frost till well after Christmas, after which you can get cold spells of -20°C and lower for short periods of time.
This does not happen every year so it adds to the problem. Slow growing plants that are marginal at that temperature may or may not make it depending on the local micro-climate.
I'd argue that you can't separate out single factors as they are all interconnected. Soil composition can make a difference in combination with temperature (even day and night differences), wind exposure and precipitation. It explains why people have quite varied reports on hardiness and also why seemingly one year a boo will do fine with a specific minimum temperature while another time much less lows cause leaf loss or even topkill. This year my aurea and violascens (potted) had massive damage with a low of not even -15°C for just a couple of nights while fargesia/borinda gaolinensis which was standing right next to them had almost no leaf damage. I have no idea why as the gaolinensis is usually much more sensitive.