How does zone matter?

I live in zone 4b.  If I plant a zone 6 tree, it will die if we have a typical winter.  Does anyone know exactly what kills it?  I wouldn't think it would be the air temperature, because the tree goes dormant.  I'm not sure how the ground temp would kill it, because it seems that once the temp of the ground gets below a certain point (within reason), that it wouldn't matter the temp.  So maybe it's the depth that the ground freezes to?  We can get frost 4 feet deep here.  If depth is the answer, it seems that by mulching a foot deep, I would be effectively changing my zone up at least one point.  Anyone have any answers as to exactly how this works?

I'm not going to speculate on causes of death, just on what I observe.

It is common for trees to winter-kill down to the roots. Then new trunks emerge from the ground.

In extremely cold winters, the small twigs on my walnut tree will freeze and die, but the main branches send out more branches in the spring.

On vegetables, it seems like it's the minimum temperature that does them in, and that the minimum varies from plant to plant. For example, last fall some favas succumbed to very light frosts, while others were still looking great after many hard frosts. Parsnips survive in my garden without protection, carrots usually don't.

If I bury root crops about 18" deep, they tend to survive my winters without freezing. Some years, they might survive laying on top of the ground.  

Climate zones stated in say Dave's garden are often wrong. Sometimes it is miusture in winter.

Angelika Maier wrote:Climate zones stated in say Dave's garden are often wrong. Sometimes it is miusture in winter.

Yup. Zones can be wrong. Micro-climates matter. Trees may be genetically diverse so some thrive while others die.

When I make a cactus bed, I make the soil very sandy, for the purpose of winter-survival of the cacti. Drier plants/roots survive better. Some cactus growers build a canopy over their cactus beds in the winter to keep rain off.

Todd Parr wrote:I live in zone 4b.  If I plant a zone 6 tree, it will die if we have a typical winter.  Does anyone know exactly what kills it?  I wouldn't think it would be the air temperature, because the tree goes dormant.  I'm not sure how the ground temp would kill it, because it seems that once the temp of the ground gets below a certain point (within reason), that it wouldn't matter the temp.  So maybe it's the depth that the ground freezes to?  We can get frost 4 feet deep here.  If depth is the answer, it seems that by mulching a foot deep, I would be effectively changing my zone up at least one point.  Anyone have any answers as to exactly how this works?

First off, as others have stated, sometimes those Zone charts are wrong.
Today, the USDA Zone chart is very wrong, the effects of the on going climatic change has rendered them mostly obsolete, but you can still use them, just add a number to where you are.
For you Todd, zone 4b is most likely now zone 5a and might even be 5b (the letters represent the slight flux of mean temperature, a stays closer to the norm than b).

There are a few reasons a tree (or any plant for that matter) dies, these are; average mean temperature is to low or high for the cells to remain viable, temps are to cold for new growth leaves to survive, humidity creates issues with respiration, ground temps get to cold causing freezing of roots, ground temps get to hot causing heat prostration of roots, conditions are just right for diseases/ insects to attack. Those are the main reasons there was a "zone" system created in the first place.

In Arkansas we can't grow citrus trees, it simply remains to cold (for now) for the newly budding leaves to survive in the early spring. We can't grow Moringa here either because we do get chill days and Moringa can not tolerate those.

Generally you can fudge one to one and a half zones if you are willing to do the soil prep and on occasion place a cover over the crown when there is going to be a cold snap/ frost. In the extreme you might have to cover and provide a heat source for a while.

Redhawk

Something else that matters is how wide the temperature swings are.  We had two freezes here this winter that were separated from very warm weather (think upper 80's) by only a few hours. I lost three young trees and several vegetables despite the lows being well above what should have been their minimum tolerance.  The plants just didn't have sufficient time to adjust.

I am sorry to hear about your tree losses Casie.

These days I watch the daily highs and lows far closer than I did ten years ago, this climatic changing time is going to have all of us "Southerners" needing eyes like a hawk so we don't loose plants to non normal weather.

Our Monthly Mean temperatures for this year are 5 degrees higher than last year so far and this is just March.
I have been recording weekly averages for the last 30 years but as of January One I started a daily record book because I can see that it will be needed over the next decade or two as we experience the change.

For those that aren't aware of it, this climatic change cycle actually started back in 1980, overall that year temps were up, nation wide by 0.25 degrees when compared to any year prior to 1980.
There had been fluctuations but not an overall rise previous to then.

Over the last few decades, ten year average temps have been slowly creeping up.
This is not something that is happening fast enough for most folks to notice but it has been heading towards this "lack of a Winter" we are experiencing now since the mid 1970's.
For anyone wanting to keep their own records for a while, you might see an interesting pattern emerge over the next 5 years.
I predicted in 1975 that Arkansas would end up with near temperate rain forest weather patterns by 2035.


Redhawk

My challenge is  temperature swings (It will be 90 F during the day and 30 F at night) and summer frosts (it is not unusual to have a hard frost in July/August).  I bought zone 4 fruit trees that didn't make it the first year because of this. We are supposed to be zone 5b, but I now ignore this and focus more on microclimates and protection for those mid-summer frosts.  We also built a greenhouse below grade to help with the temp fluctuations.  It's been a challenge to learn gardening here in Central Oregon that's for sure!!!

Kathleen Driscoll wrote:My challenge is  temperature swings (It will be 90 F during the day and 30 F at night) and summer frosts (it is not unusual to have a hard frost in July/August).  I bought zone 4 fruit trees that didn't make it the first year because of this. We are supposed to be zone 5b, but I now ignore this and focus more on microclimates and protection for those mid-summer frosts.  We also built a greenhouse below grade to help with the temp fluctuations.  It's been a challenge to learn gardening here in Central Oregon that's for sure!!!

That's rediculous. We are in a zone 3-4 here in Alberta and we might get an early June frost or really late August frost ( usually very mild ones) not every year though. I don't recall ever getting a July frost though.

Those of you that pointed out the zone inaccuracies are right of course.  The last couple years we have had much milder winters, but with truly hellish temperature swings.  I'm hesitant to count on things like that though.  I most often try to plant trees that are good to at least one zone colder than I am, just on case.  I would hate to plant a bunch of trees rated good to zone 5 and have them do well for several years and then have a "typical" Wisconsin winter and have them all killed off.  I'm trying to plant many more trees than I need in case I have some losses, but I'm getting too old to start over to many times

Thanks all for your inputs.

Exactly Todd,

Right now we don't really have an idea of what comes next.
Geological research indicates that this weather we are now experiencing came along just before the last Ice Age began, so who knows, we just might be inline for that to happen.
The last Ice Age started with Winter cold swinging in right behind unusual Winter warmth, the seasons would have moved from what we know them by the months of the year.
This created a stall in the warm ocean currents which led to those currents simply stopping, once heat can't travel around the globe, we get the deep freeze effect and that spreads from the poles down towards the equator.
Right now the polar caps are melting at an alarming rate. Land that used to be under feet of ice at the south ice cap is exposed now.
This is something not seen in at least one thousand years, though there is one map that shows an ice free Antarctica.

Earth operates in cycles, many of which we simply don't know the timing of.
That means it is a roll of the dice as far as "knowing" what comes next.
My people say we are now in the time of the fifth shaking of the earth, that this shaking will be the great spirit using both hands instead of just one to do the shaking with.

Fortunately many trees have a 4 to 6 zone spread when it comes to what they can live in, weather wise.
The wise thing to do now is find those that your area falls in the middle for suitability.

Redhawk

If found that the more mature my fruit trees are, the less chill hours seem to matter.  In my zone, we're lucky to get 300 chill hours, yet I've got trees that are supposed to get 600 or more.  Almost everything blooms, but perhaps not all at once or as aggressively as you'd hope for if they had received adequate chill hours.  But I've got bees, so pollination isn't my concern.  Generally, people want a big flush of blooms all at once so the tree attracts enough bees.  But in my case, there are so many bees hanging around, even trees with a fairly minimal flush of blossoms still get good fruit set.

I've never had the experience of something dying because it's too hot or cold.  But in our Mediteranian climate, things are relatively stable in a reasonable range year round.

Marco Banks wrote:If found that the more mature my fruit trees are, the less chill hours seem to matter.  In my zone, we're lucky to get 300 chill hours, yet I've got trees that are supposed to get 600 or more.  Almost everything blooms, but perhaps not all at once or as aggressively as you'd hope for if they had received adequate chill hours.  But I've got bees, so pollination isn't my concern.  Generally, people want a big flush of blooms all at once so the tree attracts enough bees.  But in my case, there are so many bees hanging around, even trees with a fairly minimal flush of blossoms still get good fruit set.

I've never had the experience of something dying because it's too hot or cold.  But in our Mediteranian climate, things are relatively stable in a reasonable range year round.

Not having enough chill hours is never going to be one of my problems