I've been observing and ruminating on emerging weather (climate?) patterns in Northern Colorado, zone 5a, especially as it relates to selecting tree species. There are two patterns that present challenges for trees.
Pattern 1: Summer temperature (highs in 70s-90s) that persist through October sometimes into November, followed by an abrupt drop in temperature (60 degrees in 24 hours). In these warm fall conditions some tree species do not exhibit leaf senescence and die when an abrupt drop in temperature occurs. The Siberian elms around here are particularly vulnerable. I've seen the same happening to apple trees. This pattern has been consistent for four years.
Pattern 2: Winter temperatures warm up for long stretches from January to early March, with March being an unseasonably dry, windy and warm month. Soil temperatures warm, and trees break dormancy early. However, we still have exceptionally cold weather March-May. I haven't seen a lot of tree death from this pattern, but it presents significant challenges to fruit bearing trees and I anticipate it could wreak havoc if the sporadic warm temperatures lengthen and persist. Last year a Silver Maple flowered in early February.
I'm planning to put a lot of trees in the ground this spring -- fruit trees, but also a lot of canopy trees. Shade is becoming increasingly important in the high and dry country. I want to think hard about which species to plant according to those patterns. Trees that lose their leaves early and leaf out late seem like a good idea. Burr oak is a good fit, and we'll plant a few of those, but they won't provide much shade in my lifetime. I understand Kentucky Coffee tree fits those criteria and maybe they are nitrogen fixers? I was planning on a lot of honeylocust, but an arborist friend has noticed they are struggling with canker (I'll still plat some, though). Hackberry is another one where there is more water. We've already got a lot of Siberian elm, which provide shade, food for rabbits and sheep, and when one goes down, two come up in its place (I really like that quality) and Russian olive, which is great habitat for birds and food for the sheep. Cottonwoods are doing OK where there is water, but where we need trees, there isn't easy water (5 gallon buckets style watering).
Also wondering how people are pushing the boundaries of zone 5. I'm intrigued by stories of almonds in western Colorado and pomegranates - thinking they could make it on the south side of the house.
Looking for inspiration: some fun risk taking and some sensible choices, with a view for the long haul.
Maybe consider doing some research into why trees break dormancy? My understanding was that it is primarily day length dependent, but I wonder if rootstock incompatability or other criteria are having an effect? You may have rootstocks (or scions) that are bred to break dormancy early. You may also have trees that require a shorter chill period responding to the warmer temperatures as they are naturally inclined to do.
The fall pattern is more difficult to work with, although I suspect that to some extent it's the same problem in reverse. Dormancy is primarily day-length dependent rather than temperature dependent, although temperatures do have an effect. I wonder if the tree varieties you've selected naturally work well in a warmer environment? Siberian elm in particular is known for winterkill, early dormancy break and a short chill period.
I am on the edge of zone 6a / 5b and almonds grow here with no problems (although our local colleges don't seem to think so). I was thinking of pistachios, but more research is needed. They are a desert tree that apparently thrives with high summer temperatures and little water. Grapes do great here with no water during the summer once established.
Zone 5b/6a, alkaline soil, 12 inches of water per year. For now the goal is a water independent urban homestead with edible landscaping and food forest.
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