Hardiness zones have shifted northward!
The USDA divides our country into hardiness zones based on historical average low temperatures, so people can pick plants which will survive in their area. In 1990, Boulder was in USDA Hardiness Zone 5. Today we are heading into Zone 6. Do not be afraid to try Zone 6 plants in town now!
Aspen already struggle on our hot Plains and will continue to do poorly. Consider other trees which tolerate our hot summers better. This is an example of Aspen Leaf Spot.
Some replacement trees for Aspen
Spring flowers, cut foliage, red berries, fall color, striking form in winter.
Yellow panicles of flowers
changing to striking seed
pods in summer and fall.
From Eastern US.
Pink flowers cover tree before leaves emerge; aspen-like foliage. Choose stock from a northern seed source which is cold-hardy.
Western Water Birch
Yellow fall color, striking bark and form in winter. Needs extra water in summer and winter. Susceptible to bronze birch borer.
Climate Zones will continue to change and may be different by the time a tree is full-grown. Redbud, boxwood, Calgary pear, sycamore, and tulip poplar may become easier to grow here.
Rocky Mountain Columbine, our state flower, is native to the high Rockies and does not like summer heat. There are other sky blue flowers that will grow better on the Plains.
Replacement flowers for Columbines
and Blue Penstemons