By Bill Cart, Colorado Master GardenerSM, Colorado State University Cooperative Extension, Denver
Most Coloradans agree - we live here because we want to! But the same can't be said of many plants.
For us, it's a matter of intensely bright blue skies, low humidity, warm days and cool evenings, infrequent, rainy overcast periods, and a definite four-season calendar.
Many of these factors, plus restrictive soil conditions and extreme weather changes in a short period of time, create an inhospitable environment for plant growth.
Soil, not climate, is our most limiting factor for plant growth. Gardeners tend to focus on plant hardiness zones based on lowest winter temperature extremes, but soil is a much more important day-to-day plant growth limitation.
Front Range soils tend to extremes, lots of heavy clay and some light sands but not many medium loams. What little space there is within tight clays is nearly filled with water when we irrigate leaving very little room for air - and plant roots need both! Introduced but adapted plants tolerant of waterlogged soils low in air such as swamp white oak perform well here.
Our soils also test high for calcium and are very alkaline. Though our soils contain lots of iron, the iron is tightly held in alkaline soils as opposed to the acid soils on the east coast. This means that many plants from wet areas, such as the East, show iron deficiency when planted here. Plants deficient in iron display yellow leaves with pronounced green veins. Adapted Norway maples rather than silver maple, and bur oak instead of pin oak are good illustrations of this.
Climate tends to dwarf the plants hardy enough to grow in this area as opposed to preventing us from growing them outright. Trees that leaf out early and hold leaves late are subject to limb breaking snow loads. Choosing trees that leaf out later (such as hackberry and oak) and ones with stronger horizontal branching habits (such as oak) are keys to trouble-free and long-lived trees.
Early leafing out also can set back trees when tender young leaves turn black after being killed by late season freezes such as we had in late April this year. When this happens with trees, it means a loss of the stored energy reserves that went into producing this first set of leaves. Trees will activate reserve buds to grow another set of leaves, but this may require more stored energy than weakened trees have. The freeze-kill of leaves can be the stress that pushes a marginal tree to decline or death.
Note that we can't "add more energy" to trees to help them recover from this sort of freeze. Trees can only manufacture energy from the process of photosynthesis when leaves are exposed to sunlight. Time is the only cure for trees caught with their leaves out in a freeze.
Areas such as ours that receive low amounts of natural precipitation aren't well-suited to tree growth. In the Denver area, we have few native trees - cottonwood, western hackberry, Gambel oak and ponderosa pine. The first two grow along waterways and the last two in the foothills. There just isn't enough water to support the growth of a large tree and so nature calls for grasslands.
Plants adapted to a semi-arid environment of low humidity and low soil moisture levels are the basis of Xeriscape gardening. Xeriscape, landscape water conservation through creative landscaping, started here in the arid West. Trees that are tolerant of less water (Wasatch maple, hackberry, bur oak, Kentucky coffee tree) and shrubs that require even less (New Mexican privet, buffaloberry and pea shrub) make sense for us.
Plant adaptation is a many faceted proposition. Choosing plants on the basis of environmental adaptation to both soils and climate will reward you well over the long term.
Photograph courtesy of Judy Sedbrook.
© CSU/Denver County Extension Master Gardener 2010
888 E. Iliff Avenue, Denver, CO 80210
Date last revised: 01/05/2010