Zone 5….why can’t I grow all zone 5 plants in Colorado?
Longmont, Colorado area landscapes {Monthly Series}
Colorado has a harsh climate. That’s a given. Often, people will say they just moved here from Massachusetts, upstate NY, or somewhere else in the mid-west that also has a zone 5 climate like much of the Front Range of Colorado and wonder why they can’t find their favorite plant for sale in nurseries. The reason is pretty simple….those plants usually won’t survive here unless you find the ideal spot and know what to do to keep them alive. Colorado is just too hot and dry in the summer for most of those plants to make it. But then they say, but the Zone map says we are in the same planting zone. True, but there is a catch.
The USDA Zone map is a Cold Hardiness Zone map. So it conveys what the predicted coldest winter days will bring to our area. Zone 5b (which is also subdivided into an “a” and “b” subzone) and 6a has the coldest winter temperatures between -20 and -26 degrees F. Below those temperatures, plants rated for higher numbered zones (5 and above) will not survive due to freezing of either plant roots or destruction of the cambium, xylem, and phloem (nutrient transport system for plants). Essentially the cold kills the plant. But, there are plants rated for zone 5 that will also not survive here in our Longmont Colorado landscapes because it is too dry and hot for them in the summer, and often too dry and windy in the winter. This map does nothing to address this. It only focuses on the temperatures.
The American Horticultural Society has developed a Heat Zone Map which conveys the average number of days per year above 86 degrees F which is where most plants begin to suffer from heat damage. This still doesn’t cover everything because many native plants easily handle temperatures over 100 degrees F. There is also a Sunset (as in Sunset Magazine) plant collections zone map showing the Front Range in Zone 1 and 2 for plantings. The problem with Sunset is that not all plant suppliers have the Sunset zones on the plant tags. All plant tags have the USDA cold zone on them, and some have the AHS heat zones on them. So, what to do?
First, if you know only a little about planting plants in Colorado, ask your nursery about plants you are interested in. Your best bet is to stick with plants that your nursery suggests, but beware….some nurseries stock plants that are marginal at best because they sell….usually to unsuspecting buyers. Things like Azalea, Rhododendron, and Hydrangeas will say USDA Zone 5 on the tag, but they need special locations out of the wind and heat to survive. Others like Arborvitae hate the dry winter wind and strong temperature swings. Weigela can be killed back to the ground every few years and have trouble becoming established (except Red Prince Weigela). Boxwood also suffer from winter sun and desiccation from wind. Redbud trees love sun back east, but in Colorado they need to be in the shade or sheltered from late afternoon direct sun or they will burn. And finally, some plants have a preference for acidic soils and/or wet conditions (False Cypress, the Rhododendrons, Heather, Heath….neither of which one will find in a Colorado nursery, and others) and our soils are generally more alkaline than acidic. So, if planting these, one will need to amend the soil with acidic treatments (fine pine bark, needles, peat, etc.) for optimal growing conditions.
Second, if you want to experiment, do some research about the plant’s needs. For example, ferns will grow in Zone 5. They don’t do well in Colorado unless you have a wet area, in the shade and protected from wind. There are many microclimates around your property, and some will be conducive to zones greater than 5. For example, an area that gets warm winter sun, especially early morning sun, out of the wind, against a brick wall of a house will remain warmer at night due to the radiative heat coming from the concrete and brick of the building. That area might be in zone 6 or even 7 if you are lucky since the coldest winter temperatures are usually late at night or very early morning and that early sun will warm the brick quickly. Bottom line is to just stick with what works first. Then as your landscape is established and you begin to understand the microclimates of your property, then experiment with other more exotic and desirable plants. Landscapes evolve, plants die and get replaced. Use that experience to better your landscape.
Lastly, have a budget to replace plants. Too often, people plant their landscapes without realizing how harsh our climate can be and then they have regrets when things die and they can’t replace them. Landscapes are a luxury and can be costly, but the value they add to the property, both monetary when the time comes to sell, and the immediate emotional and spiritual value are worth every penny. A home with a well maintained, lush, but appropriate for Colorado landscape will sell faster and for more money than one without. Enjoy your landscape. Be easy.
Some photos by Keith Williamson from Little Valley Wholesale Nursery
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