Fun Facts: Planting Design ‘at Altitude’

This short article attempts to bring some clarity to the complicated discussion of “designing at altitude.”

Everyone acknowledges that elevation plays a role in plant growth. The most widely known and visible example of this is the phenomenon known as Treeline. (Check out this article for a great explanation). But there is significant confusion among landscape professionals as to how and why elevation affects the plants you can grow in a cultivated setting. In fact, many commonly held beliefs are misconceptions or generalizations applied erroneously to all high elevation areas.

What truly changes as altitude increases, and how do these changes affect plants?

There are four factors that are universally tied to increased elevation:

  • Decreased temperature (generally)
  • Decreased atmospheric pressure
  • Increased exposure to overall solar radiation (on clear days)
  • Increased UV-B Radiation

All of these affect plant growth to varying extents. Decreased atmospheric pressure can translate to increased transpiration rates (the loss of water through plant leaves), however the lower air temperatures at high altitudes often negate this. Increased exposure to solar radiation can result in higher photosynthetic rates. Conversely, increased exposure to UV-B radiation impairs photosynthesis in many species of plants. And while exposure to both types of radiation increases with elevation, this is only on clear days, the number of which varies significantly by location, regardless of altitude.

Of the above elements, decreased temperature is the most universal change, and most likely the one that has the biggest impact on ornamental plant survival. Temperatures decrease by about 3.5 deg. F for every 1,000 feet in elevation gain. These cooler air and soil temperatures can affect many facets of a plant’s growth including seed germination, bud break, photosynthesis rate, flower season, and even pollen formation.

What isn’t a factor of elevation change?

Many other things attributed to increased altitude are not actually direct factors of elevation change, but rather location-specific environmental characteristics, such as:

  • Increased exposure to wind
  • Decreased available moisture
  • Shallow or poor soils

These are often applied generally to all high elevation settings, but in fact are highly dependent on the individual location.

Other environmental characteristics are actually a combination of latitude and altitude together, such as the perception of a shorter growing season. In central latitudes there is very little difference between the growing season at sea level and the growing season at a higher elevation. However, the closer you get to the poles, the more pronounced the difference becomes.

Sourcing plants from outside your elevation: Does it matter?

It is not a surprise that the same plant grown in a different area may appear different. Distinct temperature ranges, soil types, humidity, etc. will cause a plant to display different characteristics as it adapts to its environment. Species sourced from alternate regions may even have divergent genetics. An example provided by the Larimer County Ag Extension is Redbud (Cercis canadensis). Trees from seeds collected in northern areas (Wisconsin, Minnesota) are much more hardy than seeds collected from warmer climates like Oklahoma or Texas.

Regardless of elevation, it is always a good idea to source your large material from a climate comprable to that of your project. The years the plant has spent adapting to a similar climate will prepare it for successful establishment when you transplant it at your site. Compounding the stress of transplant with the stress of a wholesale change in growing environment will only increase the chance of death or failure to thrive.

The Takeaway:

At its core, successful establishment of ornamental plants depends on day length, season length, high and low temperature tolerance, and moisture/nutrient availability. The environmental factors that your site experiences must be taken into account when creating a design and selecting plant material, but elevation does not need to be added as a mysterious compounding factor.

If you think a plant can withstand your temperatures, moisture level, water, and exposure: give it a try, and let others know what you find out!

References: (Georgraphical variation in Sitka spruce productivity and its dependence on environmental factors)

Larimer County Ag Extension

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