Planting and Zoning: Knowing What Not to Try

Southwest Yard & Garden | Marisa Thompson, PhD

Question:

I have a spot with southern exposure in my yard where I would like to plant a quaking aspen (5,792 ft elevation).  Is quaking aspen suitable for this environment?

-        John R., NE Albuquerque, NM

Answer:

I traveled all over New Mexico in the past few weeks, enjoying the striking fall colors. It is no surprise that you are feeling inspired to plant such a sensational tree.

Spoiler alert: quaking aspen (Populus tremuloides) prefer the cooler climates offered above 7,500 ft. Some say they can be planted above 6,000 ft, but only with very special care and only in a very cool spot along the north or east side of a building where the soil remains mostly shaded.

Even if you babied your tree by giving extra water, it would likely suffer from heat stress every summer. Retired NMSU Extension Horticulture Specialist, Dr. Curtis Smith, says he has seen just that in the NE heights, where a small stand of aspen trees in a south-facing residential landscape is watered daily, and although it is surviving the clump of trees is not thriving at all. Right now it is showing some fall color, but the edges are all browned.

Another reason to resist the urge to for urban plantings of aspen trees is described by Tom Zegler, NM State Forester for the Socorro District. Zegler warns that “their biological strategy is at odds with what we want from a tree. As an aggressive pioneer species with a short lifespan, they budget energy for height growth and root suckering at the expense of defense and maintenance of individual stems. In fact, above-ground biomass is a dispensable part of the organism. This is why aspen in a horticultural setting – even a setting with suitable environmental conditions – are dependable for nothing but eventual disappointment.”

When selecting plants for our gardens, the number one concern is usually cold-hardiness, which refers to the minimum temperature a plant can be exposed to and still survive. The USDA developed cold-hardiness zone maps of the US to help gardeners match the cold-hardiness of a particular plant with the zone in which they live. In NM, cold-hardiness zones range from 4b in which average annual extreme minimum temperatures are -20 to -25°F (way up in Rio Arriba county close to the Colorado border) to 8a, with minimum temperature from 15 to 10°F (down in southern NM closer to El Paso and the Mexican border).

USDA Plant Hardiness Zone Map of New Mexico
from http://planthardiness.ars.usda.gov/PHZMWeb/#

If you look up the USDA plant profile for quaking aspen, you will find that they can withstand minimum temperatures of -70°F, which is off the US charts.

However, cold-hardiness is not the only limiting factor when deciding what to plant and where to plant it. Heat tolerance also needs to be considered. The American Horticultural Society created a tool akin to the USDA cold-hardiness zone map. This Plant Heat-Zone Map defines zones based on the average number of days per year above 86°F.

According to NM State Climatologist, Dr. Dave Dubois, “Average temperatures in the Albuquerque heights have been increasing over the years. Not only has the overall average been increasing, but the average high temperatures have also. This is from a National Weather Service cooperative observer's weather station positioned at an elevation of 6,270 feet in the Sandia foothills that started collecting data in 1991. This follows the trend all over NM of increasing temperatures due to climate change.” Dr. DuBois can be found using the twitter handle @NMClimate.

Moreover, water needs are a major concern when selecting the appropriate species for NM. A plant may survive in a location, but not look so great because it is constantly struggling no matter how much care is provided. Take Russian sage, for example. In northern NM, Russian sages thrive and are easy, low-water, landscape plants with long-lasting, lush purple blooms. Whereas, in southern NM (at least in my yard) they tended to looked piqued all season. This is what I am afraid of with the aspen idea. The tree may technically live, but be an unhappy resource hog that performs at a level far below expectations.

Depending on the desired tree qualities (e.g., low water requirement, fall color, shade, low maintenance, etc.), here are some helpful online resources for species selection in our area:

Shade Trees for New Mexico – NMSU Guide H-426

Selecting Ornamental Trees for New Mexico – NMSU Guide H-328

If only we could grow our favorite plants wherever we wanted. I know what I would plant: Bougainvillea, in a heartbeat.

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the Desert Blooms Facebook page (@NMDesertBlooms)

Please copy your County Extension Agent and indicate your county of residence when you submit your question!

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms.