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Silverleaf nightshade flowers are beautiful, but parts of the plant are toxic to humans and animals. Dr. Marisa Thompson courtesy photo
Question: What ornamental or fruit tree could I safely plant in a west-facing 8-foot by 8-foot niche outside my kids’ bedroom windows without worrying about it destroying the surrounding concrete and foundation? Stacy I., Albuquerque
Answer: I like this type of question. Not because I know the answer—I do not. And not because there are many possible correct answers—there are. But it is the process behind the decision that grabs my attention. What do we need to consider when selecting a new plant for a specific spot? For sure cold hardiness, heat hardiness, drought tolerance, soil type, and size are all important when creating a most-wanted list.
I have gotten similar questions in the past, and these are the first follow-up questions that come to mind:
How do you plan to water the area surrounding this new plant? Unfortunately, in our warming world and current drought, even desert plants native to our region require supplemental irrigation as they become established and thereafter. So, a watering method and plan are a must. The plan does not have to be set on a fancy irrigation system. Irrigating the entire root area and beyond can be done with a five-gallon bucket and monitoring so that the soil dries slightly between soaks is a great plan.
How attached are you to only planting a tree? Or only one plant? The niche you are describing is small, but a cluster of smaller or medium-sized plants offers diversity, both aesthetically and ecologically. Consider a small tree, a compact shrub, three small ornamental grasses, and a few perennial flowers. The idea that plants compete when placed close together has some validity in certain conditions, but this idea is being challenged by researchers from all sorts of plant disciplines. Like humans, sure, we compete, but we also benefit from living in communities. The above- and below-ground benefits of growing among neighboring plants, all nestled together, may outweigh the negatives.
How often do you walk by? If this is a spot you and your family pass frequently, consider something that smells wonderful, like a butterfly bush (Buddleja species), or that blooms profusely, like a crape myrtle.
Which native plants could you incorporate into this area? Crape myrtles are not native to the Southwest. And neither are most butterfly bushes. But there is one species, the wooly butterfly bush (Buddleja marrubifolia), that is endemic to the Chihuahuan Desert and might be cold hardy enough for a warm microclimate in Albuquerque like the one you are describing. The pale-yellow blooms of this little shrub are not as noticeable as the pinks and purples of other Buddleja plants, but the smell is pure honey heaven, and the bees know it—both native and honeybees. The Native Plant Society of New Mexico website (www.npsnm.org) offers helpful lists with plant descriptions and tips for urban landscapes.
How long do you plan to live there? Or, more pointedly, can you select a plant for longterm sustainability so the next owner inherits something they can maintain? This brings up the consideration of size too. I understand the temptation to select the tree you really want based on sentimental or aesthetic attraction but planning to continually prune a plant down to maintain a size smaller than its natural habit is a recipe for disaster. “Right Tree, Right Place” is a mantra of arborists for a reason. Pruning cuts injure plants and induce a stress response. Or course, some pruning is necessary sometimes, but planning for fewer cuts is better. By the way, pruning season is upon us, and I will be focusing on that in the upcoming weeks.
How long have you lived there? I ask this because it was not until after six months at my new house, when the windy season hit, that all of a sudden, my front entryway became a gathering spot for leaf litter and random lightweight trash, mostly Doritos and Funyuns bags. My point is that a shrubby tree might trap more debris in your landscape niche and removing windswept items from a spiny plant might become annoying, painful, or both.
Do you need this plant to provide shade? If so, I have some tough news for you. You might be better off with a shade structure and some beautiful climbing vines. Of course, growing trees for shade is worth the time it takes to get there. But in a restricted area, such as yours, there’s not enough rooting space to support a large shade tree. And stress from root restrictions invites secondary problems like pests and diseases. It’s just not sustainable planning. Here is how I described root needs and how to plan for success in a column from January 2019:
“The rules differ by tree species and soil type, but the larger the tree trunk and canopy, generally, the larger the root zone needs to be. The International Society of Arboriculture defines the critical root zone (aka critical root radius) for a given tree as the area equal to a one-foot radius from the trunk base for every one inch of trunk diameter. Trunk diameter measurements should be taken at 4.5 feet above ground (or thereabouts, depending on tree age and whether there are huge, knobby lumps in the trunk) … The quickest way for me to estimate tree trunk girth without a measuring tape is to visualize a whole pizza that is the same size as the trunk diameter—personal pan pizzas tend to be six inches, and a large pizza is usually around 15 inches. So, if your tree trunk is a medium pizza size, you can guestimate that the trunk is 12 inches in diameter and translates to an approximate 12- foot rooting radius. That is a 24- foot diameter of rooting area for a model tree to have room to breathe, but for the tree to continue to grow without failing, it will need even more space.
“This equation also comes in handy when you’re selecting a tree for a small patch of ground or a courtyard. If you only have a space that is 10 feet by 10 feet, the rooting radius is only five feet, and that means the area will only support a tree whose trunk gets up to five inches in diameter at maturity. No wonder trees fail in small spaces like medians, mall parking lot, and hell strips. (Hell strips are the aptly-named, narrow patches between sidewalks and roadways.)”
So, what we have learned is that your eight-foot by eight-foot space can be expected to support a tree that grows to a maximum trunk size equivalent to an English muffin. Of course, these rules are based on models, and caveats exist. I am reminded of George Duda who said, “Trees make liars out of all of us.” A cluster that includes a small tree with a few other well-suited friends is my preference. Good luck!
For more gardening information, visit the NMSU Extension Urban Horticulture page at Desert Blooms (http://desertblooms.nmsu.edu/) and the NMSU Horticulture Publications page at http://aces.nmsu.edu/pubs/_h/.
Marisa Y. Thompson, Ph.D., is the Extension Urban Horticulture Specialist in the Department of Extension Plant Sciences and is based at the New Mexico State University Agricultural Science Center at Los Lunas.