Being observant and brutally honest, I was thrilled to choose the topic of this post, Reality Check/Don't Do This. I hope you find my interpretation insightful as to where I live and work - the arid, high desert southwest.
May you will look deeply at your own locale's horticultural practices, including the why's, the how's, and the many possibilities.
I could write an extensive book about this topic with heavy imagery, but unfortunately, I have limited time. For the concepts lacking photos, the reader will have to visualize supporting images. For plants improperly designed or improperly maintained, you are encouraged to look at my countless past posts or conduct online searches on any plant in question.
My partial list was narrowed down to the following:
Buying and using plants without considering mature size (and thinking they can always be "trimmed" or "shaped" to stay in-bounds)
Buying plants based on their flowering season appearance, not dormant season appearance. ("Design for summer & it's pretty in summer. Design for winter & it's pretty all year." - Tara Dillard) I opt for 1/2 to 2/3 evergreen in any landscape where I live, having 4 seasons, with summers and winters too extreme for much plant or flower growth.
Above, in spring. Below, in winter; all gray and brown - powerless.
Primary succession tree species (softwoods or short-lived Arizona Ash, cottonwoods, willows) chosen over climax tree species (hardwoods like mesquites, oaks, pistaches, etc); speed over quality. The former have invasive surface roots, weak wood, and have a drinking problem - trash trees. Like oversized tumbleweeds. OK on large lots that collect water, far from pipes, hardscape, and structures. These are not trees that have much of a place in our outdoor living spaces, just open space and some park areas.
They did everything right according to some "experts" here...cool microclimate (root zones NE of wall), high water perennials underneath, and water-sucking bluegrass in the property behind it. Guess water doesn't compensate for forcing a plant from 4000'+ higher into the desert, with desert soils, desert heat, etc. That is 2 to 3 of the earth's life zones colder and wetter than anywhere in Albuquerque, to find where aspens grow naturally...similar to 5-6 actual climate zones different.
And logic does not compensate for willing ignorance.
The bias in Las Vegas is to push warmer. Oops. This after a quick low temperature 15F above the all-time record low. Dead queen palms are from Brazil, and quite happy in Orlando. Orlando...Las Vegas. Hmmm.
Not using native plants, especially sculptural accent plants
Too much clutter, no unity - a one-of-everything, "stick-gardening" approach (and you know what's about to die w/ white trunks):
Covering ground without creating beauty or an inviting space. Landscaping to just cover ground is _____.
Confusing popularity with one's expertise (it didn't stop after high school; some are shocked at those who fit this description)
Confusing common with appropriate or native
Ponderosa pines above were once dizzyingly popular in Abq, and you can see how well they perform above. They are not from anywhere near the desert where this photo was taken. They are popular once more, and aspens have become popular in the last 10 years. I posted a few photos of how poorly aspens grow here. Yep, those are flatbeds with some...people here salivate over having a bunch of each, and the seller will plant them if you buy in quantities. I can't make this up!
Letting lawns and gravel expanses constrain planting bed sizes; should be the other way
Fads (trends) over fashion (classic design principles)
Higher water-use trees & an understory of xeric plants, in the desert - instead of a tougher desert region tree of the same family. A little extra water does not help most times.
The above didn't work, so let's keep doing it. "Who cares about David's opinions, anyway. Didn't he move?"
More Honeylocust trees in the median, above, hanging on. And below, the next series of medians - same soils, same aspect, same climate, same irrigation, etc. Desert Willow (often not considered a "real tree"), along with Texas Red Oak, were planted at the same time as the honeylocusts; they are thriving and outgrowing what is considered "a real tree".
Plants used randomly with no relation to the architecture of buildings or space; plants, including natives, do not have to be used naturalistically
Implementation without an actual, drawn plan
Above is the latest rage in terrible pruning...limb up every shrub. Below is what a Russian Sage should look like.
Above was a Beargrass, and below was a Sotol. Remember what those are supposed to look like, in all their glory?
Above, an older Desert Willow; below, one under 10 years old. They grow quickly, allowing them to be pruned with a habit and interior open enough, even retail signage can be seen through the branching and foliage.
Expecting instant gratification without a massive budget for refined materials and the largest plants