Girdling trees with a weed-eater or mower because they were not
properly mulched with a ring of mulch where none of it touches the
trunk.
Using just one sprayer, which could result in diluted herbicide being sprayed on valuable plants already under stress due the need to spray an insecticide.
Improper pruning, besides topping and crepe-murder covered elsewhere, ranging from leaving hat racks or worse cutting to close and improper cutting techniques, not using the 3 cut method, resulting in damage such as were a heavy branch falls before the cut is finished and pulls a strip of bark off of the trunk. Failure to prune can be just as bad if co-dominate leaders and crossing branches are ignored, but an absurd case of unnecessary pruning took off at least four grafted branches on a 5-in-1 fruit tree.
Planting to deeply especially plants such as dogwood, azalea, and camellia, which are shallow rooted and/or prefer constantly moist yet never wet soil.
Watering to much (especially any on pavement) or to little, relative to the ideal amount equivalent of about an inch per week including rain, where standing water is obviously to much while runoff could be because the rate is to fast, but may still not be enough if not soaking several inches into good soil. Clay soils become saturated with less water than is required for sandy soils. Infrequent deep watering is better than frequent shallow watering thus an irritation timer should support weekly rather that just daily watering. Morning is the best time to irrigate since everything is already wet from the dew because there is no additional risk of fungal diseases prolonged exposure to excessive moisture. A moisture sensor can help, but I have seen this fail, thus cutting off the irrigation system during a prolonged drought only to turn it back on shortly after the first rainfall in months, resulting in the death of several large caliper trees in a newly installed landscape.
Using a treatment for a
water feature meant to eliminate algae, which results in all the fish being killed instead.
Doing things without knowing what (it is), how (it can be moved), and/or why (it was left in the way) have resulted in potential state champion trees like American Chestnut (2) and red maple being cut down and left to rot.
