Of course there are many reasons why a tree might languish. The duds often have poor root systems. Try as one might to free the roots when planting, a badly potbound tree faces an uphill climb. Living on the main raccoon thoroughfares is another challenge. Sometimes the ring-tailed bullies paw branches off my young trees even when there is no fruit.
Often, though, languishing seems related to water. Hillside shrubs and trees did much better after I terraced them. Ornamentals that were barely hanging on took off after transplanting away from nearby oak trees. I hadn't really considered water as a possible problem for Kari, Suebelle, Bonnie, and Ross, since they are irrigated. But finally it dawned on me that those four trees lagging trees are all close to large oak or pine trees. Some soil probing indicated that those titans had noticed the moist, well-fertilized soil around the upstarts, and had not been shy in inviting themselves to share the treat.
I decided to try trench warfare. The big trees have highly redundant root systems, and can easily part with a chunk of them. With help from our muscular MRFC Secretary Josh Starry, I dug trenches and cut the invading roots. They proved to be near the surface, and the slash pine seemed more aggressive than the oaks. Here's a trench with some crossing oak tree roots, actually not very many here compared to some locations. Dig, chop, and refill, to give the finished product shown below.
We'll have to wait to see whether it helps, and if so, how long it lasts. Horticulture is too complicated, with too many factors at play to carry out many controlled experiments, so much of the time we must rely on what seems to work for us and those we know. And on perseverance. If this doesn't work, we'll try something different.