I’ve seen very few fatally over-watered garden plants in New Mexico. It tends to occur with new plantings, a time of more enthusiastic watering, but even then it seems limited to ‘touchy’ desert plants - young yucca and native sagebrush, for example. What I do see is a lot of is chronically under-watered plants.

When people show me an ailing plant, and ask “What do you think is wrong?”, I’m almost always looking at long term drought effects: stunted plants with small off-color leaves, and little new growth. There’s often a physical history you can ‘read’ on the plant of multiple near deaths from sun scalding, twig and branch die-off, etc., and so while a plant may have received some ‘extra’ water, it clearly wouldn’t have been enough per watering, and far too much time between waterings. It can be such a source of pain that I think some would almost rather know a plant is suffering from an incurable disease. Yes, nutrient deficiencies and diseases are common, but even many of these problems can be traced to insufficient watering.

Good news: watering issues are easily testable without sending out to a lab. Water better using deep water cycling for 2-3 weeks, and see if there’s a good response. That’s the test. Definitely do this before before turning to ‘soil amendments’ or fertilizers, and never fertilize a dehydrated plant: https://extension.unh.edu/resource/fertilizing-trees-and-shrubs-fact-sheet

There are so many things that can be said about plant needs, so many exceptions to general rules, and so many disasters that will remain a mystery, but assuming your plants are appropriate for given light and soil conditions, aren’t sitting in a concrete pocket of caliche, aren’t getting peed on by the dog, etc., their most basic need is water, which they require for the following reasons:

Photosynthesis

Water is an essential ingredient in photosynthesis, the process by which plants use sunlight, water, and carbon dioxide to capture energy in the form of sugar (glucose). It is also the process responsible for storing all the energy we extract from fossil fuels, crops, and all of our food. https://www.ocean.washington.edu/research/gfd/has222-archive/lecture7B-06ClaraCarboncycleOutline.pdf

Transport of Nutrients

Water moves nutrients and minerals from the soil into the plant through its roots and up to the leaves. Water is the delivery system that allows plants to absorb, transport, and use nutrients. Without it, nutrients are unavailable - even if they're present in the soil. Water stress also reduces root activity, further blocking nutrient uptake.

Maintaining Turgor Pressure

Turgor pressure is the force exerted by water inside the central vacuole of plant cells, pushing the cell membrane against the rigid cell wall. It’s maintained when cells are full of water, creating internal pressure that keeps the plant upright and strong and enabling new growth. Without it, plants can’t grow effectively - even with nutrients and sunlight - because turgor pressure helps move sugars and other compounds through the phloem, the plant's nutrient transport system.

Cooling

Plants lose water through pores (stomata) in their leaves during transpiration. This helps cool plants while pulling more water and nutrients up from the roots. “A fully grown tree may lose several hundred gallons of water through its leaves on a hot, dry day. About 90% of the water that enters a plant's roots is used for cooling under warm dry conditions.” https://my.ucanr.edu/sites/marinmg/files/152980.pdf

Biochemical Reactions

Water acts as a solvent inside the plant. Nearly all of a plant’s biochemical processes require water to occur.

Deep Watering Cycling

Deep watering is equivalent perhaps to a good hour long rain, or about an 8” depth of soil saturation. It is much more than most people imagine is necessary per each watering. For general maintenance, and especially when talking localized drip irrigation timing, basic requirements are usually expressed as a few gallons a week for smaller plants, a few gallons more for larger shrubs, and about triple more for larger trees (tree root zones extend over a much larger area as well). However, those numbers in gallons, across X number of days, aren’t fully useful since they don’t account for specific plant needs, or the changes in temp, humidity, wind evaporation, etc., that result in faster/slower soil drying per week. The true amount necessary - which can only be determined by the visible health of your plants - will be a combination of deep watering (8” depth is a good consistent measure) followed by a drying cycle. The time to water again is when the top 2 inches of soil are again dry and crumbly, and certainly just as soon as you see leaf wilting - even if it’s in the middle of a hot day.

During the drying period, oxygen and other gases flow back into soil pore spaces which plants need at their root zone for energy metabolism. Soggy, oxygen poor, soil is referred to as anaerobic or anoxic (not exactly the same thing). In any case, plants adapted to low oxygen soils - wetland plants, for example - can tolerate both low oxygen levels and the types of microorganisms found there. Xeric dryland plants cannot.

So, deep watering forces oxygen and gases out, and a drying period let’s them back in. It’s the optimal cycling that brings everything into balance, and allows the most effective use of water. Shallow watering, such as when you lightly wet a soil surface on a hot day, does little more than force a desperate plant to develop shallow surface roots that are then exposed to more extreme heat and deadly drying.

The term ‘over-watering’ is bit of a misnomer to me because it’s really a matter of under-oxygenating soil as part of a wet/dry cycle. I don’t worry about applying too much water as long as I get the second part right. I probably should be more concerned about over-watering because heavy flooding can cause problems with soil compaction and nutrient leaching, but that’s another story for another day …