Quick answer: Mulch and Irrigation — Mulch is the cheapest thing that happens to a tree: a flat layer over the root zone holds moisture through the dry season, keeps the soil temperature steady and feeds the soil life the tree depends on. Almost every mulch problem is a depth problem or a contact problem — too thick, or piled against the trunk where the bark has to stay dry.
Field note. What we see most on this coast is not too little mulch but too much, in the wrong place: the volcano against the trunk that keeps the bark wet through the fog season and invites rot where the tree cannot show it until the canopy thins.
On the other hand, a thin ring of the wrong material is worse than none at all — matted, water-repellent mulch sheds the rain it was meant to store.
The claims made for mulch and water are usually qualitative. They can be made quantitative with a trowel, a watch and a couple of dry days, and doing it changes the irrigation schedule in ways that matter on a metered supply.
Number one: how fast water enters
Bare soil develops a surface crust from rain impact and from the drying of the top few millimetres. That crust sheds water. A mulch layer intercepts the impact, keeps the surface open, and holds the water long enough for it to infiltrate.
The field test: push a bottomless tin or a large ring into the ground, pour in a measured litre, and time how long it takes to disappear. Repeat on the mulched area and on the bare area beside it. The difference is usually a factor of two to five on a crusted soil, and it is the reason a mulched bed can absorb a storm that runs off the lawn.
Number two: how fast water leaves
Dig a hole a foot deep in the mulched area and in the bare area on the same morning, three days after watering. Feel the soil at six inches and at twelve. The mulched profile is usually still cool and moist at twelve; the bare one is dry at six.
That single comparison is worth more than any general advice, because it tells you how deep the water went, which is the only thing that matters for a tree.
What the two numbers change
Better infiltration means you can apply more water per session without runoff. Faster loss on the bare ground means the same volume has to be applied more often. Together they mean the mulched bed needs fewer, deeper cycles — which is the pattern that builds deep roots rather than shallow ones.
A practical adjustment: if the schedule was a short watering twice a week, the mulched version is a long watering once every ten days to two weeks, applied until the trowel test shows moisture at a foot.
The irrigation hardware question
Drip under mulch is the efficient arrangement, with one caveat: the emitter is now buried in a moist, dark environment, and roots will grow into it. Check the emitters each spring and clear them.
Drip on top of mulch is worse, because the water runs along the surface of the layer and evaporates before it reaches soil. If the tubing is above the mulch, it is in the wrong place.
Sprinklers over mulch are the least efficient: the water is intercepted by the layer and what reaches the soil is uneven. They are also the arrangement that wets the foliage of the plants, which on this coast is how salt burn and fungal leaf disease both get worse.
Where mulch does not help
On a slope, water runs off before it infiltrates regardless of the surface, and a mulch layer can slide. The answer there is contouring and planting rather than depth of chip.
On a soil that is already saturated, a mulch layer keeps it saturated longer, which is the opposite of what a waterlogged root zone needs. Where drainage is the problem, mulch is not the tool.
The seasonal check
Once a year, in late spring, do the two tests again. The layer has thinned, the crust has re-formed at the edges, and the schedule that worked last year is probably a little too generous. Adjusting then rather than in August is the difference between a plant that sails through the dry season and one that is rescued.
How deep the layer should be for each of these effects is the depth question.