Quick answer: Mulch on the Coast — 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.
Inland, a mulch layer does three things: it holds moisture, it moderates soil temperature and it suppresses weeds. On this coast it does a fourth, and the fourth one changes what material to use.
Salt lands on the surface
Aerosol arrives with the onshore wind and settles on whatever is facing it. On bare soil it is flushed down and through by the first rain. On a mulch layer it is intercepted, and the mulch then dries, concentrates the salt in its own tissue, and holds it there. The next rain carries a slug of concentrated brine into the root zone rather than a dilute solution.
Which is why a coastal bed can be saltier under a mulch layer than beside it, and why a plant that scorches in a mulched bed sometimes looks better in bare ground a metre away.
Materials that shed salt and materials that hold it
The variable is how much the material itself absorbs and how fast it dries. Fine, soft, absorbent materials — compost, grass clippings, straw — take up salt solution and hold it. Coarse, hard, waxy materials — bark nuggets, pine chip, gravel — shed most of the run-off and dry quickly.
Gravel is the extreme case: it holds nothing, it does not decompose, and salt runs off it into the soil where the rain can flush it. On a bed within sight of the water, gravel is often the right answer and it is almost never the first one considered.
Placement matters more than the material
A ring of mulch around a salt-sensitive shrub should stop short on the windward side. That sounds odd — the windward side is where the plant needs protection — but the mechanism is that the mulch is what concentrates the salt, and the windward roots are the ones already under the most stress.
Leave a bare, slightly dished strip on the windward edge so that run-off and rain move the salt away from the root zone rather than into it.
The wind problem
Anything light leaves. On an exposed lot the material that stays is the material with enough mass and enough interlock: bark nuggets, coarse chip, gravel. Straw and grass clipping mulches are gone by February and end up against the fence.
Which means the practical coastal mulch list is short. Coarse arborist chip for the root zones and the beds, bark nugget where appearance matters, gravel where salt and wind both matter, and nothing fine on an exposed site.
The depth, and why it is lower here
Two inches, not four. On a coast where the soil is often sandy and fast-draining, a deep layer creates the perched interface described in mulch on beach lots, and the salt concentration is greater in a thicker layer because there is more material to intercept the aerosol.
Keep it back from trunks and stems, and re-check the ring each spring, because the winter wind will have rearranged it.
What this is really about
A coastal garden is a garden with a salt budget. Every surface that intercepts salt and holds it is borrowing against that budget, and mulch is one of the few surfaces you get to choose. Choosing a material that sheds rather than stores is the whole of the intervention, and it costs nothing extra.