Quick answer: Mulch on Beach Lots — 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 standard instruction — three inches of coarse arborist chips over the root zone, kept back from the trunk — was written for loam. On a beach lot it fails for two reasons that have nothing to do with the chips being wrong and everything to do with the ground underneath them.
Reason one: the chips leave
Dune sand is uniform, fine and light, and it does not interlock the way a graded soil does. A layer of chips sitting on it has nothing to key into. The first northwesterly of the season moves the surface sand, the sand carries the chips, and within a winter the mulch ring has migrated to the fence line and the leeward corner of the lot. What is left is a bare patch where the tree was, and a drift of chips against whatever stopped them.
This is not a cosmetic problem. A mulch layer that has moved has stopped doing the three things mulch does: it is no longer moderating soil temperature, it is no longer holding moisture, and it is no longer suppressing weeds. The tree is back to bare sand with the added insult of a buried fence line.
The fix is not “more chips.” It is a heavier, larger-particle mulch that stays put: coarse bark nuggets, or a two-inch layer of gravel over the root zone, or chips under a jute mesh pinned at the edges until the tree’s own litter takes over. Gravel sounds wrong for a garden and is exactly right for a dune: it does not decompose, it does not blow, and it holds the sand beneath it.
Reason two: the layer becomes a lid
Sand drains fast, which is the one good thing about planting in it. Put two or three inches of fine organic matter on top and you have created a boundary between a coarse, fast-draining layer and a fine, water-holding one. Water moving down through the chips slows at the interface, spreads sideways, and the sand immediately under the mulch stays saturated far longer than bare sand would.
On a beach lot that is already a few feet above a salt water table, a saturated layer is the beginning of root rot. The tree that dies two summers after a well-intentioned mulch job usually dies of this, and the mulch is blamed for the wrong reason.
What the salt is doing while all this happens
Salt spray lands on the leaves and on the soil surface. On bare sand it is flushed down and out by winter rain. Under a mulch layer it accumulates in the top inch and stays there, because the mulch intercepts the rain and the salt crystallises in the chips instead of being carried away. Plants that tolerate salt on their foliage can still be killed by salt concentrating in the root zone, and a mulch ring is a very efficient concentrator.
What to do instead, in order
Plant species that expect sand: Monterey cypress, shore pine, the native dune grasses, sand verbena, and the low ceanothus that grows on the bluffs north of town. Amend the planting hole and nothing else — a wheelbarrow of compost mixed into the backfill, not spread across the surface.
If a mulch is wanted for weed control, use gravel or a pinned jute mat and skip the organic layer entirely. If organic matter is wanted, incorporate it rather than layering it.
Water deeply and rarely, and water at the drip line rather than the trunk, because on sand the water moves down fast and sideways slowly. The rest of the coastal planting problem is drainage, which is the same subject as soil and root zone remediation seen from the dune side.