Most garden plants want more summer water. Many established natives and Mediterranean plants want less. This difference is the easiest thing to get backwards, and it will undo any good those natives were planted for.
Once they are past their first year or two, regular summer irrigation is what kills them, not drought. Most established natives want only a deep soak every two to three weeks in the heat; that is the Theodore Payne Foundation's summer care advice. Some want none at all.
The first year or two is different
New plants need frequent water to get their roots down, and then they need to be weaned off it. Natives especially need hand-holding the first year or two, then need the water backed way off. Skip that step and a plant that should have become self-sufficient stays on life support instead, struggling to breathe.
This is also why natives go in the ground in fall and early winter. Cool weather and winter rain do the establishment watering for you. Summer is the worst time to plant them.
Water according to plant community
Where a native grows in the wild tells you how to water it. Plants from coastal strand, coastal sage scrub, chaparral and alluvial wash go dormant in summer, and summer water rots them. Riparian plants, from along a stream or river, want more. Desert scrub and montane conifer plants take an occasional deep soak.
Southern California has eight of these communities: coastal strand on the beach and dunes, coastal sage scrub on dry coastal hills, chaparral on dry mountain slopes, oak woodland under a native oak, riparian next to water, alluvial wash in an old dry riverbed, desert scrub in the low and high desert, and montane conifer in mountain forest. Microclimates shift these lines, and plenty of plants grow in two or three of the eight.
Mediterranean plants, and a few specifics
Mediterranean plants such as rockrose, lavender, rosemary and thyme are killed by wet roots, not by summer dryness: winter waterlogging in poor drainage, or summer water in warm soil.
No summer water for manzanita, flannel bush or most native sages. Ceanothus in a garden takes one or two deep waterings a month. Leucadendron wants regular summer water, and Leucospermum an occasional deep soak, weekly only in a hot inland summer.
Native oaks are the hard stop
Keep any supplemental water at least ten feet from an oak's root crown, or out in the outer two thirds of the root zone, whichever is farther. No lawn, no irrigation and no summer-watered plants inside that zone. The dry zone sits inside the drip line; the feeding roots run well beyond it.
This watering program helps control Armillaria and Phytophthora, and the oxygen starvation that comes with a wet collar. According to UC IPM, four to eight hours of saturated soil is enough for Phytophthora to take hold.
Give them their own valve
A native sharing a valve with a lawn is on death row. Zones should be used for plants with similar water needs, according to plant type and microclimates.
When you do water, water long and deep, not short and often. Long, slow soaks get the water deep, and the roots follow it down, where the soil stays cool and damp and the plant can ride out heat. Set run times for early morning, when wind and evaporation are lowest.
The full rule, with its illustrations of each plant community, is in Southern California Landscape Maintenance: Ecological Maintenance, Simplified, at rootandbranchdesign.com/book. Its appendix lists the plants in each community and the summer water each plant needs. The first chapters are free to read at rootandbranchdesign.com/book/sample.
A native sharing a valve with a lawn is on death row.