In this climate plants can grow almost year round. We have had to get a new stand for our Christmas tree - American trunks are thicker on a 2 m tree than are Danish trunks. And our back yard is a statement to the fecund conditions ... our Redwood trees top 40 m and the oak trees are 1.5 m in diameter.
Valley Oak resemble the oaks in northern Europe. The most common kind is Live Oak, an evergreen. And it is a good thing that is doesn't drop all the leaves in the fall, because they are such horrid, spiky, unpleasant leaves that nobody will enjoy jumping in a pile of them.
It is common to see tree houses. We would have loved to have such a house for the kids but our trees start branching out at the hight of 5-7 meters above ground. Call me a chicken, that is OK.
Trees like that require professional help when they need to be pruned. Even if you dared climb around 30m/100 ft above ground, the branches are hoisted down, they are so heavy. So check that your tree climbers are insured - not only if they drop a branch through your roof but also if they themselves get hurt in the process. Otherwise you may be responsible for their hospital bill.
If you don't prune your trees, they may fall over or drop a branch on you. Fortunately, it doesn't happen too often but people have been killed or maimed by dropping branches - one of the victims a long time acquaintance of ours. Another incident was when our neighbor across the creek lost a big tree. It suddenly decided to lie down - in the next neighbor's back yard. Nobody got hurt but it crushed all small trees and bushes in the neighbor's yard. We try to look after our trees, not the least because if they fall, they will probably land in the dining room.
In the summer, you may find your patio furniture covered in honeydew, a sticky substance that supposedly is aphid digested oak sap. Don't let it stay on the car too long, it is hard on the laquer. At that time of year, small acorns and leaves stick to your shoes whenever you venture outside. We have left our old 10 sqm umbrella over the furniture. The umbrella looks accordingly, but the furniture doesn't stick to you if you forget to bring out soap and water every day.
A month later the acorns start dropping. When an acorn drops from 30m and lands on the top of the chimney, it sounds like somebody threw a huge firecracker. We have gotten used to the annual bombardement but when we have visitors from overseas, they fly out of the bed if an acorn really hits well. Annoying? A little, but it could have been worse: The Buckeye chestnut trees are not close to the house - the chestnuts are the size of tennis balls.
Our fence line to one of the neighbors is a row of orange and lemon trees. The pleasant part is naturally to harvest the golden orbs - less pleasant is the task of covering the trees when you suddenly realize that it is going to freeze a lot tonight - the trees have 3 cm long thorns. Silicon Valley is one of the few places on earth where you can grow cherries - that need a touch of frost - and oranges - that can't take too much - side by side. If the oranges freeze, the little juicy cells in the fruit pop and when they thaw again the insides are mush. The insulating rind offers a little protection but not if it freezes 5 below. If it really freezes we have a couple of big tarps that we spread over the trees. We may even string a chain of old fashioned Christmas lights under the tarp; the heat from the bulbs is enough to add those 1-2 degrees and save the harvest but the modern LED lights don't give off that kind of heat. While we pick the fruit in February-April we can enjoy the smell of new flowers, the start of next year's harvest.
There are two schools for laying out a garden. One is focused on water conservation. If you grow local plants, they are adapted to the seasonal climate with rain in winter-spring and summer-fall all dry. The oaks suck up all the water they need during the wet season. If you water them - by having a lawn to close to the base of the tree for example - they will wither and die. Redwoods are good at leading the water rolling in with the morning mist down to its roots, you can't plant anything that needs watering under a Redwood neither. Other plants are active during the spring and otherwise stand like dry sprigs. Many gardens have cactus, dry, exotic grasses and little else for much of the year.
As bacteria need moisture to thrive it takes a long time to break down the dry sprigs and waxed, thorny leaves into mulch. Consequently there isn't much mulch to speak of. Below the sparse topsoil you find adobe clay and a pickax is a normal garden tool around here.
The best use for adobe is to make bricks. Mixed with water, grass, and cow patties adobe bricks provided material for many of the buildings from the Mexican period. But the bricks weren't burned, just dried in the sun, and unless you keep the houses covered in protective lime - all the Missions and churches from that era among them - they will slowly return to their former components.
In spite of the water issue most gardens have lawns, hence the propagation of sprinklers. You build a lawn by shoveling off 10 cm topsoil/adobe, install irrigation pipes, spread new topsoil over the pipes, spread a plastic net out that will keep moles and gophers from popping up, and roll out the lawn on top. If you for whatever reason decide to dig through the lawn - putting out stepping stones or digging down a pipe from the gutters - you can rest assured that same moles or gophers will know exactly where the plastic netting is no longer whole and leave their mark.
With the drought going on its 4th year, it is more likely that your will be tearing out a lawn than installing one. If remodeling, considering installing gray water recycling (any water but the toilets) for watering the garden is also an option.