This is the time when people ask what there is to do in the garden now that it's raining and plants and trees are going dormant. Winter is a busy time to do many tasks in the garden now that weed growth and falling leaves have slowed and rain is soaking the ground.
It's a good time to turn the soil and spread mulch. Turning the soil with an amendment like compost adds air and nutrients to hard, compacted soil, which plants need for good root health. Spreading mulch helps preserve soil moisture around plants. It also keeps weeds down and reduces the need for irrigation watering once the rain slows and spring returns. Be sure to keep mulch away from the wood at the base of plants and trees. Burying the base of plants and trees in soil and mulch allows fungus to grow which can cause disease and death.
Winter is also a good time to prune plants and trees when they're dormant, because they will keep their pruned shape longer than pruning during the growing season. Some plants and trees will ooze a milky sticky sap if pruned at the wrong time of the year, opening their wood up for diseases to form.
Here in the East Bay, the threat of freeze usually happens in December through the middle of January, so I don't like to plant at that time. New plants that haven't rooted into the soil could die in freezing temperatures. Other than that, as long as the soil isn't too muddy, we can plant any other time of year. It's great when plants are dormant, the soil has been turned and rain is regularly soaking the soil. The irrigation system can be shut off now, as cool temperatures keep plants dormant and regular rain ensures the soil doesn't dry out.
You can purchase and plant "bare root" now, which is a healthy and optimal way to start a plant or tree:
Winter is also a good time to assess the irrigation system. During the warmer growing season when the irrigation system runs regularly, it's not as easy to repair and make upgrades. During the winter months when the system is shut off, more time can be taken to determine what the system needs and parts can be ordered and installed.
There's many more tasks I could discuss but I'll end here and just mention if you still have tomato plants hanging on from summer in your garden, go ahead and pull them out. Store the cages for next season and amend the soil. If you have winter greens and squshes in your garden, check the leaves for pests like aphids and ants that increase with rainy weather, and harvest to enjoy during the holiday season.
Finally, if you have a chance this winter to get muddy and splash in a puddle, do it! Waterproof boots make that especially enjoyable...