Jan 1st marks the beginning of California's recreational marijuana laws, meaning adults over 21 can purchase and enjoy their cannabis on private property, but can you smoke in your backyard? If your backyard is private property then yes you're welcome to smoke, however rental properties and multi-unit living situations can become more complex if other tenants complain of smoke or smell, possibly breaching a rental contract. A suggestion to keep these risks minimal would be using a vape pen which emits no smoke or scent like combustion.
Smoking on private property, including in the backyard, is legal under state law -- this will apply to marijuana as well.
Neighbors may complain to the landlord about the smell or use of marijuana (it's still illegal under federal law and is a Schedule 1 drug).
This in turn could lead to a ban at a complex if one is not already in place, just to keep the peace. At which point a transgression or complaint could mean you are in breach of your rental contract, even if you do not face state criminal liability.