Disclaimer: This is not a legal advice but only general information.
California law requires that ashes are unrecognizable to the public and unobtrusive.
Public land such as parks, public gardens and streets all have their own rules and regulations that have been set by the town. No state or federal laws exist about spreading ashes on these types of property. Check with the town officials if you want to spread ashes in these areas. Spreading cremated ashes on uninhabited public land such as rural woods is generally allowed, but you should also check with town officials. Follow general etiquette and refrain from scattering ashes on trails or obvious places because some people find scattering cremated remains offensive.
To scatter cremated ashes on private land, ask the owner for written permission first. If the owner refuses, do not attempt to access the land, or you can be charged with trespassing and misdemeanor. You are allowed to spread ashes on your own private property. You are also allowed to keep cremated ashes in an urn or container on your own property or in your home. If you are not sure if an area of land is private or public, check with your town hall first or consult a town map.
Federal law states that spreading ashes on the sea, which is considered a burial at sea, must be done at least 500 yards away from the shore. Federal law also requires that you report the burial at sea to the closest Environmental Protection Agency office within a month of the burial. Spreading ashes in inland waterways, such as rivers and lakes, falls under the Clean Water Act and may require a permit from the local government. Tthese laws are designed to protect waterways from pollution and because cremated human remains are not a health hazard, it is not usually enforced in these cases.
Scattering cremated ashes in public graveyards is usually allowed, although some towns have recently passed laws banning the practice, so check with your town officials. The ashes are generally spread on a grave, in a crypt or in a scattering garden within the graveyard. To scatter ashes on old family graveyards found on private property, you must ask for permission from the landowner. If you are scattering the ashes on another person's grave, such as a friend or relative, ask the immediate family of that person for permission first.
National parks are protected and have minimal human influence, helping them retain their natural beauty and making them an appealing location for scattering ashes. The common policy for national parks allows the scattering of ashes with the permission of the park superintendent. However, some parks, such as the Joshua Tree National Park in Riverside County, California, prohibit all ash-scattering for fear of disrupting or contaminating the existing human remains or other historical elements of the properties. For information regarding each park's ash-scattering policy, the National Park Service provides individual park superintendent contact information at NPS.gov.
While it is lawful to follow the rules and regulations of local, state and federal governments regarding the scattering of human ashes, some public lands - such as many rural wooded areas - have no laws regulating the scattering of ashes. In such cases, one is under no obligation to obtain permission to scatter human ashes, but should refrain from scattering ashes in a way that could be offensive to others. For example, it is suggested scattering ashes at least 100 yards from public trails or other populated areas.