Answer: California is an agricultural state. As such the USDA has restrictions on certain plant material or soil that might harbor destructive insects or disease pathogens. Consider how devastating a population of the wrong insects could be to a food crop, and you'll understand the reasoning behind a plant quarantine. Some growers in other states can have their nursery stock inspected and certified safe for importation into California - it just depends upon the grower and the restrictions of the USDA.
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