R.I. Gen. Laws § 2-18.1-7: Certificates.

Nursery inspection certificates issued by the director shall bear the date of issue and be valid until the fifteenth of September following their issuances. The director limits the expiration date whenever it is necessary to insure freedom from pests of the nursery stock so inspected and certified, and, in the case of stock that is shipped out of state, it is permissible to designate a different expiration date when the date has been agreed upon in writing between the director and the plant regulatory officials of the state of destination.