The Postal Service™ has recently received inquiries regarding the placement of plant seeds and other extraneous materials on or under postage stamps when they are affixed to a mailpiece. This is a reminder that extraneous material is not to be placed in, on, or under postage stamps on mailpieces. Any type of substance that may adulterate the postage stamp is prohibited. This includes plant material, samples, or any type of HAZMAT material.
— Product Classification, Pricing, 12-24-15
