Amazon has banned the sale of foreign seeds to its customers in the United States, a move that comes after thousands of U.S. residents received unsolicited packages of seeds over the summer, most of them postmarked from China.

The company updated its policy Wednesday, advising its foreign sellers that it “would no longer allow the import into the U.S. of plant or seed products.” Amazon prohibits the sale of products the U.S. Department of Agriculture labels “noxious” and those that are subject to government quarantine or can be fatal when touched or consumed.

“Moving forward, we are only permitting the sale of seeds by sellers who are based in the U.S.,” an Amazon spokeswoman said in a statement.

For sellers who use Amazon to fulfill orders and store items in the company’s warehouses, the updated policy will go into effect Sept. 30.

The update to Amazon’s policy comes a little over a month after mysterious seed packets bearing Chinese characters started arriving in mailboxes across the United States in July, prompting all 50 states to issue safety warnings.

The U.S. Agriculture Department’s Animal and Plant Health Inspection Service disclosed an uptick in reports of unsolicited packages of seeds starting in late July.

“USDA is also working with e-commerce companies to remove online sellers that are illegally importing or facilitating the illegal import of propagative materials, including seeds,” the agency said in a statement on its website. “We have also intensified our engagement with e-commerce companies to ensure they and the sellers who use their platforms are complying with USDA regulations.”

The Animal and Plant Health Inspection Service said the unsolicited packages appeared to be a part of a “brushing scam,” in which an online seller ships an inexpensive product to an unwitting receiver and then submits positive reviews on the recipient’s behalf with a goal of boosting the seller’s ratings and visibility online.

As of Aug. 31, the inspection service had collected more than 8,500 packages of seeds and had examined more than 2,400 individual seed packets. It said it had identified more than 300 seed species, including cabbage, hibiscus, lavender, mint, morning glory, mustard, rose, rosemary and sage.

The seeds pose no known health risks, the inspection service said. The main concern is that the seeds could negatively affect U.S. agriculture by introducing seed-borne viruses and other diseases.

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