(Reuters) – Amazon.com has banned the sale of foreign seeds in the United States after thousands of Americans received unsolicited seed packages in their mailboxes.

The U.S. Department of Agriculture (USDA) in July identified more than a dozen plant species ranging from morning glory to mustard in bags of unsolicited seeds. The Americans were warned not to plant the seeds.

According to plant experts, seeds from other parts of the world may be non-native varieties that are harmful to commodity crops.

“Going forward, we only allow US-based sellers to sell seeds,” Amazon said in an email statement Saturday.

The company on Wednesday changed its policy on seed sales. The policy change was first reported by the Wall Street Journal.

The company added that sellers who do not follow its guidelines will be subject to action, including the removal of their accounts.

According to Amazon’s Policy Web Page, the ban extends to plants and plant products.

The USDA said in July that packages are often part of a “breeding” scam, in which people receive items they do not want from the seller, and then post false positive customer reviews to boost sales.

Osama L. Lissy, deputy administrator of the USDA’s Animal and Plant Health Inspection Service (APIS), said in an August 11 update that experts analyzing some seeds from China found very few problems. El-Lisi added that the two countries were working together on the investigation.