In northern China, folded jiaozi may be placed on bi made from dried sorghum stems.
Bi (箅) is a traditional support used while preparing dumplings. Its lattice-like surface helps keep filled jiaozi from sagging or losing their shape before cooking. The dried sorghum stems can also leave a distinctive mark on the dumpling’s underside.
Sorghum is a cereal crop long associated with northern Chinese agriculture, where its stalks have also been used for practical household purposes. The answer is not wheat or rice: wheat is commonly used to make the dumpling wrappers, while rice is not what the bi itself is made from. Bamboo can form steamers and baskets, but it is not the plant specified in this tradition.
Jiaozi are Chinese dumplings filled with meat, vegetables, or both. They may be boiled, steamed, pan-fried, deep-fried, or baked, and the Japanese version is called gyōza.