From Dragon Ball Encyclopedia, the ''Dragon Ball'' wiki
The Ajissa (アジッサ, Ajissa; FUNimation "Ajisa", Viz "Hydrangea Plant") is a tree that grows on Planet Namek.
Kuririn asked Dende why he had not eaten his food. He replied that Namekians did not eat their kind of food and only needed water. Kuririn asked about the crops he had seen being cultivated as he thought they were consumed by the Namekians. Dende said that those were actually Ajissa seedlings. According to Dende, Planet Namek was once beautiful and had an abundance of Ajissa trees. The Ajissa forests and most of the Namekians died out, however, due to a dramatic climate shift. The Namekians were trying to make Planet Namek beautiful again by growing more Ajissa trees.