When Chihiro discovers the recipe for a resurrection potion, the main ingredient was found to be the poisonous Hydrangea plant. More specifically, the ones found in the series are the Hydrangea macrophylla. From Wikipedia:
Hydrangea is a genus of 70-75 species of flowering plants native to southern and eastern Asia (China, Japan, Korea, the Himalayas, and Indonesia) and the Americas. By far the greatest species diversity is in eastern Asia, notably China, Japan, and Korea. Most are shrubs 1 to 3 meters tall, but some are small trees, and others lianas reaching up to 30 m (98 ft) by climbing up trees. They can be either deciduous or evergreen, though the widely cultivated temperate species are all deciduous. 
Role in SankareaEdit
Due to the effects of the resurrection potion wearing off over time, hydrangea leaves have become a part of Babu's and Rea's daily diet, due to their ability to prolong the effects of the resurrection potion. Jogoro is also seen to have eaten hydrangea several times throughout the series, despite it being poisonous to his health and a probable cause for his loss of sanity. At the beginning of the series, there is a plentiful amount of hydrangea throughout the area in which Sankarea takes place. As the series progress, however, Chihiro must find an alternative to keeping the Zombies alive as the season for hydrangea growth comes to an end. The hydrangea's effect of preventing the zombies from decomposing is only limited and lasts for a brief time.