Culture Medium (培养液; pinyin: Péiyǎng yè) is an item in the Chinese version of Plants vs. Zombies 2. It has different types of rarity similar to the rarity of the plants, and 10 units of this item is required to upgrade a plant with its corresponding rarity to Level 5. It can only be bought from Endless Challenge event. From the event, you will get Time Tokens that would let you buy them.
White (Common) Culture Medium
Green (Uncommon) Culture Medium
Blue (Rare) Culture Medium
Purple (Super Rare) Culture Medium
Yellow (Legendary) Culture Medium
The Yellow Culture Medium is displayed in the shop for the Endless Zone event
The upgrading menu for Level 5, which shows that Culture Medium is required for upgrading