While rune dust is created automatically after creating one, rune shards and strengthened plant cure are not.
Along with rune dust and rune shards, strengthened plant cure is one of only three quest items from Garden of Tranquillity that are dropped rather than destroyed.
The player can create rune shards and rune dust without starting Garden of Tranquillity, but nothing interesting happens when attempting to put rune dust into plant cure to create strengthened plant cure. After the quest, the plant cure can still be created.