The Blightmoon

The moon is the source of all life and according to many theories the source of all magic. Regardless it has a very close link with Briar and can heavily influence it.
Under the influence of Mopestia, once every 50 or so years, tho this time can vary greatly, the pristine white moon turns a deep putrid green for a night that lasts 24 hours. During this time creatures of natures are emboldened and more hostile. Plants grow faster and tend to be more slimy and putrid in nature. Many tree's start secreting thick poisonous ooze and growing roots that trap any intruders. In general nature grows incredibly inhospitable and dangerous.
After the Blightmoon ends the changes it has made to plants disappear but any damage that was done remains and the appearance of many plants stays altered for months or years after.