Wraith Roses

Wraith Roses are a subvariety of roses that have grown with magical properties. Their thorns are laced with a kind of poison that will sap at an individual's strength over time. Individual Wraith Rose encounters are rarely fatal, and are moreso a bother to many who encounter them. However, walking through a field of them could absolutely be deadly. Large fields of these roses have grown by killing those who walk in, and fertilizing themselves with the bodies of those they kill, then growing larger.
Wraith roses have in some cases been used as a way to show affection, as a quest to find, safely extract, and return a rose to a lover is a dangerous feat. Many fields or concentrations of Wraith Roses are also accompanied by predators, looking to take advantage of weakened prey. 
Wraith roses are light pink in colour, and grow at all times of the year in temperate climates.

Mechanically, brushing a Wraith Rose's thorns will reduce your Max HP until your next long rest by 1d8 unless a DC 18 Constitution save is made. 
WARNING: this is for a few thorns. Walking through a bush, or gods forbid a field of them will incur increasing penalties.