Hard thorns can pop 2 bloons each and pop any Bloon type.
~ In-game description
Hard Thorns is the first upgrade of Path 1 for the Druid in BTD6. With this upgrade, Druid thorns pop 2 bloons per thorn. Thorns with this upgrade can pop Frozen Bloons and Lead Bloons, and Spirit of the Forest on-track vines can pop any bloon type. It also makes the Druid's thorns have a slight purple tint.
Hard Thorns increases the pierce of each thorn by +1 and allows the Druid to pop all types of bloons, excluding Camo Bloons without Camo detection. Usually, its main use is for the early-game, although Hard Thorns crosspath can be useful in a few cases where there is a lack of external pierce buffs available. That being said, if lead-popping is unavailable then this upgrade can help make up for this limitation.
It is a useful early-game option for countering grouped bloons with Druids.
When paired with Thorn Swarm, the 1-1-0 Druid becomes an effective option against early grouped and spaced bloons.
However, with Obyn Greenfoot and the Hot Magic MK, this upgrade is only going to be really useful for popping Leads.
Try setting the Hard Thorns on the Last targeting to thin out as many of the stronger early-game bloons as possible, plus Radar Scanner from the Monkey Village and most DDT's will be destroyed quite easily.