Cholla

The nether needs to be more interesting to explore. We need more to see, and do when there. This nether plant is dangerous. Touching it causes thorns damage and it is full of lava.  If broken without a silk touch axe it would erupt into a flow of lava that would quickly spread.

                It is wide at the bottom with tendril like protrusions coming off the top.

(inspiration for appearance:)

                The plant would not be broken when the block below it is removed. It would be affected by gravity. (It would drop like an anvil or armor stand.)  It would not only be dangerous to touch, but also from just getting too close.  Similar to a jumping cholla it can sense when something moves nearby. When it senses this movement, this plant shoots quilled projectiles in the direction of the movement, knocking back any players within a three block radius and doing a small amount of damage.  This knockback would interrupt any mining, making it difficult to remove. It cannot be broken by hand. Only a tool can break it and only a silk touch axe can harvest this plant.  It is fire and blast resistant.  They should grow in small groups on netherrack.