I would recommend going for the fruit and flower achievements first, but if you still need this one, here is the best way to do so:
A root cutting is when you clip a plant from the very base node.
-Start a new free play tank, or find a tutorial that has you clip a mutation from algae (might be easier)
-Fast Forward until the algae mutates a tall grass-like node
-clip this node and plant it
-Place the speed at x1 and wait for a seed to come off of the plant
-clip the plant and re-plant it, this essentially duplicates it
-repeat until you have 20 or more plants
-now go to town and spam
and
to clip and replant them as fast as possible
These plants should only be one node high with no leaves or any other flair so 100% of your clippings should go towards this achievement