DMs need to think when this spell is used:
What plant life is present where the spell is being cast? What characteristics does it have?
What effect will a fireball, burning hands, alchemist's fire, etc ... have on the plants that are holding onto the victims?
If there is a single plant (ie; a single tree, bush, etc ....) wrapping around the victim, how sturdy is that plant? Could it be ripped out by the roots? Could it be hacked with an axe?
The final line of the spell says it all: the spell may be altered by the nature of the plants. Drop the DC if there is only tall grass present. Drop it a lot if the grass isn't so tall.
This spell is incredibly simplified in CRPGs. In a Pen and Paper game, you *must* use your own judgment to make it work well.
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With regards to the argument that this spell is bad because it is overpowered when it can be used to full effect, most spells have that problem ... they are more or less useful in certain situations. After all, most 7 CR monsters don't fear a burning hands spell ... unless they have fire vulnerability. This spell just happens to have a wider swing. A druid that memorizes it may find that he wasted a spell slot if he never fights in a location where there are plants or wildlife ... or he may single handedly best the foe with one spell and a few missile weapon shots.