This are the parts I am interested in (bolding mine):
If this spell is cast on an area that is already affected by any spell or effect that enhances plants, such as entangle or wall of thorns, any DC involved with these spells is increased by 4.
Any ground-covering vegetation in the spell's area becomes very hard and sharply pointed without changing its appearance.
Seems like spike growth could/should be one of the spells whose DC is increased by 4. What do you think?
I would rule it so.