The spell says:
choose a point within range. All normal plants in a 100-foot radius centered on that point become thick and overgrown. A creature moving through the area must spend 4 feet of movement for every 1 foot it moves.
It sounds like your question is: "the spell doesn't say that the plants are required in order to impede movement, so technically this spell impedes movement even without plants, right?"
That's technically correct, but I think any DM would interpret the spell to require plants to work. If you cast this spell in a place where there are no plants, you should expect the DM to tell you that the spell has no effect.