[...] When a creature moves into or within the area, it takes 2d4 piercing damage for every 5 feet it travels.
The transformation of the ground is camouflaged to look natural. Any creature that can't see the area at the time the spell is cast must make a Wisdom (Perception) check against your spell save DC to recognize the terrain as hazardous before entering it.
I'm uncertain how recognizing the terrain works:
Does succeeding on the Wisdom (Perception) check reveal the entire
area of effect, or only the square(s) you were about to enter?
If you take damage from spike growth, are you now aware of the area
of effect, or is it still camouflaged?
Part 1: Succeeding would reveal the entire area as being hazardous as long as the entire area can be seen by the creature in question. -the logic behind this is that the spell states "the terrain is revealed as hazardous" which implies that the whole terrain is revealed. If it was meant to apply only to the 5 foot cube you were stepping on the spell would of said so.
What its saying is that if a creature did not see you cast the spell they have to roll for perception when they see the area you casted spike growth on to determine if they notice the spikes.
Part 2: If they take damage they are aware that they got hurt but not that spikes are there. - the logic behind this is that the spell does not state that the area is revealed as hazardous if it damages you. Therefore you would have to roll for perception again or maybe investigation(up to your DM) to determine if the area that they are on is hazardous. They could also roll for medicine to see if they could determine if the wounds are spike wounds but that's up to the DM.