I good way to balance against this spell in my opinion is instead of nerfing it, is first through narrative. If a creature would recognize that the something is amiss with the ground (extra dense foliage in the forest or lots of detritus in a building or street) make them check and then if they succeed, have them jump over it reducing the total damage, or all of it if the creature has the speed and strength to clear the growth.

Quote from Further_Reading >>

This spell is pretty fun with the new genie warlock. Get grasp of hadar/repelling blast. Drop the spikes at the right spot and each beam does +4d4 damage from the enemy being pushed/pulled across. 

 Wow, I never thought of playing a warlock with a Dao patron, because I prefer Dijinni, but now I realize getting Spike Growth is a huge boon.  I really want to see the look on the DM's face when their beloved baddie is been turned into a yo-yo or soccer ball.  Plus, you get resistance to bludgeoning damage, which is extremely common in any campaign that's not too intrigue-ish.    

first level druid? spike growth is a 2nd level spell.

EDIT: to @CooperS

"When a creature moves into or within the area..." 
The damage has to trigger either when you enter or when you exit a square with spikes, you can't have it both ways (otherwise you would be taking the damage twice every time you move through a spiked square). For this spell it's when you enter, maybe for other spells it's different. 

Alright alright. You all have some really overpowered strategies with this already but let me introduce you to mine: 

Build: Aarakocra, at least level 3 in Astral Self Monk and at least level 3 in Druid (both focus on Wisdom, making this a good multiclass anyway) 

Turn 1: Bonus Action activate Arms of the Astral Self, giving 10 ft range and the ability to use Wisdom to grapple, action to cast Spike Growth

Turn 2: Action grapple target (from 10 ft away using WIS mod), movement+bonus action (step of the wind) to fly above the spike growth with the grappled target dragging along the floor 10 ft below for 60 ft of movement ((50+10)*2/2) gets you 24d4 or 60 damage.

Repeat for following turns, adding in action to dash giving 90 ft movement or 36d6 or 90 damage (1 ft=1 damage on average). Any increases in movement speed increase damage greatly, for example with Haste from a friend, Boots of Speed and a higher level Monk (one of my characters who is at level 10), you get average 560 damage (base 70 *2 from Haste *2 from boots *4 from action, haste action, bonus action, and movement /2 from grapple /5*2*2.5). Also, you're 10 ft away, meaning that they cannot melee attack you with most weapons. If the target escapes the grapple, well they're in the middle of the spike field where it's difficult terrain and it's pretty easy to catch up to somebody when you fly at 127 mph.

Anything they can do to you, you can do to them. Cast it where they're standing then move to a place where you can't be seen so they have to move. Use creatures with misty step or flying, use ranged creatures. Bring along a caster that can dispel it. It's concentration so target the spell caster until it drops. Throw a few Flameskulls at them that not only fly but have resistance to piercing damage so their ranged weapons are going to be less effective and gets magic missile and fireball. Heaps of stuff you can do.

Heck, if they're regularly fighting against some group of enemies or an organisation have that enemy learn and bring along counters like enchantments that give them resistance to piercing damage or steam powered pogo sticks that allow them to only touch 2 squares as they move across. Maybe they bring along a specially designed steel golem or armored wagon that has immunity to piercing damage and doesn't lose half movement from difficult terrain?

i'm only wondering if its a 20ft Sphere or circle, the icon next to the 20 ft seems to suggest its a sphere so the soil undernead is also spiky and that air above is overgrown into a dome of spikes or is it just a circle on the surface of everything thats just mildly inconvenient from those that can fly up an out without taking the damage

If you read monster stat blocks you'll notice that it says that they have resistance to bludgeoning slashing and piercing damage from non-magical attacks.

Since this is not an attack, it doesn't count for resistance. This is also why monsters with those types of resistances take full damage from falling.

Too bad it doesn't upcast....

Not really, that'd make it even more broken.

*

Some of the comments are over using the spell.  Just like Opportunity Attacks only happen if the target moves themselves out of melee range, this spell says:
"When a creature moves into or within the area"
Not 
"When a creature moves or is moved into or within the area."  

If you force them to move they wouldn't take damage just like with Opportunity Attacks.

--
Opportunity Attacks text:
"For example, you don't provoke an opportunity attack if an explosion hurls you out of a foe's reach or if gravity causes you to fall past an enemy."

I feel like some of the comments here are forgetting or maybe not recognizing that, this spell is dangerous only if its cast out of line of sight of people, which is unlikely.  The perception check to determine if its dangerous is only a requirement if you didn't see it get cast, which means anyone that sees the druid cast this spell knows something is funky with the ground and would not intentionally walk through it unless it was necessary.

I like to reflavor this spell as "giant patch of LEGOs" or "field of d4s"

Quote from Lizz123 >>

I was wondering, if I cast this spell on a zone that I'm on it, do I take dammage?

 yas

Does the Perception check count as an action to recognize the terrain as hazardous?

With an extra +4d4, since you can trigger both Grasp of Hadar and Repelling Blast on the same shot.

 they would still take damage . if they are moving within the area willing or unwilling .

This spell is amazing. I once had a lvl 1 character take down a huge creature in one shot using nothing but a spell scroll of Spike growth, a trip-wire chain, and some really good rolls to avoid getting chomped as it chased me.

If you have a beast that will cover the entire 20 foot area of the spell and it trips on top of it, that's an automatic 32 d4.

Would the air within the area also be affected? The area icon looks to be a sphere which seems to suggest that it would. 
If it does, then you could have a fairy Druid cast the spell, then fly over the area, cast thorn whip to pull the target up 10 feet, dealing 1d6+4d4 piercing damage. Then on their turn they would fall 10 feet back down, taking the 4d4 piercing damage again and taking 1d6 bludgeoning damage due to fall damage.

This spell mixed with plant growth would be insane. The plant growth would force a creature to spend 4 feet of movement to move one foot. If this spell was first cast and then plant growth, a person would be taking 2d4 damage every round and would be unable to move regardless of damage, leaving huge leeway for additional attacks. One creature I love that combines these two spells is the grung wildling. If he were to cast spike growth for an ambush and then plant growth, whatever creatures walking into their trap would be taking major damage.