The ground in a 20-foot radius centered on a point within range twists and sprouts hard spikes and thorns. The area becomes difficult terrain for the duration. 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.
"That day, the druid cast spike growth where a gargantuan, half-burrowed Sandworm stood… and for the next half hour, everybody stopped playing and started frantically browsing through the manuals to figure out what to do."
So, the question does size matter…?
I take for granted that you can choose as the epicenter of the spell the point where the creature touches the ground: it won’t influence the space where the body of the creature is, nor anything below, but the surrounding terrain on the ground level should be influenced (although you don’t really see such point, the spell doesn’t require you too, contrary to the usual routine). So, the token of this sizeable creature occupies a 16 (4×4) squares space on the grid (12 hexagons if you’re into that). At the start of its turn, it’s gonna find himself in the middle of a semi-hidden spike field. Since the spell only hurts whoever moves into or within the area, I infer that creatures who find themselves already in it and decide not to move won’t get hurt, all the more if they’re half-burrowed. Now, if the creature notices the danger (and it should since it was there) but still decides to move above the terrain (although I guess he could return underground where half his body lies without repercussions), how much does he get hurt? The spell mentions a damage x movement ratio, and with smaller creatures it’s no problem. But what about bigger monsters? Is this 2nd level spell a colossus bane, which indirectly does x4 damage to large monsters, x9 to huge ones and x16 to gargantuan ones per square (=5 feet)?
RAW, I’d rule against it: bigger creatures aren’t affected multiple times by effects that target more than one of their squares (think fireball: no matter the size, if the spell hits just a square or the whole circumference of a token, the damage only hits once). That said, seems to me like this huge AoE spell should indeed scale with the size of its victims as more spikes pierce through their flesh. Also, it wouldn’t be the first time that low level spells were hugely effective against specific creatures (heat metal against full-plated enemies comes to mind).
What do you think?