Spike Growth area question

[Emerald]

In the spell Spike Growth, the area is 20 ft square/level. There are times where 20 feet would be useful, but more would just make things to difficult. Can you forsake part of the area? In other words, if I am 5th level do I have to cover 100 ft or can I choose any area up to 100 ft? It seems to me if you have to cover the total area possible, this spell actually get less useful the higher level you are.

 

[Mordeth]

I see no reason why you couldnt layer the squares on top of each other. It wouldnt do more damage or raise the DC any, but would get rid of the extra squares.

Meteor Swarm talks about overlapping areas... and it even does more damage per area, but that is a 9th level spell too