.Net Form Garbage Collection Behavior?

I'm creating a windows forms program and have recently noticed a large amount of memory slowly being eaten by the application. I went through my program and tried to tie up as many loose ends as I could in relation to object creation but noticed that the program would still keep memory for every form loaded until it eventually crashed.

I decided to make a simple test program which sole purpose is to pop-up a new form with a few controls, wait for 200 milliseconds, then close the form and repeat the process 30 times every time I hit a button. I found it caused the same behavior, even with the only new object creation being that of the form. So, I decided to manually invoke the garbage collector in a subroutine which handles the formclosed event. During tests I would get some memory back occasionally, but not enough to make a difference in the long run. So I start setting the form variable to nothing before I called GC.collect(). The program will initially eat a few 1000K of memory before leveling off and using only 20-40K for every 30 forms created.

Now what I'm wondering is, what is the point of an automated garbage collection system if it requires constant manual evocation and explicit dereferencing of all new objects to be effective? Have I completely lost grasp on the scope of VB.Net forms and it simply never marks forms as available memory to be cleared?I would have thought object memory references created within a form would be marked as usable when the form's close or dispose routine is called and no references to objects created within the form exist anywhere outside the closed form.