Everything's a mess.

fixed :D

x = Rnd(1,640)
y = Rnd(1,480)

this looks wrong (always use superstrict - it helps find stuff like this)

i think it should be :

newgrass.x = Rnd(1,640)
newgrass.y = Rnd(1,480)
DrawImage newgrass.image,newgrass.x,newgrass.y

Method UpdateGrass()

You should probally make this a function too.

Check out the tutorials about types n stuff in the tutorials section - there is a great one about someware...

... here

edit:

also if ur gonna do it this way - MAKE sure you remove it from the list once its been drawn otherwise ur gonna have all kinds of problems.

Great, it's working now :D

Well, most of it. I can't get the grass to stay where it was originally put. That was the point of the UpdateGrass function (I changed it back like you suggested). Any ideas?

UpdateGrass is never called in your example so only the new PlantRandomGrass grass is drawn. You're using keydown(KEY_G) which might be stacked and calling PlantRandomGrass more times than you expected.
You could change it to Keyhit.

Wow, forgot to put that back in. It works perfectly now :D

Thanks so much.