Where to use plants as diffusors?

I'd think about trying them on the side walls at the point of first order
reflections. Have your wife or a friend sit in the listening position while
you take a medium sized (not too large to handle) mirror and place it
along the side wall. Starting at a point even with the speaker with the
mirror against it, walk forward along the side wall. At some point your
partner will be able to see the speaker reflected in the mirror. Mark this
spot on the wall. This is the point of first order reflection, and you can
try placing a plant here. Repeat on the other side wall. You can also
tape or pin reflective Mylar on the wall and do the same thing yourself
without help. The plants would need to be at least the height of the
tweeters.

i've got them at the first order side wall points and have fairly well stuffed my big glass bay window full. helped a lot more than you'd expect. I have to add that most of the plats were at least half up the wall if not floor to cieling. i'd put short ones up on a table so they are at the level of your tweets

Being only "a couple of feet tall" they might not do much, but suggest putting them in the corners.
Good listening!