Opinions needed
 

I made these leaf blocks but they don't look right with out a stem. What do you think , should I add stems and how , with out adding more bulk to the seams. I already have 12 done this way and need 18 more.

There's a product called "Quick Bias" which is a fusible bias tape. It comes in different colors. You could use this to make stems. You could also make your own fusible bias, or, as the area is short, you could just make a straight stem and attach with a fusible.

....... or do an applique stem?
.............. or embroider the stem?

Though once you add the stem, you are losing the view of the secondary pattern that is forming where the HSTs meet.

Having gone as far as you have, I would leave them as is, and join them up. Kind of like autumn leaves on the forest floor, with bits of evergreen scattered amidst.

I think they are beautiful just as they are. I recognize them as leaves and has been pointed out, there is a secondary thing going on there. Very pretty colors.

I think they look absolutely fine and leaflike without stems, just maybe a tad less "literal".

Totally agree, they look fine as they are.
That being said, if you really want stems.. I like the idea of emoroidering them.

If you want stems buy some bias tape to form them or find a bias strip maker and make small strips that can be hand sewn or machine sewn on. No need to do much more. you might rip a couple stitches on the leaf to tuck in the stem end and another tiny hole on the green triangle.

You have several suggestions on how to add a stem. You might take a piece of bias tape and place, or use temporary adhesive to see if you like the blocks with a stem.

I think they are very pretty, just the way they are.