Try a mid-grey background, don't work from black.
You can use slightly darker outlines to make it stand out from the background, but straight grey stalk is boring and unnatural anyway.
Don't dither unnecessarily, just add another colour.
Shape is too blobby.
Try drawing from reference.
Pick trees or a mushrooms you like the look of in pictures and draw them.
Sketching lines on paper first if you don't have a wacom.
Draw many, not just one.
Pick the best one/s then scan in if you can and trace over, or try to do the line art as best you can.
Then you can start looking at colours.