Hi there, sound similar to one of my projects.
I once kept frozen leaves for more than a year and the quality was good. Keep your samples at -80 degree Celcius, after you snap freeze the samples in liquid Nitrogen for storage.
I wouldnt grind the leaves and store them. Ground tissues easily thaw during handling or transfer, and this may cause DNA degradation.
Unless lysis buffer can be added immediately then you will get bad samples.
I wouldn't store the leaves at -20 C make sure its -80 degree Celcius.