There is already a whole in the botom of the cup to allow drainage. once the plants are too large for the cups, transplant them and use the cups agai...
My mother came up with this fantastic idea to reuse her K-cups instead of just throwing them out. (and goodness knows we use enough of them around here.
Step 1: Keurig k-cup
After a coffee (or other beverage) is made from the cup, dont throw it out. Set it aside and wait for it to cool down. You may want to set it on some paper towel because they do have some water in them still so they will leak a bit.