Planted Tank. What is the SHORTEST aquatic "gr" I can get for my tank?

I have a 20 gallon tank with 2 small driftwood pieces and one large one. I have a few plants, even growing on the driftwood, but I want something that will cover the bottom but won't grow so tall. I don't care about the price, or how hard it can be to find it. The bottom of my tank has those little TINY pebble looking ...things... (I'm not sure what to call them...mini rocks). No CO2 but once I get the gr, I will purchase some... :)