The SI unit would be grams, however considering a blade of grass is pretty small I would probably measure it in mg, milligrams.
That one!
Metric ruler
liter
The gram.
Centimetre
That one!
Metric ruler
You would use meters.
Grams.
Kilogram
a millimeter
Millimeter
The kilogram.
Millimetres
Millimetres.
The gram.
liter