Ice cream sticks of birch works great as plant labels. Do you write with a waterproof pen on the sticks so they long.
One disadvantage of wood can be that they suck up so much water and that the texts to write be blurry. These sticks by björk sucks up water not as easy as they are absolutely ideal for this purpose and they are pretty cheap, 37:- for 200 St. The sticks I use is 11,5 cm long but there's both shorter and longer.
Available to buy here CChobby