A grease (wax) pencil is perfect for this task. Water and sun proof, the marking can also be removed if you like.

I like the Listo brand of grease pencil the best.....by far. But do your own research and review study.

LISTO grease pencils

I like the Staedtler Garden Marker pens for home use. 2-paks are available through Amazon. But for nursery signage (I worked in a garden center for many years) we always had a dedicated sign printer that produced laminated copies and could include photos. These last for years and offer a very professional looking presentation. If available I would always opt for these over something that was hand written.