To grow or cultivate (something) on someone or something (transitive): e.g., to grow a beard, to cultivate plants or moss on a surface.
彼は最近ひげを生やし始めた。
He recently started growing a beard.
庭にバラを生やして花を増やした。
I planted roses in the garden to increase the flowers.
屋根に苔（こけ）が生やしてしまった。
Moss has grown on the roof.
To let something grow (often by leaving it alone), for example letting weeds or facial hair grow unchecked.
面倒で庭の雑草をそのまま生やしておいた。
I was too lazy and left the weeds in the garden to grow.
役者は役づくりのために長いひげを生やした。
The actor grew a long beard for his role.