Thanks for this, I didn't know the proverb. Here's what I found on the web:
Ill weeds grow apace.
Nettles are never frostbitten.
Vices are learnt without a master.
Proverbs > Weeds
If you want to sound erudite, you can also use a Latin maxim:
Herba mala (mala herba) cito crescit.