My young cosmos plants have been outside hardening off but have now developed brown edges on leaves. Not sure if its wind scorch or sunburn. Should I trim off the burnt edges or leave well alone?. New growth coming through ok from centre of plants.
Cosmos are quite tender until established and the cold weather has passed. The scorch marks could be due to cold temperatures/frost overnight. If they've been watered or rained on, then exposed to sun (sun rays act like magnifying glasses on water droplets causing burns), that will cause scorching but not usually on the outer edges alone, like you mentioned.
My guess is cold temperatures and if you've already planted them in beds, cover them up overnight for the next few days as a nasty cold front is on the cards for most of the UK.
The new growth is promising though!