hawthorn/blackthorn

we have recently planted some hawthorn and blackthorn we want to create a thick hedge they are about 3 foot high when is the best time to cut them back and will this make them thicker from the base??

 

I cut my new hedges in May and September to thicken out. I have just cut old one now, before Birds start nesting. I always leave one third of the growth on each cut. This cutting will thicken out but be cautious of rabbit protectors. I really didn't need mine and it benefited by taking them off, even if a year late.

 

Just hedge trim the sides lightly this year then lay it next winter.