New hedge

jamielwellsJguwzBAJ

I'm looking to plant new mixed hedge (Blackthorn, Red robin, Hawthorn & Hornbeam) between existing conifer hedge, which I have cut down to 1.2m and removed all branches from trunks (hoping trunks will eventually die off). Does anybody envisage any problems.

JennyJ

The conifers will have taken all the goodness out of the soil, so dig in as much organic matter as you can get - well-rotted manure or garden compost (not potting compost) if you can get it. You might also find that the soil is too full of conifer roots to dig the good wide planting holes needed to give your new plants a good start. Possibly worth seeing if you can rock the trunks enough to get the main roots out.

Fairygirl

You can also just pile the soil high enough for planting - assuming it's bare root whips you're using?
I've done that before -planting hornbeam in among mature tree roots of a maple and a lime, although we don't have to worry too much about keeping them moist. I didn't lose any. A mulch after they're thoroughly soaked is also beneficial.
Blackthorn can thrive in almost anything. Mine is in a tiny space beside the concrete footings of the adjacent pavement. 
Red Robin is an odd choice among the others. I presume it's Photinia? Not sure how well that'll fare. The others will be fine once established.