Nothing at this time of year - chemical feeds should only be applied during the growing season. Doesn't sound as if you prepared the ground beforehand - the addition of humus rich materials whilst doing the preparatory digging prior to planting a hedge makes a huge difference to the health of the plants, particularly in poor soil. Suggest you at least apply a good thick mulch of humus rich material, such as well composted manure, leaf mould, garden compost, soil conditioning compost, anything like that, next Spring, having first applied a good sized handful of Growmore around the base of each plant.

A good, well aged (48 hours) compost tea this spring should give it a jump start, along with the solid compost for long term effect.