It all depends how old and established the plants are. Dig them up with as much rootball as you can manage. It is best to prepare the new holes before you start to dig them up. This way they will be out of the ground for as little time as possible. Water into the hole before you plant them and let it drain through. Then thoroughly water again after planting. Make sure to tread around them firmly to get the soil into contact with the rootball. Water all spring and summer, even if it rains.

Thanks for getting back to me. I only planted last year when I bought them at 4' and very healthy looking. They seemed to be ok but over winter the leaves have started to go brown and fall off. I did risk planting them in only 2' of soil on top of hardcore. Sounds silly now but it was to extend my four year old hedge which is doing really well. I thought I would make room for them in amongst the ones doing ok and have bought some chicken pellets to give them a boost. I really don't want to lose them. Could you please give me an idea of how deep they need to be and how close in width they can go.
Thanks

You might be best to dig them out, dig out the hardcore where they were and refill with topsoil from elsewhere. Then replant. If you leave them they will probably die of drought in the first dry spell of weather.A bit of a job but worth it. If it will take more than a day to dig out the hardcore you can heel the plants in somewhere else in the garden temporarily. To do this dig a trench and put the plants in at a 45 degree angle, backfill and firm down the soil. Water in. Plants can stay like this for a couple of months without harm. Or tuck the rootballs into plastic bags and keep moist, but just for a week or so.

Thanks Volunteer

Could I increase the height of the soil instead of breaking through the hardcore and find something to retain the soil. If I could what depth will they need ? sorry to be a pain but want to try and save them !