This is the fastest way to create a hedge that can replace fencing, Plant 2 year old bare rooted whips, 6 plants per m, mainly Hawthorn (whitethorn), but once every 2 m replace one with another hedging species that tolerates trimming such as Blackthorn, Holly, Hazel, Spindle, Guelder Rose, Dog Rose, Rowan and Willow.
Therefore for every 100 m of new hedge you will need 550 whitethorn and 50 other these other species Mark out the line of the new hedge using wire, twine or lime.
To make it easier to plant and give plants a great start, you can dig over with a mini digger but this is not strictly necessary but can speed up growth.
Plant to the same depth as was previously planted Don’t bury the stem Or expose the roots Firm in Identify a few whitethorn, rowan or crab apple or other native tree (maybe 5 / 100 m) to be retained as trees and protect with tree guards or shelters Prune all other plants except holly down to 2-3 inch or so above ground level with a sloping cut to leave a sharp point Protect roots from drying out during planting by keeping plants in the bag until needed Plant in a double staggered rows with 330mm (just over a foot) between the rows and the same between the plants within the rows, often the length of a boot Push a 4 foot or 1.2m wide strip of used silage plastic down neatly over the cut stumps.
Press the edges of the plastic into the ground with a spade to hold it down Protect from livestock including rabbits. The aim is to remove that wire in a few years.
Thanks to Teagasc for this technique it is very effective If you want to see this in action have a look at this video from Teagasc https://youtu.be/CgtUP2VdILI The other option is to plant the trees and let them grow naturally for 7 years and then lay the hedge this way you will have a stock proof hedge after 11 years all going well, for this way look for strong plants at least 60-90cm tall