Posted: 25/04/2013 at 12:47
If you want a hedge, or any plant, to grow well you have to give it good soil. Digging isolated holes in tarmac and planting into compacted clay is unlikely to work as the roots will find it hard to penetrate and grow and then they'll find it short on air, nutrients and water.
You could dig the same holes and plant fence posts in concrete or you could clear a trench the length of the boundary at least 2' wide and then dig over the soil to a depthe of about a foot and work in plenty of well rotted manure and/or garden or bought in compost. If the clay is very solid, you'll have to dig deeper to loosen it up and add some fine pea grit so the roots don't end up sitting in a sump and drowning
The best time to plant a hedge is autumn when you can plant cheap, bare root whips. They'll grow roots over the winter and will then need little maintenance apart form an anuual top dressing and a regular trim when they get big enough. Hornbeam does well in heavy soils.