Answer: Unfortunately, privet hedges are very difficult to garden under or next to. The privets not only cause shade but their roots also steal water and nutrients from the soil so other plants are unable to survive there. Rather than planting next to it, you might consider hard pruning the hedge and retraining it.
When the hedge grows wider at the top than at the base, it shades itself and loses foliage on the lower half of the plant. Privet should be pruned at an angle from top to bottom so that the top of the hedge is narrower than the base. This allows light to reach the bottom of the plants and keeps them leafy at the bottom as a result. Frequent pruning also keeps the plant densely branched and thick.
With a neglected or overgrown hedge, renewal pruning can be effective. Renewal pruning should be done in late winter or early spring before it leafs out for the year. To renew, cut it down very short, close to the ground. The new growth is then trimmed often, every time it grows about 6 inches it is sheared or trimmed off again. The more often it is cut, the better. This forces lots of branching at the base of the hedge. Eventually as it grows taller it is trained at an angle -- narrow top, wide base -- to keep it leafy at the bottom. (The hedge grows back quite fast because it already has a big, established root system.)
I hope this helps.
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