The answer to 'when to hedge' is that the best time to hedge depends on the type of plant and your landscape goals. In general, the ideal time to trim and shape hedges is in late winter or early spring before new growth begins. This allows the hedge to fill in and grow evenly over the spring and summer.
For fast-growing hedges like privet or yew, you may need to trim them 2-3 times per year to maintain the desired shape and size. Slower-growing hedges like boxwood or holly may only need trimming once a year. It's important to avoid trimming during the hottest months of summer, as this can stress the plants.