Cold air is heavier than warm air. If your garden is near the top of a slope, it may miss some late spring frosts or early autumn frosts because cold air tends to slide down the slope and settle in depressions. If your garden is in a depression, the total frost-free season there could be shorter than the average for your area. On gentle slopes, a tight fence, hedgerow, or vine-covered fence will block some of the air flow. Thus, the advantage of air drainage is lost.
Strong winds are a handicap to some tender plants.
Such plants are injured by cold winds and frequently they are blown over. Around the house and garden there are many little swirls of wind and some quiet places. Tight hedges reduce wind velocity considerably. If the wind strikes a wall, it may swirl vigorously around the corner of a house. You can identify these spots of low and high wind by observing where snow or dry leaves are blown away and where they accumulate.