Because cold air, being denser than warm air sinks, low lying areas of the garden can be several degrees colder. Consequently frost may occur in these areas when there is no frost evident anywhere else in the garden. Plant tender species on higher ground or on slopes where the cold air will flow past the plants as it moves to the low point. Any sloping area is less prone to frost, because the cold air can't settle there as easily. Precondition your plants to withstand cold temperatures by discontinuing fertilizing in early September, so that no new foliage is on the plant when cold temperatures arrive.
Older leaves are much tougher and more able to withstand a frost.
When the inevitable occurs and a frost is predicted, there are several things which you can do to protect your plants. Water the garden thoroughly before nightfall. The soil will release moisture into the air around your plants during the night, keep the air somewhat warmer. Even a slight breeze will prevent cold air from settling near the ground during the night. You can help keep frost from forming by providing this breeze artificially with an electric fan.