The autumnal equinox occurs during the third week of September. It is the time when the sun crosses the equator making day and night of equal length on all points of the earth. After the equinox in Autumn, the days grow shorter.  In September we stop fertilizing plants, but we continue deadheading our fall bloomers, as we want flowers to continue coming until frost. The purpose of cutting off the spent flowers is so the plant won’t produce seeds. Once seeds are set, a plant shuts down because it has completed its primary purpose, which is to reproduce itself.  When we deadhead, we snip the spent flower off right above a leaf axil where the topmost leaves join the flower’s stem. When deadheading roses, cut just above a group of five leaves on the stem, as this stimulates the development of new growth.  Carry a bucket around the garden to collect the flower heads as you cut them off the plants. Then set the bucket of spent flowers in a cool dry spot out of the sun, and when the flowers are completely dry, place them in an airtight container and add some fixative such as orris root to which you have added a few drops of perfumed essential oil available at hobby shops. Your potpourri will be then ready to use this winter.