Month of September Gardening Tasks
It is best to stop fertilizing all your perennial plants and trees in September. You want the stems and foliage to gradually harden off to prepare them for early frosts. If you continue to feed, this will encourage new young and tender growth which can be zapped by the soon to come cold air. Do not bother to feed annuals since their life cycle is about finished.
Now is the time for all plants to finish their reproduction as flowering and seeding and prepare themselves for the dormant season. Food storage in roots or bulbs is essential for its survival until next spring. Stop giving them the wrong signal and cut back feeding, but maintain very good watering so that the plants are not stressed.