Houseplant Care in September
Bring outdoor plants in when nights dip consistently to around 50°F, and keep them apart from your other plants for 2–3 weeks. Give a final fertilizer feed in early September, then stop until spring.
Light in September
Days shorten through the fall equinox. Move light-hungry plants closer to windows. Plants coming in from outdoors go into bright, indirect light; Iowa State Extension notes that even the brightest indoor spot gets a fraction of outdoor light, and that some species drop leaves after the move.
Watering in September
Water less often as growth slows and the soil takes longer to dry, and confirm with the finger test. Plants coming in from outdoors need a new routine too, since light, humidity and temperature all change (Iowa State Extension). Illinois Extension puts the fertilizing season between March and September.
Pests and problems to watch in September
Plants coming back indoors can carry insects or disease, Illinois Extension warns, and it advises keeping them apart from other houseplants for 2–3 weeks. Inspect under leaves, in soil, and along stems for spider mites, scale, and mealybugs.
What to plan now
Stock up on insecticidal soap and neem oil before winter problems show up.