TL;DR: apparently the management at my apartment complex considers plants to be "Seasonal decoration." Here's the email chain:

My landlord: You are in violation of your lease with the condition of your patio/deck area. The only thing allowed on your porch is patio furniture. Correct the issues shown in the attachment ASAP! If you have any questions please contact the property Manager M-F 9 am – 5 pm.

Thank You from our office staff and maintenance team! [Attached photo: My balcony seen from below, showing my one plant.]

Me: I've just re-read my lease agreement and could not find the clause you are referring to. The closest thing I could find is this, from the Community Rules Addendum, Use of Unit and Common Areas:

"3. It is the Tenant’s responsibility to keep balconies and patios in a clean and orderly condition. Storing of cartons, garbage, boxes, appliances, or other visually objectionable items on the patio or balcony is prohibited. Clotheslines or clothes hanging over balconies are not permitted. Hanging clothes and towels out to dry or use of the patio or balcony as a general storage area is not permitted. Seasonal and holiday decorations are not allowed without prior written permission from Landlord. If allowed, seasonal and holiday decorations must be reasonable, must not interfere with or disturb other tenants’ quiet enjoyment of the Community, and must be promptly removed within a reasonable time after the holiday, not to exceed 14 days or as otherwise directed by Landlord. Grilling or barbecuing is not permitted on balconies or patios."

It does NOT say "The only thing allowed on your porch is patio furniture," nor does it make any mention of prohibiting plants. If you are referring to another clause, please let me know.

My landlord: While there is no direct mention of potted plants, they fall under the category of seasonal and holiday decoration.

Potted plants are considered seasonal items not currently permitted by the property manager. If you would like to, you may stop by our leasing office during the week to speak with the property manager regarding the use of unit and common areas.

Me: Plants are not "seasonal and holiday decoration." They are plants. They do not get put away for the winter, they do not celebrate a holiday, they just sit there. All year long. If potted plants are not permitted, then the lease agreement needs to be rewritten to reflect that. The lease which I signed does not prohibit potted plants.

My landlord: If you have any comments or concerns regarding use of unit common areas, please stop by our leasing office to speak directly with our property manager.

Me: If you have any further concerns, please contact me via email. Otherwise, I consider this matter resolved.

Thank you for your understanding

(That last email was sent after closing time. I guess I'll find out tomorrow how that goes over.)