A potted hibiscus on our west facing fifth floor balcony has had some leaves turn yellow and drop off during the last two weeks. It has not been fertilized. It had a number of buds which just started to blossom again.
There are a number of reasons why your hibiscus has yellowing leaves and it may require a bit of investigating on your part. Yellow leaves typically occur when the plant is stressed and this could be caused by:
- Under watering
- Over watering
- Too hot
- Too cold
- Too much direct sunlight
- Not enough sunlight
- Too much wind
- Improper nutrition
- Insects- particularly spider mites
Re watering: ensure that your pot has proper drainage. During the warmer summer days and when your plant is in bloom you will most likely need to water daily. You can check your plant by sticking your finger about an inch into the soil to see if it is dry. Conversely as the weather cools and when you move your pots inside, you will need to water less.
Re temperature: as mentioned above, in the extreme heat more water will be needed. As tropical plants, hibiscus like temperatures between 18-29 degrees C. Although this may not be the cause of your yellowing leaves now, plants will also drop leaves if in a drafty window where they are too cold.
Re sunlight: too much sun can cause stress on hibiscus that are not used to it and they can respond with yellow leaves. Conversely if they do not get enough light, they will drop some of their leaves so that they don’t need to support as many.
Re wind: you mentioned you are on a fifth floor balcony. This could expose the plant to more wind than it is accustomed to. Wind can dry plants out quickly.
Re fertilizer: you mentioned you have not fertilized. According to a grower, Hidden Valley Hibiscus, a condition known as chlorosis can occur when there is nutrition deficit. Leaves will remain partly green and partly yellow when there is a nutrition problem. If they do not turn completely yellow nor fall off, then it is likely that the problem is a lack of essential nutrients. Fertilizing your plant and/or amending the soil can help this.
Re insects: have you had a close look at the leaves of your plant? The most common insects on hibiscus are spider mites and aphids. Further information on how to treat infestations can be found at the University of Maryland https://extension.umd.edu/hgic/topics/aphids-houseplants and the University of Nebraska https://byf.unl.edu/e1a12689-6543-4199-a5f8-39a30fcbb37f.pdf
From the above information, I hope that you are able to determine what is causing stress for your hibiscus. Best of luck!