Wendync,
Honestly, I hate these plants from big box stores. You could have root congestion, lack of nutrients, lack of TLC. The company forced 3 plants to live very close for life.
Ed

First off welcome :) Your hibiscus needs water daily with temps above 70 and being in the sun. When the temps reach 80 and above water it 2 times a day once in the morning and once at night. Hibiscus are water lovers so they need alot lol. misting also helps especially in the morning hours when plants are wanting to feed. Fertalizing should be done once every two weeks with a Bloom booster from March to August to help them out. Winter overing Hibiscus are not that terrifying bring them in when temps reach anything under 45 degrees, I would spray them with an insecticidal soap before bringing them in, aphids love hibiscus and can destroy your plants. make sure you face your plant in a southern facing window so they get plenty of sun they will loose leaves its normal in the winter. -Evie

I think it is something else that no one has mentioned - the color of your pot. Almost all plants hate to have their roots cooked and a dark colored container will absorb more heat. Being in a container is not the same for the roots as a natural environment where the roots are below ground. I purchased a plant a couple weeks ago and until I got around to planting it, it sat in its black nursery pot out in full sun. Well, the second week, when the really high heat hit (90s), it started almost immediately with the yellow leaves. I got it in the ground and the yellow leaves stopped almost immediately. I bet recently true summer has started for you, too.

I realize your zone and that you cannot plant the hibiscus in the ground. So you can do several things: move it to a light-colored container; move it to where other plants shade the container (put it in between some low shrubs or bushes where the trunk/leaves still gets full sun); or plunge (bury) the pot in the ground until winter. All three will help tremendously.

Carol in Jacksonville

Carol,
This is a great point. I have moved several of mine to light colored pots for that reason. I will also be adding light colored 1" gravel on the top of the soil this summer to protect those surface feeding roots. I guess I take this part for granted as it took me all winter to find a light grey replacement for 4 plants living in an all black pot.
Ed

Carol, that's some really good advice and something I've never thought about. In future I'll remember to look for light colored pots to buy. Thank you :)

Oh, when I first started gardening a few years back, I just could not figure out why lots of plants were struggling, even though I had them in the right conditions and was giving them the right amount of water. Then I heard/read/thought about plant roots and how anytime a plant is in a pot, the roots are in different conditions than if the plant is in the ground. The roots are forced to encircle and are at a much higher temp than if in the cool ground. I now keep any plant waiting to go in the ground in the shade - with the exception of that hibiscus - I wanted it to have plenty of sun. In the second week, the temps shot up, the roots cooked and the plant got mad, LOL.

I hope Wendy, the original poster, will post back and let us know if any of these suggestions helped.

Carol

I will try your suggestion and repot the plant in another container. To give you an update, I have noticed that the plant looks wilted almost all of the time, despite if it has just been watered or not. However, I do not think it is *overwatered*--- I only water when it feels dry, not just at the surface but a little ways down in the dirt. It certainly never goes more than 2 days without water and never seems bone dry.

The leaves really seem to yellow drastically overnight, however it is still blooming and setting buds just fine. Maybe the roots are getting too hot..