? what are the names of HARDY hibiscus?
You are VERY limited in zone 4. This will be limited to any species within Hibiscus moscheutos, or Rose Mallow/Swamp Hibiscus.
Also, One bush's leaves are yellow
Sounds like powdery mildew. It is very hard to get rid of once it takes hold. Here is more:
https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/hibiscus-has-white-fungus.htm
Any useful information on best care for the plant Lord & Ladies. I was just given a start of this.
Those plants do best in part shade so an east-facing location would be ideal.
only one plant, but newer planting
This could be for many reasons. Unfortunately, your photos did not come through, so I am unable to pin down the exact cause, but I can tell you some common reasons why this could occur.
This could be because of planting in the heat, since this would very much affect the plant, negatively.
This could be due to poor phosphorus and potassium levels in the soil, though it isn't likely since the plants nearby are fine.
This could be due to disease at the time of planting. Sometimes, there are diseases that affect the plant, such as Botrytis, Fusarium, and Verticillium.
Another reason would be Potassium and Phosphorus deficiencies. These can cause blooming issues.
These articles will help:
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/testing-soil.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/soil-ph-plants.htm
https://www.gardeningknowhow.com/ornamental/flowers/hibiscus
Why are the blossoms on my Hybiscus plant falling?
Unfortunately, there are a few causes to your blooms falling off. There are several insects that will attack the flowers, along with many environmental factors. These articles will help:
https://www.gardeningknowhow.com/ornamental/flowers/hibiscus
. I have 5 different hibiscus plant and bought this for hydrangea plants and used it on my hibiscus plants. It caused one of the hibiscus plants leaves to turn yellow and then die off. just part of the plant. I assume I should not use this on my hibiscus. I believe it will be alright as we are at the end of out season and it is still blooming beautiful flowers.
I shouldn't kill it. It may be that there was an issue, and that after the fungicide killed the issue it may have taken a portion of the plant with it. It is best to spray a small portion to check sensitivity before spraying the entire plant to avoid potential damage.
This collection of articles will help you with hibiscus care:
https://www.gardeningknowhow.com/ornamental/flowers/hibiscus
This collection of articles will help with hydrangea care:
https://www.gardeningknowhow.com/ornamental/shrubs/hydrangea
while those pests attack more of the garden?
Aphids are easily controlled, luckily. Things like Neem oil and attracting natural predator insects to the garden can reduce their number drastically! Here are some articles that will help:
https://www.gardeningknowhow.com/plant-problems/pests/insects/homemade-aphid-control.htm
https://www.gardeningknowhow.com/ornamental/flowers/hibiscus/pests-on-hibiscus-plants.htm
https://www.gardeningknowhow.com/ornamental/flowers/hibiscus