Have not been able to save my gardenia plant. Please give me the steps to follow.
You did not mention how the plant looks -- whether the leaves have dropped or if you have an infestation of some kind. Here's an article about growing gardenias.
https://www.gardeningknowhow.com/ornamental/flowers/gardenia/gardenia-plant.htm
You may find other articles here that specifically address the problems you're having: https://www.gardeningknowhow.com/search?q=gardenia
My gardenia is the shape of a tree; it has black stuff on top of the leaves. I would like to know what this is and how to treat it. This plant is I know 5 years old and I don't want to lose it so please help.
Your gardenia likely has sooty mold, which forms on the honeydew left by sucking insects. If you examine the undersides of your leaves you should find the culprit. Often it is whiteflies, aphids or scale. Here is an article that tells you how to get rid of the insects. Then the mold will go away or can be wiped off.
https://www.gardeningknowhow.com/plant-problems/disease/how-to-get-rid-of-sooty-mold.htm
https://www.gardeningknowhow.com/ornamental/flowers/gardenia/gardenia-pests.htm
lant growth. I keep it moist. In the beginning I noticed some leaves turned yellow and brown but lately I noticed that new green l eav es are growing and looking healthy. Which makes me very happy. However, the only bud it had just fell off before blooming. What did I do wrong? What can I do to make for this doesn't happen again.
I see multiple issues, here. I notice symptoms of infection, likely from saturated soil. I see signs of spider mites, as well.
Both issues together will be very difficult to correct, and they often lead to a plant's death, fairly quickly.
The give it the best chance, it will be necessary to treat for spider mites, then infection.
Here are some articles that will help with that:
https://www.gardeningknowhow.com/plant-problems/pests/insects/spider-mite-treatment.htm
https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm
Here is an article that will help you to know what care this picky plant requires:
https://www.gardeningknowhow.com/ornamental/flowers/gardenia/growing-gardenias-indoors.htm
All bought at same time, all same type, size, etc. All get equal amounts of sunshine & water. All equally spaced between the next one. So why are 4 of the 5 growing & blooming wonderfully & the one on the end is the runt of the litter? It doesn’t bloom nearly as much as the others & the shrub/bush/leafy part of it is not nearly as full & filled out as the others.
What a mystery. The first thing I'd do is check the smaller plant for bugs, mold, leaf issues or other problems. I'm assuming they're all being fed the same way. Could this one be exposed to something the others aren't? Is it getting a little less sunlight - a little more shade during the day? Perhaps it has a root problem? It may be just the nature of that particular plant, but I'd examine it, regardless. Here's an article that may help:
https://www.gardeningknowhow.com/ornamental/flowers/gardenia/no-flowers-on-gardenia.htm
Our website has quite a few articles about gardenias, but I think this one might be most helpful.
I live in McLean, VA, US zone 7b. I bought a Semi mature gardenia plant in a pot and reported it in a big container with a drainage hole and saucer. Its on a sunny spot on my deck (~6 hours of sunshine daily). While I see new buds and leaves growing, I noticed the stems being covered with a thin papery brown skin and some leaves are turning yellow. I water it every other day and drain the excess water from the saucer. Is the brown stem covering a disease? Thank you!
Watering on schedule will lead to root rot, usually. Watering should be done as the soil dries, completely, down to about 1 or 2 inches or so. This is a delicate balance, since they do not tolerate dry soils, nor wet soils. The action of drying and re-wetting is what oxygenates the roots, and letting it remain constantly moist will cause the expected. Lack of oxygenation to the root zone. Conditions, such as this, will lead to the environment that promotes microbial growth.
It is important to let the soil dry out, just a little, between watering.
I would treat with a fungicide. This article will help:
https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm
This article will help you to grow Gardenias, which can be quite finicky:
https://www.gardeningknowhow.com/ornamental/flowers/gardenia/gardenia-plant.htm
I have changed them to a larger pot, put in gravel at the bottom, added more soil, and placed them in a semi shaded spot on my front porch.
There are quite a few reasons why this could be happening. This article will help you to address those issues: https://www.gardeningknowhow.com/edible/helping-a-gardenia-bush-with-yellow-leaves.htm
d desinfection gel dried on them. Will this kill my plant?
You plant will likely not notice. If you are seeing signs of distress, it will likely be due to environmental conditions. These plants are, notoriously, finicky.
Proper care will be necessary. This article will help:
https://www.gardeningknowhow.com/ornamental/flowers/gardenia/gardenia-plant.htm