Gardenia indoors Winter in Montana
My Gardenia and I live in Montana. I was thrilled to have a potted gardenia on my deck that actually flowered this past spring/summer/fall. I moved the pot inside for winter. I continued the care I had given it through the warmer seasons...i.e. watering with the appropriate amount of vinegar/water mixture, working coffee grounds into the topsoil occasionally and placing the pot in a tray with pebbles with water. I also gave it a sulfer treatment and bought a grow light for it as there is very little sunshine in the winter. It is sitting by a window with the grow light on for approx 8 hours a day. I am noticing that the new foliage is struggling and just yesterday noticed little white spots on the underside of the leaves. It tried to bloom (one bloom around Thanksgiving) but failed. What should I do to keep it alive and thriving until Spring when I can put it outside again? Thank you!
Gardenia's are sensitive; with primary concerns for growing conditions being protection from cold temps, slightly acidic soil, watering (not too much or too little), and light.
Vinegar, coffee grounds and Sulfur - is a lot of acidity. Don't over-do it, Check the soil pH and keep it to 5.0 - 6.5.
Once you get the pH into the optimum range, discontinue the concentrated substances, as they may be too harsh. Use a slow release complete organic fertilizer for acid loving plants, like this:
The white spots on the underside of the leaves could be mealy bugs or another pest. You can wipe them off and/or use a botanical bio-pesticide:
Under no circumstances should you water this with vinegar. This is toxic to plants. Adding this along with sulfur will certainly kill the plant. I'm going to take a stab as guessing the pH of your soil.. It is probably sitting at about 4.... It needs to be between 6.5 to 7.0. You should add dolomitic lime to buffer the soil back up.
Along with this, you may have a spider mite infestation. I am going to give you a few articles that you will want to read...
This next one will give you ways to get rid of the infestation: https://www.gardeningknowhow.com/special/organic/