Answer: Sometimes plants go through an adjustment when they are brought indoors while they try to adapt to the lower light levels, the drier air, and shock of being moved. The colder it is outside when they are finally moved indoors, the bigger the shock, as they will have begun to adapt to the colder temperatures by slowing their growth. Also, if exposed to temperatures below about 45 to 50 degrees, they may have suffered cold injury. Sometimes cold damage does not cause an immediate effect but instead becomes apparent over several weeks. If your plants did well last winter, place them in the same conditions again, and hope for the best. Also inspect them carefully for signs of pests such as spider mites, these can cause a plant to defoliate (raise the humidity level and treat with insecticidal soap if you find these). Meanwhile, take care not to overwater them as they will be using less water due to the lost foliage. Be very patient waiting for them to leaf out again, sometimes they will sit until the days begin to lengthen in the spring, say the end of January, without doing much. Gardenias are notoriously fussy, so congratulations on keeping one for as long as you have! Here is some general care and troubleshooting information to try to help you sort that one out. Good luck with your plants!
Unfortunately, the gardenia is often a disappointment to gardeners because it can be very, very demanding. In some cases it is better to consider it as a florist bouquet and dispose of it once the flowers have faded. However, you might be able to keep it going as a container plant. Here are some general care notes and instructions.
Gardenias are finicky, and any change of location or humidity or sun can set them back, so do not be too surprised if it suffers some initial shock from being moved.
In general, bright light is essential, but avoid direct mid-day sun in the summer. It may summer outside when temperatures are settled, place it in a bright location out of direct sun, in dappled light all day or in gentle morning sun. Buds that turn black and drop, and bottom leaves that are yellowed are sure signs that gardenias aren't getting enough light.
For flower buds to form and thrive, night temperatures need to be between 60-65F. During the day, temperatures should be 70-75F. A very constant, even temperature within these ranges is required or buds will drop. Also avoid drafts or moving the plant. Simply moving the plant often causes bud drop, as may any imbalance in the growing conditions.
Keep the soil evenly moist (but not sopping wet)at all times except in winter when watering should be reduced slightly to compensate for the seasonally slower growth.
Fertilize regularly but lightly during active growth periods (spring and summer) with a water soluble fertilizer for acid loving plants according to the label instructions. Check the label also to make sure it includes micronutrients or "minors" to assure a broad based supply of minerals.
Gardenias are very susceptible to spider mites, which can distort the buds and cause leaves to yellow and drop. Look on the undersides of the leaves for tiny black specks and whitish webbing. If necessary, use an insecticidal soap spray according to the label instructions to control these, be sure to repeat to control subsequent generations and treat all surfaces of the plant. Washing the plant with a spray of tepid water can also help rinse them away.
Routine misting with water helps raise the humidity and consequently discourages spider mites but it is better to set it on a humidity tray of pebbles and water or use a humidifier especially during the winter heating season when indoor air is very dry.
Finally, yellow leaves can be a natural occurance since the old leaves normally turn yellow and fall off, or could indicate inadequate light, or can signal insufficient water or even the use of hard water. Leaf drop can also be caused by improper soil pH (gardenias require 5 to 5.5 pH - on the acid side) or a nutrient deficiciency.
As you can see, these are not easy plants. Occasionally we get lucky and a gardenia will thrive. If not, don't take it personally, they are very difficult.
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