how come the leaves on my gardinia are yellow and falling off?

This site can help.

Gardenias do not like drafts or fluctuations in temperatures and they react by dropping their buds. They like cool weather.

http://knol.google.com/k/how-to-care-for-a-gardeni...

I use to have gardinia's.

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Is yours in a flower container or in the ground?

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That can happen if the roots cannot grow preventing the gardinia's from getting the nutrition

it needs and the leaves will start to fall and the flowers will turn yellowish in color.

Usually gardinia's are planted in the ground. If your isn't in the ground, you might wanna try yto plant it in the ground. Just make sure that you loosen the soil really good first.

a number of possibilities ranging from...too much heat, not enough water, too much water, not enough light, temperature fluctuation..etc..etc..

you need recommended amount of fertilizer and make sure it gets enough water