It's incredibly frustrating when you work really hard to take care of a plant, but no matter what you try its leaves turn yellow. Before you chuck it away in frustration, try and figure out what may have happened. If you can figure that out, you may be able to restore it to its former magnificence.
Leaves can turn yellow if the temperature suddenly drops, like with a sudden frost. Prevent this from happening by either transplanting your plants indoors before the cold weather season arrives, or by providing your outdoor plants with adequate cover during the winter months.
Conversely, leaves can also turn yellow in cases of intense heat and too much direct sun. The leaves will respond by weakening and yellowing. Move them to an area where they can receive more shade and partial sun.
Overwatering also causes yellowing. The roots of the plant become overwhelmed with water, and the leaves react by turning yellow. Ease up on the water if you suspect this is the case. Make sure you're giving your plant the adequate amount of water for its variety.
On the flip side, a plant's leaves can turn yellow when it doesn't receive enough water. The stems don't have the strength to support the leaves, and it will start to shed them.
A condition called decline occurs when there's a sudden and rapid yellowing of leaves follow by an intense shed. This can be caused by a plant living in one environment, like a nursery, and suddenly being brought into another environment, like your home. There's nothing you can do except continue to give the plant the living conditions it requires. The yellowing leaves are done for, but the new ones should grow in fine.
If you buy a plant at full bloom, it may already have reached its peak, so your yellowing plant may have passed its prime. Buy plants in bud, not bloom.
If you've had the plant for a while, and none of the above conditions apply, you may have a pest or disease problem.