Bougainvillea is a tropical plant that can bloom year-round in areas of the world where night and day are nearly the same length. Because of this preference, it blooms best in fall and spring in areas of North America and Europe but cannot thrive in freezing temperatures. The plant features rich green leaves and brightly coloured flowers, and yellowing of leaves can indicate a serious problem--from insects to vitamin deficiency. Treating yellowing flowers on bougainvillea depends on the cause.
Inspect your leaves, top and bottom, for webs and small-moving dots. These are indications of spider mites, which feed on the leaves and turn them yellow.
Spray your leaves with forceful jets of water to dislodge spider mites. Such a spraying every two to three weeks can prevent spider mite infestations, as the mites cannot cling to the leaves under water spray.
Treat your leaves with natural horticultural oil or insecticidal soap or oil. Follow specific product instructions regarding applying these products; some will require dilution with water.
Coat the leaves completely with the oil or soap. These products kill on contact, so you must thoroughly coat each leaf to ensure the mites die. Repeat applications of oil as necessary every two to three weeks to eliminate a mite infestation.
Refrain from watering your bougainvillea for a week. Overwatering with water sitting in the roots can drain colour from the leaves. Consider mixing peat moss, compost or another organic material at a rate of 1 to 4 with your soil to increase drainage.
Fertilise your bougainvillea with a nutrient-blended fertiliser containing magnesium and iron. Make sure these nurtients are balanced in the fertiliser. Yellow leaves can indicate a deficiency of either of these materials, but too much of one will cause a deficiency of the other.
Mix 2 tsp of Epsom salt into 1 gallon of water and drench the leaves and soil to increase magnesium and iron for the leaves. Do not use Epsom salts if you use fertiliser. Combining treatments can create an unbalance of other nutrients with the magnesium and iron.
If you are unsure of the cause or treatment for your bougainvillea, consult an expert at a garden centre, nursery or extension service.