Answer: What you describe can indicate a nutrient disorder, cold weather damage, or nothing more serious than the natural aging of older foliage. Broadleaf evergreens such as bougainvillea eventually shed their older leaves to make room for new foliage. Some leaves remain for 2-3 years, by which time they can begin to look a little shop-worn. Since you're watering and fertilizing regularly, we can probably rule out nutrient deficiency. Cold winter weather can cause the foliage of fuchsias, azaleas and bougainvilleas to take on a reddish cast. Sometimes this goes away and sometimes the off color remains until the leaf falls off the plant. As long as the new growth is normal and your bougainvillea appears otherwise healthy, I'd chalk up the odd colored foliage to natural senescence.
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