Agapanthus is a beautiful perennial lily plant that is grown prolifically in the Deep South and parts of the Southwest. They are evergreen plants that produce tall colorful blooms in shades of white to deep purple in early- to mid-summer. Even when they're not in bloom, these plants make an excellent border plant, resembling the commonly used liriope plant throughout much of the year.
Gardeners often buy agapanthus plants from their local garden center only to discover that a few months after planting them they bloom in unexpected colors that are different from the plant they purchased. Many logically guess that it must have something to do with the pH level of the soil, as this is a factor in the changing color of other plants, most notably hydrangeas. However, this is not actually the case; when agapanthus appear to change colors, it is actually not the original plant but its offspring that is blooming in a new and different color.
New cultivars of agapanthus are regularly developed in various shades of blue and purple. Cultivars are cuttings derived from a single plant and then cloned--absolute mirror images of other plants. When agapanthus seed after blooming, they are spreading seeds that don't necessarily have the same genetic makeup as themselves, and therefore may bloom in colors that are different than their parent plant. Agapanthus color is not affected by the pH of the soil, it just propagates often, and the offspring of your original agapanthus will inevitably appear in different colors. What appears to be a single plant growing and blooming in a different color is actually a new plant growing next to it.
It takes a little bit of determination to maintain a monochromatic agapanthus bed, but it's certainly possible. Keep careful watch of your agapanthus blooms. As soon as any given bloom begins to lose its vibrancy and begins to wilt, dead head it, or cut the bloom off before it has a chance to seed. You not only will prevent new agapanthus of different color varieties from popping up in your bed, you will also encourage more blooms from that particular plant over the course of the flowering season.
If you'd like to maintain and expand a monochromatic agapanthus garden and have a few large, healthy plants growing already, you can propagate them in the same manner nurseries do. Pull your agapanthus plants out from your garden and divide the root system underneath into however many separate plants you would like to have. In doing this, you can both enlarge your agapanthus bed and ensure that your agapanthus will all be exactly the same color as one another. This cannot be done regularly, as plants need time to grow to a reasonable size before being divided. This size is usually obtained in four to five years.