Cooperative Extension: Garden and Yard

There are a couple possibilities: one is that the plant is “reverting.” Many of our interesting horticultural cultivars come from genetic mutations. These are not always stable and parts of a plant (like a whole branch or part of a branch) can “revert” to the pre-mutation form of that plant. Another possibility is that the purple lilac was grafted onto a rootstock with white flowers and that particular branch is a sucker from the rootstock. Either way: you can either prune off the branch growing white ones if you prefer to encourage the purple blooms, or enjoy the combination!