SHORT ANSWER: Japanese knotweed flowers are usually white, but dwarf Japanese knotweed - a related species - sometimes grows pink flowers. Additionally, there are several other pink-flowered plants that are commonly mistaken for Japanese knotweed.
Japanese knotweed can be identified by a number of distinctive characteristics: the bamboo-like stems, the heart-shaped leaves, and the clusters of flowers that appear in late summer.
These flowers are quite small, and they're usually white or cream in colour.
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Is It Japanese Knotweed?