Any time when the plant is actively growing. I am busiest in April and May, and this is a good time to plant, but plants also establish well in September, after flowering, when there is less stress on the plant. It’s best to avoid very hot or very cold periods. Water lilies can only be successfully established from bare rooted plants from April to August. After August and before April I will only send them in pots. Choice may be more limited out of season. I generally don’t advise buying, dividing or transplanting plants in the coldest months of December, January and February, even potted ones; they will get going better from March onwards.