This section is from the book "The Gardener V3", by William Thomson. Also available from Amazon: The New Organic Grower: A Master's Manual of Tools and Techniques for the Home and Market Gardener.
If your Lily of the Valley has produced foliage and no bloom, the crowns must have been too weak and immature. Grow it on carefully this season, and if the crowns and leaves are very thick in the pots, thin out the weakest of them and top-dress with rotten manure. When well hardened off plunge the pots outdoors in a sunny place, and attend carefully to their watering, and keep the foliage healthy as long as possible; and unless it is now very weak indeed, it will make fine blooming crowns for next year. Do not expose it till all danger from frost is past.