The flowers are sometimes affected by a grey, fuzzy mold called botrytis. Under humid conditions, it is often found growing on the older flowers. Treatment is to simply remove the old flowers, so the mold does not spread to the new flowers. Cannas grow best in full sun with moderate water in well-drained rich or sandy soil. They grow from perennial rhizomes, but are frequently grown as annuals in temperate zones for an exotic or tropical look in the garden. In arid regions, cannas are often grown in the water garden, with the lower inch of pot submerged. In all areas, high winds tear the leaves, so shelter is advised. The rhizomes are sensitive to frost and will rot if left unprotected in freezing conditions. In areas with winter temperatures below in the winter (< USDA Zone 8b), the rhizomes can be dug up before freezing and stored (above ) for replanting in the spring. Otherwise, they should be protected by a thick layer of mulch over winter. See the List of Canna cultivars for photographs of Canna cultivars. Cannas became very popular in Victorian times as garden plants and were grown widely in France, Germany, Hungary, India, Italy, the United Kingdom, and the United States. Some cultivars from this time, including a sterile hybrid, usually referred to as "Canna" × "ehemannii", are still commercially available. × "ehemannii" is tall and green leafed with terminal drooping panicles of hot pink iris like flowers, looking somewhat like a cross between a banana and a fuchsia. As tender perennials in northern climates, they suffered severe setbacks when two world wars sent the young gardening staff off to war. It took many years for the frugalities of war and its rationing subsequences to change to the more prosperous times of the late 20th century. The genus "Canna" has recently experienced a renewed interest and revival in popularity. Once, hundreds of cultivars existed, but many are now extinct. In 1910, Árpäd Mühle, from Hungary, published his "Canna" book, written in German. It contained descriptions of over 500 cultivars. In recent years, many new cultivars have been created, but the genus suffers severely from having many synonyms for many popular ones. Most of the synonyms were created by old varieties resurfacing without viable names, with the increase in popularity from the 1960s onwards.