A cool period that mimics what would occur in nature.
Treatment of Subtropicals
Subtropicals such as Chinese elms, Ulmus parvifolia, have little if any dormancy requirements. In colder areas they drop their leaves, go dormant and act like deciduous trees. In milder, non freezing environments, they are evergreen and exhibit continuous growth except for occasional 'quiescence'. They require fairly high light levels and that will be the most difficult factor to maintain. A sunny window is usually insufficient and supplemental light, such as a fluorescent lamp six inches above the plant, is strongly recommended. Most subtropical plants that do not have strict dormancy requirements, still seem to perform better if they have a brief cold dormant period that allows them to lose their foliage. Both Chinese elm and Pomegranate, Punica granatum, fall into this category.