Species such as Populus deltoides Bartr. which are difficult to root and have a long period of seed maturation are not amenable to controlled crossing using floral cuttings in water culture. A controlled breeding technique were studies involving accelerating rooting of floral cuttings by warming the flowerpot using a constant water temperature and at the same time keeping flower buds in dormancy by low temperature environment. The floral cuttings were planted in flowerpots with soil and grown in a cool (3℃) room, the pots being soaked in water at 3 different temperatures. After 25 days, the pots with the floral cuttings were moved to the greenhouse for controlled crossing. The floral cuttings of P. deltoides with internal soil temperature of 22℃ were much superior to the others. Not only did it ease maintaining the vigor of floral cuttings, but also result in more and better seeds, which had 25.5 root hairs, root length of 31.8 cm and root weight of 16.9 g on average. The ones with 18℃ treatment had 12 root hairs, 10.4 cm root length and 2.7 g root weight on average, and the ones with 14℃ treatment had 2.2 root hairs, 4.2 cm root length and 0.4 g root weight on average. Efforts to improve rooting of P. deltoids by warming the flowerpot proved to be successful in controlled crossing with species of sections Aigeiros and Tacamahaca. All the floral cuttings in soils at 22℃ produced seeds (100%), with an average of 1 800 seeds per floral cutting. 60% floral cuttings at 18℃ treatment survived, but the flowers fell because the cuttings had so weak root hairs that they were not enough to support blooming, with an average of 1 000 seeds per fruited cutting. Of those at 14℃ treatment, 20% of cuttings produced seeds, with an average of 500 seeds per fruited cutting, and 80% of the floral cuttings were infected by canker. Other female parents such as P.×canadensis showed similar results to P. deltoides, but floral cuttings of section Tacamahaca have good rooting in water culture, and showed similar results to the rooting treatment of floral cuttings in controlled crossing. Thin floral cuttings are easy to root and can maintain more catkins than floral cuttings in water culture. This is also helpful for female trees.