Poplars are always propagated from cuttings. Cuttings should be taken during the dormant season when the trees are leafless, in February. They may be taken from branches, epicormic shoots or root suckers, but should not be taken from the crowns of older trees, as such cuttings may produce curved boles or a branchy habit The trees from which cuttings are taken should be completely healthy, as any diseases present in the old trees will be carried on to their offspring. Care should be taken to select parent trees of good bole form and rapid growth.
The simplest method is take the cuttings from trees, and plant them directly where the trees are to be grown. This is the method used in the Muktinath Valley, Mustang, where cuttings approximately 25 cm long are planted, frequently in groups of three, as this is said to produce a larger, thicker trunk. When the branches reach the desired length of 3-6 m the trees are coppiced or pollarded. They are planted along canals (Pyers, 1985). In community forestry and other small nurseries in Nepal the practice has been to root cuttings in polythene bags filled with soil in February, and to plant them out in the monsoon in the same way as other plants raised in containers. This technique is simple, and for raising a few plants for use by farmers in the Middle Hills it may be the most practicable method, especially when irrigation is not possible.
However for raising poplars on a commercial scale more elaborate nursery techniques are needed; the following account is based mainly on Arendt and Lindgren (1990). Best results are from cuttings from one-year-old plants raised in the nursery in the previous year. The cuttings are taken between late January and the end of February, and should be well lignified, 15-20 cm long, with a mid-diameter of between 10 and 40 mm. The bottom end should be cut at an angle about one centimetre below, and the top end about one centimetre above, a bud; this is because the top end of the cutting is likely to dry out. The cutting should include 3-5 buds. If the cuttings are likely to remain more than an hour before being planted, as for instance during transport, the ends should be sealed with wax; for less than an hour this is unnecessary. The sealed ends should be removed before planting.
Directly after the cuttings have been prepared they should be soaked in water for 48 to 72 hours. After this they are first immersed in a fungicide (Emisan or Dithane M-45) and then an insecticide (Aldrin) for 30 minutes (Aldrin is a persistent organochloride, and should be used with great caution. It is banned in number of countries). They are then planted in nursery beds at 60 cm x 60 cm spacing, with their tops about level with the soil surface.
Nursery beds should be well manured; at Jogikuti near Butwal 20-25 t of farmyard manure ha-1 are used. This manuring should be repeated each year. If farmyard manure is not available inorganic fertilizers can be used. In addition the application of 100 kg urea ha-1 , in two split doses in early May and early June, is recommended. The beds should be irrigated within six hours of the cuttings have been planted. Until the cuttings are well established irrigation about twice a week will be needed; after this a seven- to ten-day watering interval, according to the weather and state of growth of the plants, will be enough until the onset of the monsoon.
Soon after sprouting, in April or May, shoots are singled to one per plant. The stems should be debudded by hand from the end of May to October to produce clean stems. Unsatisfactory stems should be culled. Thorough weeding, by hand, is essential for good growth, and should be done at least monthly; it can be discontinued after September. An alternative to weeding is the cultivation of crops between the poplar plants. Such crops should be short-lived and should not produce bulky masses of vegetation; tomatoes, chillies and gram (avoiding twining varieties) are suitable.
The plants are ready for planting after a year, when they should have reached a height of at least 3 m. They are planted as entire transplants. They are lifted carefully, avoiding damage to the roots; after lifting the roots should be trimmed to form a ball of a maximum diameter of 20 cm. All fine hair roots should be removed. The stems should be pruned by removing all branches and secondary sprouts. After lifting, the plants should be stored in fresh water, preferably under shade. The water should be changed frequently. Plants can be stored up to 2.5 months in this manner. In any case they should be soaked in water for at least 48 hours before transport to the field. (This nursery practice is suitable for the Terai, where commercial plantations of poplars are most likely to be attempted. In the Middle Hills longer periods in the nursery may be needed, and consideration might be given to the two-stage system proposed by Sharpe (1983); see Napier and Robbins (1989). However it is doubtful whether the importance of poplar plantations in this region is likely to be great enough to justify such complicated and expensive nursery practises.)