Large trees. Leaves (in Nepal species) heart-shaped, 3-5-nerved at base. Male and female flowers nearly always on different trees, small, in pendulous catkins. Two species of poplar are native to Nepal, Populus ciliata and P. jacquemontiana var. glauca. In addition a number of exotic poplars have been introduced, of which clones of P. deltoides and its hybrid with P. nigra (P. x canadensis or P. x euramericana) are the most important.
Poplars are deciduous, strongly light-demanding trees, often found naturally on alluvial land or near streams. Some are very fast growing. The seed is very small and light, and propagation from seed is rarely attempted except for research purposes and hybridization. Cultivated clones are of one sex only, and so propagation of these from seed is in any case impossible. Poplars are readily established by the use of cuttings. Most of them coppice and pollard well, and produce abundant root suckers.
Poplars are sensitive to day length; that is, many species and clones will not grow well unless there is an appreciable difference between the length of the shortest day in winter and the longest day in summer. This photoperiodic sensitivity has hindered the cultivation of poplars near the equator. In Nepal in midsummer the daylight lasts about 13.4 hours, and the shortest day is about 10.6 hours; fortunately this is sufficient to enable southern European cultivars of poplars to be grown, as can be seen in the poplar avenues of Kathmandu. Also cultivars which have succeeded in northern India are likely to be adapted to Nepal day lengths.
Poplars generally grow best in soils with a pH of between 5.5 and 7.5, on loamy soils, preferably with the water table within a few metres of the surface. Most will withstand temporary flooding but will not tolerate waterlogged soils with stagnant water. Fast-growing poplars have high demands for water. To obtain good results from poplars high-quality plants and good tending are essential.
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.)
Planting is in the dormant period, usually in January-February. The transplants are made into bundles of 25, with their roots wrapped in jute sacks soaked in water. During transport great care must be taken that the plants do not dry out. On arrival at the planting site they are again soaked in water for a further 2-4 days. Before planting the bottom metre of the transplant is dipped into a 0.25 per cent Aldrin emulsion (250 ml Aldrex 30EC in 100 l water) for 10 min, to protect against insects especially termites, and then into Emisan (250 g of Emisan-6 in 100 l water), to protect against fungi. In India the planting pits are prepared by a soil auger and are 22 cm in diameter by 1m deep. The upper soil of the pit is mixed with 50 g single superphosphate and 5 ml Aldrex 30EC. For Nepal, Arendt and Lindgren (1990) recommend adding 2 kg of farmyard manure to each pit before planting, and to apply the Aldrin as 2 l of Aldrex emulsion. The soil is not compacted round the roots at the time of planting, but after the first irrigation, which will cause the soil to settle, the pit is filled up to ground level, and then the soil compacted.
Planting in India is along irrigation channels spaced 5 m apart and the plants are spaced 4 m apart along the channels. This spacing can be varied to suit different crops, for instance. In the first and second years irrigation is needed every seven to ten days from the time of planting until the onset of the monsoon, twice a month from October to January or February, and from March to the monsoon every 7-10 days. Thereafter a minimum of two irrigations per month during the hot season, and one per month during the winter post-monsoon season, will be needed. The duration of irrigation may be reduced if there is a permanent water table near the surface. During irrigation care should be taken not to cause waterlogging.
Intercropping, involving weeding and soil working, is essential for good growth of poplars; the alternative is manual clean weeding, which can usually be ruled out because of its cost. A number of crops can be used, depending on farmers' needs, though maize is undesirable as it is the host of a disease which affects poplars, and paddy because it grows in standing water. Tall perennial herbs, such as palmarosa and lemongrass, which compete with the trees for water and nutrients should be avoided during the early life of the plantation. Summer crops include peanuts, sesame, pigeon peas and tomatoes for the first three years, and later, when the canopy has closed, such crops as turmeric and ginger; during winter, when the trees are leafless, a wider range can be grown including wheat, barley, oats, potatoes, sweet potatoes, sugar cane, and many other vegetables. Mint (Mentha arvensis), a perennial, has been used successfully at the Medicinal Herbs Farm, Tamaghari, Bara District (Adkins, 1988). Sometimes however there are conflicts between the needs of the crops and the trees; farmers do not irrigate ripening wheat, for instance. Fertilizer may be needed for the crops; if so the trees will benefIt as well.
To produce high quality timber for veneers, including matches, the stems must be pruned. Immediately after sprouting the lower third of the stem should be debudded. In the fIrst dormant season double leaders should be removed, and beginning in the second dormant season side branches should be removed by cutting them off flush with the bole, up to a third of the total height of the tree. From the end of the third year pruning can extend to half the total tree height, and should be continued until a total height of 8 m has been pruned. Normally pruning will not be needed after five years. Poplars are planted at the final spacing, and no thinning should be needed.
Cultivated poplars are subject to auaeks by numerous pest and diseases (FAO, 1979) but fortunately nothing serious has yet appeared in Nepal, apart from defoliation of avenue trees by insect larvae. A close watch should, however, be kept for outbreaks of pests or diseases.
In some countries hybrid poplar plantations produce very high annual yields, up to 40 m3 ha-1, and give economic returns comparable with, or even higher than, agricultural crops. To obtain this sort of yield and return the plantation must be on fertile soils, and high inputs are needed, including irrigation in Nepal. In Uttar Pradesh, in India, poplars grown for match production are expected to produce a mean annual increment, on an eight-year rotation, of about 20 m ha under bark.
Poplars produce a white, soft, even-grained timber, which is not durable., In industrial countries it is used for boxes, shelving, and other purposes for which strength and durability are not important. Much of it is made into veneer, used both for plywood and matches; it is one of the best match woods. It is also used for paper pulp. In some arid countries (Syria, Iraq, Aghanistan), and in the Muktinath region of Mustang, Nepal, the poles are used for house building. It is not a good fuelwood, as it is light in weight (300 to 450 kg m-3) and has a relative low calorific value, about 19,500 kJ kg-1. It is somewhat difficult to ignite and tends to smoulder rather than to burn with a clear flame. It is used as fuel in some countries, but generally where no better fuelwood is available. The leaves are used for fodder in some countries including parts of Europe. The crude protein content on a dry-matter basis is about 15 per cent (FAO, 1979).
The indigenous poplars have not been planted on a large scale, and little information is available on their performance in plantations. They, together with willows, are among the most important trees in the and Mustang region. In the Middle Hills exotic poplar clones have grown very well m places as avenue trees, but results in conventional plantations have generally been poor, probably from a combination of poor soil, inadequate water supplies, and lack of care. In a few places, for instance in parts of Solokhumbu District, farmers have planted a few trees near their houses and along irrigation channels, with some success. However for practical purposes large-scale poplar plantations can be ruled out for this area.
In the Terai prospects for poplar growing, under irrigation, are much better, as has been shown under similar conditions in Uttar Pradesh in India. Raising poplar plants in nurseries requires a good deal of care and expertise, certainly beyond the capabilities of a semi-skilled nursery naike, so specialized nurseries would be needed. However from these nurseries plants could be provided to farmers; technical advice would also be needed.
In India the match company Wimco supplies plants to farmers and guarantees to buy back the trees, when they have reached 90 cm girth, at a fixed price. Farmers are also given help in getting loans. Ninety-five per cent of the plantations are in blocks of between one and five hectares, and most of the rest are in lines along field bunds, or as avenues. Similar schemes could be tried in Nepal.