Methods

Current literature on available methods of bamboo propagation (seed and vegetative) were reviewed to establish the key limiting factors involved in the propagation of bamboo.

Results

The vegetative propagation of bamboo via culm cuttings was found to be the simplest and most cost-effective method developed over the last few decades.

Conclusions

Additional work is required to increase the success rate of culm cuttings, and finding improved substrates for the propagules could be one area of investigation.

Introduction

The demand for bamboo is increasing steadily because of its multipurpose utility. Twenty million hectares of bamboo (equivalent to 80% of total world stock) are available from India, China and Myanmar. In most of these countries, including India, 99% of bamboo products are supplied from natural forests. China is considered as the largest bamboo resource of the globe, having 500 species in 40 genera distributed in 7.6 million hectares of forest land. The current taxonomic authorities for all the species referred to in this review are available from the Plant List. Phyllostachys edulis (Moso bamboo) is the most economically important bamboo species, but other species, namely Chimonocalamus delicatus, Dendrocalamus membranaceus and Fargesia yunnanens, are prevalent in China. India is considered as the second largest bamboo resource in terms of species diversity after China and of production. In India, 18 genera and 128 species of bamboo have been reported by Seethalakshmi and Kumar (1998), among which 41 species are exotic to India. Bambusa balcooa, Bambusa bambos, Bambusa tulda, Dendrocalamus hamiltonii, and Dendrocalamus strictus are the main economically important bamboo species of India. In India, according to Yengkopam (2013), 8.96 million hectares of 63.3 million hectares of forest land is covered by bamboo. However, a forest survey of India estimated that 13 million hectares of India’s forest land are covered by bamboo growing stock, which yields around 169 million tones of ‘wood’. In India, bamboo is also considered as an important economic resource for nearly 7.5 million bamboo artisans. The total value of bamboo forest produces is expected to reach USD 48 billion within 2020 in China. The economic value of total bamboo harvest in India is USD 0.30 billion and its products have a potential export value amounting to USD 4.46 billion by 2025. Human activities such as deforestation, industrialization, and illegal extraction of bamboo, have resulted in the loss of natural bamboo resource. Therefore, it is necessary to find cost-effective methods for large-scale propagation of bamboo for planting in new bamboo forests to reduce the existing gap between demand and supply. This review discusses the advantages and disadvantages of seed and various vegetative methods for propagating bamboo and the key factors affecting success.

Seed-based propagation

Seeding in bamboo is very irregular and unavailable for most of the species. Bamboo flowers gregariously or sporadically after long intervals to produce seeds. In gregarious flowering, the entire local populations of the individual species die after flowering. However, in sporadic flowering, only a minority of culms (the hollow main axes of upright shoots) in clumps produce flowers and die. Sporadic flowering is reported in B. balcooa, B. nutans, B. vulgaris and B. longispiculata while gregarious flowering is reported in B. tulda, B. bambos, B. polymorpha, Dendrocalamus longispathus, D. strictus and Oxytenanthera nigrociliata. Some species do not even produce seed (e.g. Bambusa balcooa, B. vulgaris), and those that do so often flower at long intervals varying from 30 to 70 years (e.g. Bambusa bambos, B. polymorpha, Dendrocalamus strictus, Melocanna baccifea and Phyllostachys spp.). Therefore, availability of seed may only be after long time intervals, seed viability may be poor and brief, storage facilities suitable for seed may be lacking, highly heterogeneous seedling populations may occur, and often overall poor seed set and consumption of seed by wild animals are the major problems with seed-based propagation. Seed-based propagation methods, therefore, cannot be relied for mass multiplication on a sustainable basis, so there is strong need to explore the possibility of vegetative propagation for mass multiplication of bamboo.

Vegetative propagation

The potential advantage of vegetative propagation is the production of true-to-type plants on a sustainable basis, which is clearly not feasible with seeds. However, plants raised through vegetative propagation maintain a definite vegetative time period before flowering, due to the physiological age of the mother plant. There is a danger that flowering, and death, could occur soon after propagation. Therefore, before collecting the propagating material, plant age and relevant physiological details need to be ascertained for each species, noting that there may be environmental effects which can change this status. Relevant establishment records for the mother plants must be developed and maintained. Widely used vegetative propagation methods for bamboo are rhizome or offset cuttings, layering and culm cuttings.

A brief account on the different methods of vegetative propagation is discussed here under the following subheadings.

Macro proliferation

Bamboo seedlings possess the capacity to proliferate. In this method, the rhizome system of a seedling is cut into pieces, each with roots and shoots. Then each piece with a shoot and root is used as propagating material. By this method, each plant can be multiplied three to seven times, depending on the species. However, seed dependency is the major drawback of this method.

‘Rhizome’- or ‘offset’-based propagation

‘Offsets’ or ‘rhizomes’ are conventionally used for propagating the sympodial bamboos. In this method, either whole rhizomes or portions each with single culm having three to five basal nodes (called offsets) are used as propagating material. Offsets, however, are difficult to handle and transport because of their bulkiness, and are not cost-effective due to limited availability and a low regeneration capacity, i.e. slow growth and/or poor multiplication rates. They are also prone to desiccation, are labor intensive, season dependent and have a low survival rate, and therefore are unsuitable for large-scale plantations.

Layering

In this method either a whole culm or a branch bearing part of the culm is bent down to the ground so that the nodes are pegged into the soil. Roots develop from the nodal regions are separated and planted in polybags for hardening. Different types of layering include:

A.) Ground layering, where a culm is bent down to ground and covered with rooting medium.

B.) Air layering/marcotting, in which the root-promoting medium (garden soil, leaf mold and commercial plant growth regulators, etc.) is applied around the nodes of the culm and often wrapped longitudinally with coconut fiber and/or water hyacinth roots.

C.) Stump layering, one to two node stumps of severed culms are covered with a suitable propagating medium. The low success rate of this layering method limits its use. Through stump layering, only 10% survival has been reported in B. vulgaris and D. giganteus. This method is also difficult to follow if clumps of bamboo are tightly bunched or culms have thin walls which limits its application for other species.

Cutting-based propagation

Vegetative propagation using ‘cuttings’ has advantages over ‘rhizome’/‘offsets’. Cutting-based propagation does not damage the mother plant and cuttings are also easily available for large-scale propagation. This could be achieved by either ‘branch cuttings’ or ‘culm cuttings’.

A.) Branch cuttings

The primary branches are easily separated from the hollow main stem of an upright shoot (culm) and reduced to two- to three-node cuttings and buried in growth medium. The basic advantages of branch cuttings are abundant availability and ease of handling. However, the method is restricted to thick-walled bamboo species only. Branch cuttings require 6–12 months for rooting and 12–30 months for rhizome development and thus are time-consuming to produce indicating that plants produced by this method have slower growth.

B.) Culm cuttings

In this method, the culm segments with two–three nodes from the mother plants are taken to propagate bamboo species. This method is applicable for both thick- and thin-walled bamboo, but the success rate is quite low in thin-walled bamboo. Rooting can be possible within 90 days of setting of cuttings in rooting media.

The lack of success of vegetative propagation methods mentioned earlier (macroproliferation, ‘rhizome’- or ‘offset’-based propagation and layering-based propagation) other than cuttings is notorious. It is evident from the literature that culm/branch cuttings are suitable for propagation because of easy handling and abundant availability. Ahlawat et al. (2002) reported that culm cuttings are 40–80% more successful than the offset method. The chief advantage of culm cuttings is that they can be collected from the stock material without affecting the rhizome system. Besides wider applicability, culm cuttings are advantageous in circumventing the limitations (discussed earlier) of branch cuttings. Culm cuttings were found to be better than branch cuttings; in terms of rooting, 88.3% rooting in culm cuttings was observed which was more than double in comparison to that in branch cuttings (40%) in B. nutans, while in B. tulda it was 23.3%, compared with no rooting in branch cutting material (Singh et al. 2011). Culm cuttings have performed better with respect to rooting response percentage (~70.6%), number of roots/node (~15.3) and number of shoots/node (~9.4), than branch cuttings which showed ~60.3% response percentage, ~2.6 shoots/node and ~6.4 roots/node (Deb et al. 2016). The success of the culm cuttings is the end-result of multiple factors that we need to address properly.

Factors affecting culm cutting-based propagation

 Several internal and external factors are responsible for successful macropropagation with culm cuttings: age of mother plant; type of cutting ; method of setting; type(s) and concentration of plant growth regulators; season and substrate used for macropropagation. In addition, environmental factors such as temperature, humidity, water balance, rooting medium, light intensity, wind speed, and pests and diseases, are also limiting factors for the macropropagation. A summary of the findings of the previous literature on macropropagation through culm cuttings in different species of bamboo is provided in Table 1. The most important controllable critical factors are briefly discussed below.