Biodegradable seedling bags could grow stronger trees, but can they replace polythene?
Grandma was right to raise her tree seedlings in that broken old gourd; this biodegradable container helped her saplings establish more successfully on the farm.
A new article in the journal Small-scale Forestry confirms the superiority of biodegradable bags over polythene ones in easing a seedling’s transition from the nursery to the farm. Upon transplanting, tree seedlings grown in biodegradable bags established with more vigour, says the article by researchers from the World Agroforestry Centre (ICRAF) and partners. The authors note, however, that the adoption of biodegradable seedling bags is not a straightforward proposition for small-scale tree nursery operators in Kenya and similar African countries.
Jonathan Muriuki, an agroforestry scientist with ICRAF’s East Africa Programme, worked alongside ICRAF colleagues and partners from the Kenya Ministry of Agriculture and Ellegard A/S Denmark in the study, which was funded by the European Union Commission. They tracked and compared—from the nursery to the field—the growth and vigor of seedlings grown in polythene bags and four types of biodegradable seedling bags. Calliandra calothyrsus, a popular exotic fodder and soil-improvement species, was chosen for the work. Three types of cellulose seedling tubing made by Ellegard A/S (types FP, VP and EP), bags fashioned from banana fibre sheaths, and polythene seedling tubing were used in the trials.
“In the nursery, seedlings grown in polythene bags outperformed those in biodegradable bags in vigor by nearly double, but when we moved them to the farm the situation switched,” says Muriuki, the article’s lead author. The superior performance of the seedlings in biodegradable containers started coming to light around the second month after transplanting. By the fourth month, all the seedlings grown in the four types of biodegradable tubing had overtaken those produced in polythene tubes in vigour. At the end of the study, 6 months post-transplanting, this trend held steady.
These findings mean that seedlings produced in biodegradable bags establish more robustly once planted on farms, says Muriuki.
‘Transplant shock,’—the stress or damage plants undergo when transplanted from one set of conditions to another—might explain this observation.
“Unlike polythene, biodegradable bags promote better drainage and aeration, which helps normal root development in the nursery. Because biodegradable bags do not have to be cut away from the roots when seedlings are transplanted, the root system remains undisturbed, which reduces the risk of transplant shock to the tree seedling,” Muriuki says.
Polythene seedlings tubes restrict the growth and aeration of roots in the nursery. When removed from the plastic and transplanted into the soil, the roots, which may have coiled in the bag, take longer to anchor into the ground. This might explain the lower vigor seedlings suffer when they are transplanted from these bags.
Armed with this knowledge, it might seem intuitive that nursery owners would pick biodegradable bags over polythene. However, the popularity of polythene with nursery owners is not unmerited.
Samuel Njenga Kimani, the owner of Juja Tree Nursery on the outskirts of Nairobi, says he has used polythene bags since he started his nursery nearly a decade ago, and they have served him well. They come in five different sizes to suit all types of seedlings, and they are cheap and easily available, he says. The polytubes transport well—their rigid walls keeping everything firmly in place—which is appreciated by customers.
The downsides of polythene seedling bags, however, are many and serious. After transplanting, the bags are discarded, burned or buried on agricultural land. Bags discarded on farms can enter waterways and cause blockage, and also suffocate aquatic creatures. Domestic and wild animals sometimes swallow them, with disastrous effects. The bags also serve as breeding sites for mosquitoes and other disease vectors. Burying the polythene bags in the soil interferes with proper water percolation and aeration of the soil, and burning them produces noxious smoke.
These demerits of polythene bags have prompted some governments to impose levies and taxes on their use, or ban them altogether.
In the research reported, polythene and biodegradable seedling bags differed in their watering requirements—an important consideration for a nursery operation. Under the shade of a tree or inside shade netting, the seedlings grown in polythene tubing needed the least frequent watering. Inside a polythene enclosure, however, the biodegradable cellulose bags needed less frequent watering (every 4.5–4.8 days) than the polytubes (every 4.2 days); in this humid environment, seedlings in banana tubing needed watering every 3.8 days on average.
Based on these findings, Muriuki says biodegradable seedling containers could be most efficiently adopted in areas where water is not a major limiting factor. “Nursery owners would do well to construct simple polythene structures to enclose seedlings grown in biodegradable bags, as this will reduce evaporation rates and improve water-use efficiency.” He is quick to point out the need to investigate more environmentally friendly materials for constructing such seedling enclosures.
The greater resilience to transplanting shock that biodegradable bags confer is a strong selling point for them, says Muriuki.
Nursery owner Njenga agrees, saying he would be willing to try biodegradable bags in his nursery if they were available and affordable. “I would educate my customers about their benefits. I’ve been trying to ask them to return their used polythene bags to us and get a discount on their next purchase, without much success. I think biodegradable bags would solve the problem of polythene waste for them,” he said.
Policy incentives might help with encouraging the adoption of biodegradable bags, say the authors of the article. For example, nursery operators who adopted biodegradable bags might be included in payments for environmental services schemes.
Muriuki says ultimately, biodegradable tubing would have to make business sense to catch on with small-scale nurseries. “Unless a nursery can turn a profit with biodegradable seedling bags, they will find it hard to embrace them.”
Download journal article
Testing Biodegradable Seedling Containers as an Alternative for Polythene Tubes in Tropical Small-Scale Tree Nurseries by Jonathan K. Muriuki, Anne W. Kuria, Catherine W. Muthuri, Athanase Mukuralinda, Anthony J. Simons, and Ramni H. Jamnadass. Small-scale Forestry: Volume 13, Issue 2 (2014), Page 127-142.
Presentation by Jonathan Muriuki on Making Agriculture Evergreen in East Africa
 This research was funded by The European Union Commission through its incremental funds to Consultative Group on International Agricultural Research (CGIAR) centres, (in this case the World Agroforestry Centre; ICRAF), geared towards development of options for tree germplasm conservation and supply systems. The authors acknowledge Ellegard A/S Denmark, especially Arne Beisland, for providing the biodegradable materials at no profit for trial purposes, and the Kaguru Agriculture Training Centre under the Ministry of Agriculture, Meru County office in Kenya for availing the land for nursery and field trials. Efforts by the data collection team involving Alexander Munyi, Silas Muthuri, Valentine Gitonga and the labourers who worked with them are greatly appreciated.