Bigger Pots Are Not Always Better: Six-Year Trial Reveals How Nursery Choices Shape Silky Oak Success in Ethiopia

In the degraded highlands of northern Ethiopia, the fate of a reforestation project can hinge on decisions made months before a single tree touches the soil. A new six-year field experiment from the Sirinka Agricultural Research Center suggests that two of the most overlooked nursery variables—pot size and seedling age—leave measurable fingerprints on one of the region’s most important plantation trees, Grevillea robusta, the silky oak. The study, conducted in the Habru District of the North Wollo Zone at roughly 1,850 meters above sea level, followed seedlings from the nursery bench through 74 months of growth in the field, offering one of the more complete pictures of how early container choices echo through a tree’s later life.

The research team, led by Andualem Ayalew Alemu and colleagues at the Amhara Agricultural Research Institute, set out to answer a deceptively simple question: does it matter whether a silky oak seedling spends four, five, or six months in the nursery, and whether it grows in an 8, 10, or 12 centimeter pot? The stakes are considerable. Ethiopia loses between 80,000 and 200,000 hectares of forest each year, driven by deforestation, poor land management, and the conversion of natural ecosystems into farmland. Grevillea robusta, an Australian native from the Proteaceae family, has become a workhorse of Ethiopian agroforestry, contributing an estimated 1.7 million tons of carbon dioxide equivalents annually and adding roughly 44 million Birr in net present value from sawn wood production.

The experiment was rigorous in design. Seeds were obtained from the licensed Ethiopian seed company Eden Seed Enterprise, cleaned of insect-damaged or rotten material, and stored at 4 degrees Celsius to prolong viability. Seedlings were raised in a standardized growing medium of forest soil, local soil, river sand, and decomposed animal manure mixed at a ratio of 3:2:1:1, then arranged in a factorial randomized complete block design with three replications. Nine treatment combinations crossed three pot sizes with three seedling ages, with 100 seedlings per treatment in the nursery. The 8-centimeter pot and 4-month seedling age combination served as the control, reflecting common nursery practice. Seedlings were watered twice daily, and destructive sampling of ten seedlings per treatment at each age allowed the team to measure root length, shoot length, and the root-to-shoot ratio.

The nursery results were telling. The interaction of pot size and seedling age had a statistically significant effect on shoot length, with several mid-range treatments producing markedly longer shoots than others, while root length and the root-to-shoot ratio showed no significant differences. The authors attribute the shoot response partly to the nitrogen-rich manure in the growing medium, which prior research suggests promotes shoot growth more strongly than root growth. Notably, the mean root-to-shoot ratio declined as seedlings aged in the nursery, from about 1.7 at four months to 1.98 at six months in the study’s measurements, a pattern consistent with the understanding that allocation between roots and shoots shifts as seedlings mature. The root-to-shoot ratio is widely regarded as a key indicator of a seedling’s ability to cope with resource scarcity after transplanting.

Then came the field phase, and with it, the long wait. Seedlings were planted at a spacing of 2 by 2 meters in 12-square-meter plots, six seedlings per plot, with treatments replicated across blocks separated by buffer strips. The team measured root collar diameter, diameter at breast height, height, and survival at three-month intervals, ultimately computing volume and mean annual increment using a form factor of 0.86 for tropical Grevillea plantations. After more than six years in the ground, the interaction of pot size and seedling age showed a significant effect on root collar diameter, while diameter at breast height, height, survival, volume, and mean annual increment did not differ significantly among treatments.

The absolute growth figures reveal both the promise and the constraints of the site. By 74 months after planting, the best-performing treatment had reached a diameter at breast height of 11.5 centimeters, while the slowest stood at 8.5 centimeters, yielding a mean annual diameter increment of 1.7 centimeters per year. Mean height ranged from 6.8 to 7.6 meters. These numbers fall short of benchmarks from more favorable environments: trees in south-eastern Queensland, with comparable rainfall, reached a mean diameter of 16.7 centimeters and a height of 8.9 meters at six years, while Indian plantations have recorded annual diameter increments of 2 centimeters in their first five years. The researchers point to the study area’s modest 945 millimeters of annual rainfall and its long dry season as likely explanations, though the clay-to-clay loam soils, with a pH of 6.4, are considered well suited to the species.

Survival was the quiet triumph of the trial. Across all treatments, survival remained above 80 percent from six months through 74 months after planting, with several treatments achieving complete survival. That figure exceeds results from smallholder plantings in southwestern Ethiopia, where around 80.6 percent of Grevillea survived three years near homesteads, and the authors credit the intensive research-station management—weeping, hoeing, and protection from livestock—that smallholders often cannot replicate. In the drylands of northern Ethiopia, where planted seedling survival is frequently undermined by moisture stress, poor site conditions, and weak post-planting care, such numbers are far from trivial.

The practical implications cut against intuition. Rather than confirming that bigger containers and longer nursery stays produce better trees, the study concludes that 8- and 10-centimeter pots combined with 4- to 5-month-old seedlings represent a sound choice for Grevillea establishment in the midland Amhara context. This aligns with earlier Ethiopian findings that five months is sufficient for the species to reach planting size, but contrasts with Kenyan research recommending larger 9-centimeter pots and eight-month-old seedlings. The discrepancy, the authors suggest, reflects differences in altitude, site quality, rainfall, and management—reminders that nursery prescriptions are not universal but must be tuned to local conditions.

The study is candid about its limits. It examined only one exotic species, omitted a cost-benefit analysis of the different pot and age combinations, and left open the question of how indigenous species with slow growth and longer nursery requirements would respond. The authors call for further research on optimal seedling age and pot size for species with poor germination and sluggish early growth, explicitly including economic analysis. Still, the core message stands: quality seedling production is a prerequisite for successful plantation establishment, and the choices made in the nursery—down to the diameter of a polythene bag—ripple outward into the survival and growth of trees planted years later. For a country racing to restore degraded highlands, that is a cheap lever with long reach.

Subject of Research: Effects of nursery pot size and seedling age on the growth and survival of Grevillea robusta in Ethiopia

Article Title: Effect of pot size and seedling age on growth and survival of Grevillea robusta (A. Cunn.) in the Eastern Amhara region of Ethiopia

Article References: Effect of pot size and seedling age on growth and survival of Grevillea robusta (A. Cunn.) in the Eastern Amhara region of Ethiopia. (n.d.). https://doi.org/10.1007/s44353-026-00076-0

Image Credits: AI Generated

DOI: 10.1007/s44353-026-00076-0

Keywords: Grevillea robusta, silky oak, seedling age, pot size, nursery management, Ethiopia, reforestation, agroforestry, root-to-shoot ratio, survival rate, land degradation, field trial