Georgia's most important vegetable crop is the short-day sweet onion marketed as the Vidalia onion, which had a farm gate value over $125 million in 2005 (Boatright and McKissick, 2006). This high value crop is produced from transplants produced on-farm in high-density plantings (Boyhan and Kelley, 2007). Onion seeds are sown in September at a rate of 800,000 to 2,000,000 seeds/acre. These plants are grown for 8 to 10 weeks, at which time they are harvested and transplanted to their final spacing of 60,000 to 80,000 plants/acre.
Transplant density, along with other factors, has been investigated in several studies with onion. For example, in Haryana, India, in a study comparing three different plant spacings and three different nitrogen (N) rates, it was found that the highest density (10 × 5 cm) had the smallest number of doubled bulbs and the thinnest necks compared with the medium (10 × 7.5 cm) and low (10 × 10 cm) densities (Gupta and Sharma, 2000). In addition, the high density had greater overall yields, but smaller bulbs. Finally, soluble solids and bolting (flowering) were not significantly affected. In a similar study, increasing plant population increased yield on clay or sandy soils (Abo-Zeid and Farghali, 1996). In addition, they found that bulb quality parameters (dry matter, bulb weight, and bulb diameter) were adversely affected at the high population density. Finally, Viloria et al. (2003) found increasing plant population from 12 × 20 cm to 6 × 20 cm spacing resulted in increased yield with maximum bulb fresh weight with a spacing of 10 × 20 cm.
In a study investigating rice root-knot nematode (Meloidogyne graminicola) and transplant age effect on yield and quality of onions, it was found that older transplants resulted in reduced yield along with reduced root and leaf weights (Gergon et al., 2002).
Herison et al. (1993) showed that increasing seedlings per cell in 200-cell flats resulted in increased yield when transplanted without separating cell seedlings with long-day onions. In a similar study comparing greenhouse-grown to field-grown transplants, Russo (2004) found no yield difference between field-grown transplants and greenhouse-grown transplants produced in 58-cm3 cells, but they had higher yields than onions produced in 36-cm3 cells.
In an experiment with the short-day variety Granex 33, increasing row numbers per bed increased yield with a decrease in bulb size (Stoffella, 1996). In addition, increasing within-row spacing decreased yield and increased bulb size. In Puerto Rico, with investigations of row spacing from 30 to 90 cm and two different fertilizer rates it was found that the closer spacing resulted in greater yield, regardless of the fertilizer rates (in kg·ha−1: 111 N + 48.4 P + 92.1 K or 222 N + 96.9 P + 184.3 K) (Mangual-Crespo et al., 1979). In the Treasure Valley of eastern Oregon and southwestern Idaho, a study of plant population and N rates showed that yield increased with increasing plant population from 185,000 to 370,000 plants/ha (Shock et al., 2004). In addition, there was a decrease in bulb size as the plant population increased. May et al. (2007) found similar results with decreasing bulb size as plant populations increased from 60 to 108 plants/m2. This trend of increasing yield with higher plant densities and smaller bulb size appears to be consistent across many studies throughout the world.
Other aspects of transplant production have been investigated. For example, Sabota and Downes (1981) reported that onion yields were reduced when onion transplant roots were pruned and when smaller transplants were used.
The objectives of these experiments were to determine the effect of plant population on yield and quality, and to determine the effect of transplant size on yield and quality.
Abo-Zeid, M.I. & Farghali, M.A. 1996 Potassium fertilization and plant density effects on onion grown in different soils Assiut J. Agr. Sci. 27 33 45
Boyhan, G., R. Torrance, R. Blackley, J. Cook, R. Hill, and J. Paulk. 2004. Vidalia onion variety trials 2003–2004. 2004 Georgia Onion Res.-Ext. Rpt. No. 3-2004:1–4.
Gergon, E.B., Miller, S.A., Halbrendt, J.M. & Davide, R.G. 2002 Effect of rice root-knot nematode on growth and yield of Yellow Granex onion Plant Dis. 86 1339 1344
Gupta, R.P. & Sharma, V.P. 2000 Effect of different spacings and levels of nitrogen for production of export quality onion bulbs planted on raised bed Nwsl. Natl. Hort. Res. Dev. Foundation 20 13 16
Herison, C., Masabni, J.G. & Zandstra, B.H. 1993 Increasing seedling density, age and nitrogen fertilization increases onion yield HortScience 28 23 25
Mangual-Crespo, G., Ramirez, C.T. & Orengo, E. 1979 Effect of plant spacing and fertilizer levels on yield and dry bulb weight of onion cv. Texas Grano 502 J. Agr. Univ. Puerto Rico 63 417 422
May, A., Cecìlio-Filho, A.B., Porto, D.R. de Q., Vargas, P.F. & Barbosa, J.C. 2007 Plant density and nitrogen and potassium fertilization rates on yield of onion hybrids Horticultura Brasileira 25 53 59
Sabota, C.M. & Downes, J.D. 1981 Onion growth and yield in relation to transplant pruning, size, spacing, and depth of planting HortScience 16 533 535
Shock, C.C., Feibert, E.B.G. & Saunders, L.D. 2004 Plant population and nitrogen fertilization for subsurface drip-irrigated onion HortScience 39 1722 1727
Stoffella, P.J. 1996 Planting arrangement and density of transplants influence sweet Spanish onion yields and bulb size HortScience 31 1129 1130
U.S. Department of Agriculture 1995 United States standards for grades of Bermuda-Granex-Grano type onions U.S. Dept. Agr. Rpt Washington, DC
Viloria, A., Arteaga, L., Díaz, L. & Delgado, D. 2003 Effect of N-P-K fertilization and planting distance on onion (Allium cepa L.) yield Bioagro 15 129 133