Onion is an important crop in Georgia, with a farm gate value over $125 million in 2005 (Boatright and McKissick, 2006). This important commodity is produced in southeastern Georgia in the Vidalia-growing region, which is protected by Federal Market Order 955 (Boyhan and Torrance, 2002). This high-value crop is produced from transplants 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.
Using transplants for onion production is labor intensive and is not practiced in all onion-producing regions. A number of different aspects of transplant production have been investigated concerning onions. Al-Abdulsalam and Hamaiel (2004) investigated planting date and fertilizer and found that 20 Oct. was the optimum planting date with moderate fertilizer application in comparison with 20 Sept. or 20 Nov. transplanting dates in Al-Hasa, Saudi Arabia. In another study from the Middle East, El-Rehim et al. (1997) found that at the Shandaweel Experiment Station, Egypt, the best yields were with transplants set on 15 Nov. compared with 5 Feb.
With short- and intermediate-day onions grown in Queensland, Australia, it was found that the highest yields occurred with April plantings (70–84 t·ha−1) compared with February/March (54–56 t·ha−1) or May/June (60–63 t·ha−1) plantings (Jackson et al., 2001). In another study evaluating transplant date and variety, the best transplanting date for total yield was on 15 July compared with 30 July, or 15 or 30 Aug. (Pandey et al., 1992).
Sowing date was shown to have an effect on onion production in Louisiana, with the best yields for onions sown on 3 Oct. compared with 23 Sept. or 10 Oct., with the former planting date resulting in increased bolting (Mulkey and Talbot, 1991).
Fall-planted onion sets had greater losses compared with spring-planted sets, with temporary plastic covers having no effect on yield in Poland (Tendaj and Gruszecki, 2002).
Herison et al. (1993) found that older transplants (12 vs. 8 weeks old) resulted in larger bulbs and increased yield at harvest. Transplant age has also been investigated in the tropical West African nation of Ghana. Kanton et al. (2002) found that maximum yield was produced from transplants that were 20 to 40 d old, and 40-d-old transplants produced the heaviest bulbs. In addition, the lightest bulbs were from 70-d-old transplants.
There were several objectives for these experiments. One objective was to determine how long onions could be transplanted after transplants were judged ready, while still having good yields of quality onions. Onion transplants are ready in 8 to 10 weeks from sowing, but typically require 8 weeks or longer for the entire onion crop to be transplanted. In addition, onions with a propensity for double bulbs were evaluated for the effect of sowing and transplanting date on reducing this phenomenon.
Al-Abdulsalam, M.A. & Hamaiel, A.F. 2004 Effect of planting dates and compound fertilizers on growth, yield and quality of Hassawi onion under Al-Hassa oasis conditions Scientific J. King Faisal Univ. (Basic and Appl. Sci.) 5 65 79
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.
El-Rehim, G.H.A., Ahmed, F.A., Shalaby, G.I. & Waly, A.A. 1997 Effect of transplanting date and planting density on bulb yield and quality of Giza 20 onion in upper Egypt Egyptian J. Agr. Res. 75 681 695
Herison, C., Masabni, J.G. & Zandstra, B.H. 1993 Increasing seedling density, age and nitrogen fertilization increases onion yield HortScience 28 23 25
Jackson, K.J., Duff, A.A. & O'Donnell, W.E. 2001 Development of a suite of onion varieties for the subtropical region of the Lockyer Valley in S.E. Queensland, Australia Acta Hort. 555 239 242
Kanton, R.A.L., Abbey, L., Hilla, R.G., Tabil, M.A. & January, N.D. 2002 Influence of transplanting age on bulb yield and yield components of onion (Allium cepa L.) J. Veg. Crop Production 8 27 37
Pandey, U.B., Qadri, S.M.H., Chougule, A.B. & Tripathi, S.P. 1992 Studies on effect of dates of transplanting on yield and quality of different onion varieties Nwsl. Assoc. Agr. Dev. Foundation 12 5 8
Tendaj, M. & Gruszecki, R. 2002 The effect of the planting time of onion sets and kind of cover on the yield of early bunching onion Ann. Univ. Mariae Curie-Sklodowska, Section EEE 11 61 69
U.S. Department of Agriculture 1995 United States standards for grades of Bermuda-Granex-Grano type onions U.S. Dept. Agr. Rpt Washington, DC