Introduction
Transplanting onions is not a common practice in the United States, but bare root transplants are used to establish stands of early maturing onions in a few growing regions. Use of plug-grown onion transplants is almost unknown. The primary reasons for transplanting bulb onions are to obtain earliness, to obtain a stand when soil or weather conditions are unfavorable for direct seeding, or to allow for multiple cropping. Transplants may provide a means for western Oregon growers to establish a stand on muck soils following spring fumigation for control of white rot and other pests. The season is usually too short to allow spring fumigation followed by direct seeding.
Growing and transplanting plugs is expensive compared to direct seeding. One means to reduce this cost is to grow multiple seedlings per cell, a practice used to some extent in the United Kingdom. This practice could lead to excessive crowding of plants and deformed bulbs at harvest, particularly at the high plant populations (80,000 to 120,000 per acre) common in production of bulb onions for storage. A preliminary trial in 1988 established the feasibility of using multiple seedlings per plug, since acceptable yields of well-formed, large bulbs could be obtained as long as plant population did not exceed four per foot, with two or three seedlings per cell. The purpose of the 1989 trial was to investigate the effects of plug size, number of seedlings per plug, and within-row spacing on the yield and quality of storage onions. Plug sizes were limited to commonly available sizes which would require minimal greenhouse bench space.
Reasons for transplanting green or bunching onions include to achieve earliness, to multiple crop, or to reduce the labor involved in bunching the onions in the field or packing shed. Seeding enough plants per plug to make a pre-formed bunch would save on labor if most plugs would produce a marketable bunch without having to separate and rebunch the stems. The major drawback is the large plant population per acre used in bunching onions and the consequent large number of plugs needed to establish a planting. The objective of this trials was to investigate the effects of three sizes of plugs, three populations per plug, and two within-row spacings on the number of marketable bunches per unit area.
Methods
For the storage (bulb) onion trial, 'Granada' was seeded in an unheated greenhouse on 28 March and 10 April, 1989, into two sizes of plugs. The smaller size was a 288-cell tray from Landmark Plastic Corp. with 5.1 cm3 per cell. The larger size was a 128-cell Landmark tray with 31 cm3 per cell. Both trays were filled with a peat-vermulite medium. Either 2, 4, or 5 seeds per cell were placed in each plug. Seedlings were thinned to 1, 2, or 3 per cell at the first true leaf stage. The seedlings were watered daily as needed and fertilized weekly with a 10N-13P-16.7K soluble fertilizer at 100 ppm N.
The plugs were transplanted to a Willamette silt loam on 8 May (first seeding) or 30 May (second seeding), to which had been applied 1000 pounds per acre of a 10N-8.7 P-16.7K fertilizer. The treatments consisted of a factorial combination of the two cell sizes, two populations per cell (2 and 3), and two within-row spacings (6 and 12-inch). In addition, a check treatment consisted of one seedling per 288 cell on six-inch spacing. For all treatments, plot size was a 10-foot section of a 4-row bed with 12 inches between rows. Target plant population per acre ranged from a low of 69,696 (1 per cell, 6-inch spacing or 2 per cell, 12-inch spacing) to a high of 209,088 (3 per cell, 6-inch spacing).
Propaclor herbicide was applied at 4 pounds/acre immediately after planting. Oxyfluorfen was applied at 0.25 pounds/acre on 13 June (first planting) or 21 July (second planting). An additional 100 pounds per acre of N was applied on 9 June to the first planting and on 22 June to the second planting. Methomyl was applied at 1.0 pound/acre on 21 July for thrips control. Plots were rated for degree of bolting on 31 August and harvested on 20 September (first planting) or 25 September (second planting), after all tops were down. Bulbs were separated into three size categories (less than 2-inch diam., 2 to 3-inch diam., and over 3-inch diam.), weighed, counted, and misshapen bulbs noted.
For the bunching onion trial, methods were similar except as noted below. 'Ishikura' onion was seeded into three sizes of plugs in an unheated greenhouse on 30 March, 1989. The smallest plug was a 5.1 cm3-cell in Landmark Plastic Corp. 288-cell trays, the medium plug was a 9.2 cm3-cell in Growers' Transplanting, Inc. 256-cell trays, and the largest plug was a 31 cm3-cell in Landmark 128-cell trays. Either 6, 8, or 10 seeds were counted into each cell.
The plugs were transplanted 3 May. The treatments consisted of a factorial combination of the three cell sizes, the three populations per cell, and either 6-inch or 12-inch spacing between bunches in the row. Target plant population per acre ranged from a low of 209,088 (6/cell, 12-inch spacing) to a high of 696,960 (10/cell, 6-inch spacing). An additional 90 pounds N per acre was applied as ammonium nitrate on 9 June. Plots were harvested on 11 July. Marketable (0.25 to 0.5-inch, no blemishes) stems were counted for each bunch.
Results
Storage Onions
Plant survival, expressed as the percentage of bulbs recovered at harvest compared to the number of seedlings planted, was decreased by greater number of seedlings per cell for both plantings (Tables 1 and 2). Larger cell size produced greater survival in the second planting but not in the first. Thus, crowding, whether due to cell size or number of seedlings per cell, appears to reduce seedling survival. The same trend was evident in 1988.
The percentage of bulbs forming seed stalks (bolting) was much smaller in 1989 than in 1988, perhaps because the seedlings were less developed at time of transplanting in 1989. Nevertheless, there was a significant effect of within-row spacing on bolting percentage for the first planting: bolting was nearly three times more likely when the spacing was six inches (Table 1). This is consistent with 1988 results. Bolting percentage did not vary significantly with treatment for the second planting, and unlike 1988, did not vary significantly with plug size or seedlings per cell in the first planting.
The percentage of small bulbs decreased with the larger cell size for the first planting but not for the second planting. Likewise, total yield and mean bulb weight tended to increase with the larger cell, but the differences were not statistically significant. In the second planting, cell size had no effect on bulb size distribution. In 1988, with a much larger difference in cell sizes, size distribution and mean bulb weight were affected to a much greater degree by cell size.
Three, rather than two, seedlings per cell increased the percentage of small bulbs and reduced mean bulb weight in both plantings, but tended to increase total yield, due to the larger number of bulbs harvested. Increasing the within-row spacing from 6 to 12 inches increased mean bulb weight and the percentage of large bulbs, but decreased yields since plant populations were halved. The percentage of misshapen bulbs was not greatly affected by treatment in 1989.
Compared to the check treatment of only one seedling per cell, all other treatments reduced mean bulb weight and the number of large bulbs produced, but only reduced total yield with 12-inch spacing in the first planting (Table 3). There were no significant two or three-way interactions among cell size, seedlings per cell, and spacing. However, the interaction means are given in Table 3 to permit comparison of each treatment combination with the control.
For the second planting, all combinations of treatments produced higher yields than did the check. Mean bulb weight was unusually low for the check, with both treatments having two seedlings per plug and 12-inch spacing tending to surpass the check in bulb weight.
This trial reaffirms the feasibility of using two seedlings per cell, halving the greenhouse bench space, trays, and media needed to establish a stand. Both cell sizes used in this trial produced acceptable seedlings and had little effect on subsequent growth.
Bunching Onions.
Plants in the 256 and 128-cell trays were not ready to pull easily on 3 May, but bunches in the 288 trays pulled readily. It may have been better to have trimmed the seedlings back and held them in the greenhouse a few more days.
Neither spacing between bunches in the row nor plug size affected the number of stems per bunch recovered at harvest (Table 4). Thus, the relative crowding imposed by the 6-inch spacing or the smaller cells did not affect seedling survival and development into a marketable stem. The actual number of marketable stems recovered increased from 5.1 when six seeds were planted to 7.8 when 10 seeds were planted. Since this reflects a decrease in the recovery from 85.7 percent when six seeds were planted to 77.8 percent when 10 seeds were planted, crowding and seedling competition increased with greater numbers of seedlings per plug.
The actual number of good bunches (those containing six or more marketable stems and not needing rebunching) recovered per unit area did not vary with plug size, but increased markedly as the number of seeds per plug increased or as the within-row spacing between bunches decreased (Table 4). The percentage of marketable bunches recovered increased somewhat at 12-inch as opposed to 6-inch spacing, but not nearly enough to offset the reduced plant population at 12-inch spacing. The percentage of marketable bunches increased dramatically at 10 seeds per plug compared to six or eight seeds per plug, indicating that plugs should be overseeded by 50 percent or more compared to the desired or minimum acceptable number of stems per bunch. However, the degree of overseeding necessary should vary with the percent germination of the seedlot and may vary with plug size for very small plugs.
The potential number of marketable bunches per unit area, assuming that all bunches having too many or too few stems are rebunched to six marketable stems per bunch, was also not affected by cell size. Potential bunches per unit area increased with 6-inch spacing and with the greatest number of seeds per plug. However, in percentage terms, the potential bunches was closest to the maximum possible for 12-inch spacing and 6 seeds per plug.
There were no significant two or three-way interactions affecting stems per bunch or bunches per acre. However, the three-way interactions are given in Table 5 so that all treatment combinations may be compared. The greatest number of stems per bunch occurred with the combination of 6-inch spacing, 10 seeds per plug, and the smallest plug size. The greatest number of marketable bunches per unit area and, if rebunching is assumed, the greatest potential yield also occurred with this combination. However, any other plug size also gave yields which were not signifantly different from those with 6-inch spacing, 10 seeds per plug, and the smallest plug.
These results affirm the possibility of growing good quality bunching onions from plugs. An economic analysis of the profitability of plug-started bunching onions is not possible from this data. The relative profitability would depend on the net effect of the increased cost of establishing a stand from plugs and the possibly reduced costs for bunching labor.
Table 1. Main effects of plug size, number of seedlings per plug, and within-row spacing between plugs on seedling survival, bolting, bulb size, and yield of transplanted 'Granada' onion, first planting, 1989, NWREC, Oregon Seedling % % bulbs by no. Total yield Mean bulb Misshapen Treatment survival (%) bolted 1-2" 2-3" 3-4" cwt/acre wt. (g) (% by no.) Plug size Small 85 0.6 16.0 79.9 4.0 331 143 5.1 Large 87 1.0 10.6 86.2 3.1 355 148 4.9 NSz NS * NS NS NS NS NS No./plug 2 96 0.9 7.5 88.7 3.7 326 155 4.2 3 80 0.6 19.1 77.5 3.4 350 136 5.9 ** NS ** * NS NS * NS Spacing 6-inch 87 1.1 18.3 80.9 0.8 409 125 6.0 12-inch 85 0.4 8.3 85.3 6.4 268 166 4.0 NS ** ** NS * ** ** NS Checky 98 0.3 1.3 85.2 13.5 306 203 0.0 ____________________________________________________________________________________ zNS,*,**: Nonsignificant, significant at the 5 % level, and significant at the 1 % level, respectively. ySmall plug, 1 seedling/plug, 6-inch spacing. Table 2. Main effects of plug size, number of seedlings per plug, and within-row spacing between plugs on seedling survival, bolting, bulb size, and yield of transplanted 'Granada' onion, second planting, 1989, NWREC, Oregon Seedling % % bulbs by no. Total yield Mean bulb Misshapen Treatment survival (%) bolted 1-2" 2-3" 3-4" cwt/acre wt. (g) (% by no.) Plug size Small 83 0.3 17.2 75.0 7.7 350 158 4.6 Large 95 0.4 18.5 73.3 8.1 384 151 4.0 ** NSz NS NS NS NS NS NS No./plug 2 93 0.5 11.5 77.0 11.6 350 171 3.7 3 86 0.2 24.3 71.4 4.3 384 138 4.9 * NS ** NS ** NS ** * Spacing 6-inch 90 0.3 24.3 72.8 2.9 445 132 4.7 12-inch 87 0.4 11.5 75.6 12.9 289 177 3.9 NS NS ** NS ** ** ** NS Checky 93 0.0 13.5 83.0 3.5 235 165 0.2 ____________________________________________________________________________________ zNS,*,**: Nonsignificant, significant at the 5 % level, and significant at the 1 % level, respectively. ySmall plug, 1 seedling/plug, 6-inch spacing. Table 3. Interaction of plug size, number of seedlings/plug, and within-row spacing between plugs on yield and mean bulb weight of transplanted 'Granada' onion, two plantings, 1989, NWREC, Oregon Plug No./ Spacing Plants/ First planting Second planting size plug (inches) acrez Yield Mean bulb Yield Mean bulb cwt/acre wt. (g) cwt/acre wt. (g) Small 2 6 139392 396 131 378 145 Large 2 6 139392 381 137 439 152 Small 3 6 209088 428 108 457 118 Large 3 6 209088 430 123 508 115 Small 2 12 69696 264 179 297 208 Large 2 12 69696 265 171 288 179 Small 3 12 104544 235 154 269 162 Large 3 12 104544 306 161 302 157 Smally 1 6 69696 306 203 235 165 LSD (0.05) 57 36 105 32 zTarget population yCheck Table 4. Main effects of within-row spacing, number of seedlings per plug, and plug size on plants per bunch, number of marketable bunches per acre, the percentage of marketable bunches, and the number of bunches obtained after rebunching of green onions, 1989, NWREC, Oregon Stems/bunch Good bunches/acrez Potential bunches/acrey _________________ ___________________ ________________________ Treatment No. % of nominal No. % of nominal No. % of nominalx _____________________________________________________________________________ Spacing 6-inch 6.4 80.2 48500 69.6 76250 82.1 12-inch 6.4 79.5 27010 77.5 40460 87.1 NSw NS ** ** ** * No./plug 6 5.1 85.7 22940 43.9 47430 90.6 8 6.2 78.1 40510 77.5 55900 80.2 10 7.8 77.8 49800 95.3 71830 82.4 ** ** ** ** ** * Plug size Small 6.5 80.9 40370 77.2 61030 87.6 Medium 6.3 78.5 35860 68.6 56630 81.3 Large 6.4 80.1 37030 70.8 57410 82.4 NS NS NS NS NS NS zActual number of good bunches of 6 to 8 onions recovered without rebunching. yPotential 6-stem bunches after rebunching of over- or undersized bunches. xPercentage of stems or bunches that would have been possible if all seedlings survived and developed normally. wNS,*,**: No significant differences; differences significant at the 5% and 1% levels, respectively. Table 5. Interaction of within-row spacing, number of seedlings per plug, and plug size on stems per bunch, number of marketable bunches per acre, and the number of bunches obtained after rebunching of green onions, 1989, NWREC, Oregon No./ Plug Stems/ Good bunches/ Potential bunches/ Spacing plug size bunch acre acre 6-inch 6 Small 5.3 33110 65340 Medium 5.0 21780 56630 Large 5.0 33110 61860 8 Small 6.7 63600 81890 Medium 6.1 54890 74920 Large 6.1 40950 65340 10 Small 8.2 66210 97840 Medium 7.5 59240 86250 Large 7.9 63600 96180 12-inch 6 Small 5.2 15680 33980 Medium 4.9 10450 29620 Large 5.5 23520 36590 8 Small 6.0 27010 36590 Medium 6.4 30490 39200 Large 6.1 26140 37460 10 Small 7.7 36590 50530 Medium 7.9 38330 53140 Large 7.6 34850 47040 LSD (0.05) 0.8 13370 13380