Plug Transplanting of Onions (1988)


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 are to obtain earliness, to obtain a stand when soil or weather conditions are unfavorable for direct seeding, or to allow for multiple cropping. Transplanting 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 diseases, weeds, and insect pests. The season is usually too short to allow spring fumigation followed by direct seeding.

Growing seedlings in cells or plugs in a greenhouse 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 the plants in the row and deformed bulbs at harvest, particularly at the high plant populations (80,000 to 120,000/acre) common in production of bulb onions for storage.

The purpose of this preliminary trial was to investigate the effects of plug size, number of seedlings/plug, and spacing of the plugs in the row on yield and quality of storage onions. The trial was conducted on a mineral soil in the expectation that this would be a more difficult test of bulb shape and quality than with the same variety on a muck soil since the maturing bulbs are thought to spread apart more easily on muck.


'Granada' onion was seeded in an unheated greenhouse on 21 March, 1988, into two sizes of plugs or cells. The smaller plug size was a 256-plug tray from Growers' Transplanting, Inc. The 0.75-inch-square cells were filled with a peatlite medium to a depth of about 1 inch. The larger plug was a 2-inch-square plastic pot, filled to a depth of about 2.5 inches. Either 2, 4, or 5 seeds were placed in each plug. Seedlings were thinned to 1, 2, or 3 per plug at the first true leaf stage. The seedlings were irrigated daily as needed and fertilized weekly with a ION-13P-16.7K soluble fertilizer at 100 ppm N.

The plugs were transplanted to a Willamette silt loam, pH 6.0, on 12 May. The treatments consisted of a factorial combination of the two cell sizes, the three populations per plug, and in-row spacing between plugs of 6 or 12 inches. In addition, a check treatment consisted of single seedlings grown in the small plugs and planted on 4-inch spacing. For all treatments, plot size was a 10-foot section of a 3-row bed with 15 inches between rows. Target plant populations per acre ranged from a low of 26,136 (1 plant/plug, 12-inch spacing) to a high of 156,816 (3 plants/plug, 6-inch spacing).

Propaclor herbicide was applied at 4 pounds/acre immediately after planting and was reapplied one month later at the same rate in combination with oxyfluorfen at 0.25 pounds/acre. Weeds were also hand-hoed once. Methomyl was applied at 1.0 pounds/acre for thrips control on 20 June, followed by an application of azinphosmethyl on 11 July. Treatments were rated for degree of bolting and percentage of tops down on 19 August. Bulbs were topped on 1 September and harvested on 8 September. Bulbs were separated into four size categories (less than 2-inch diameter, 2- to 3-inch diameter, 3- to 4-inch diameter, and 4- to 5-inch diameter), weighed, counted, and misshapen bulbs noted.


Plant survival, expressed as the percentage of bulbs recovered at harvest compared to the number of seedlings planted, was increased by the larger plug, and by 1 or -2 seedlings/plug compared to 3 seedlings/plug (Table 1). In-row spacing between plugs had no significant effect on plant survival, but there was a trend toward greater survival at the 12-inch spacing. Thus, crowding of the seedlings reduced seedling survival. Seedlings in the smaller plugs and, especially, with multiple seedlings/plug, were noticeably less developed at time of transplanting. There were no significant interactions between plug size, seedlings/plug, and in-row spacing, so only main effects are given in the tables.

Maturity, as reflected in the percentage of tops down two weeks before harvest, was greater for the larger plug size, for 2 or 3 seedlings/plug as compared to 1 seedling/plug, and for the 6-inch in-row spacing (Table 1). Thus, higher plant populations favored earlier maturity. However, this was not strictly an effect of crowding, as plants from the larger, less crowded plugs, were also more mature. There were no significant interactions, so only main effects are given in the Table.

The percentage of bulbs forming seed stalks (bolting) was increased by large plug size, by increasing number of seedlings/plug, and by the closer in-row spacing (Table 1). The same factors affecting maturity were also apparently affecting seed stalk formation. Generally, crowding was associated with greater degree of bolting; however, the delayed maturity in the small plugs somewhat offset the effects of crowding.

Main effects of treatments on yield are presented in Table 2. The larger plug size, when averaged over the other treatments, produced more bulbs in the largest size category and reduced the number of bulbs in the smallest size categories. Total yield and mean bulb weight were greater for the larger plugs and there were fewer misshapen bulbs with the larger plugs.

Increasing the number of seedlings/plug increased the percentage of bulbs in the two smaller categories, at the expense of the larger bulbs, and mean bulb weight decreased markedly with increasing number of seedlings/plug. Nevertheless, total yield increased with increasing number of seedlings/plug as the much greater plant population more than offset the effect of reduced bulb size. The number of misshapen bulbs also increased sharply with multiple seedlings/plug.

Increasing the distance between plugs in the row greatly increased the percentage of bulbs in the larger-sized categories and increased mean bulb weight, but reduced yield as the effect of the larger bulbs was more than offset by the reduced plant population. Increasing the distance between plugs slightly reduced the percentage of misshapen bulbs.

There was a significant interaction of seedlings/plug and in-row spacing affecting the yield of large bulbs and mean bulb weight (Table 3). Increasing the number of seedlings/plug decreased large-bulb yield at 6-inch spacing, while the opposite was true with 12-inch spacing. The excessive crowding at 6-inch spacing and multiple seedlings/plug reduced bulb size. Bulb size decreased with increasing seedlings/plug much more at 6-inch spacing than at 12-inch spacing.

The interaction of all combinations of treatments on total yield, yield of large bulbs, mean bulb weight, and percentage of bolters and misshapen bulbs is given in Table 4. The greatest total yield occurred with the combination of large plugs, 3 seedlings/cell, and 6-inch spacing, but the percentage of bolters was also highest for this treatment and the percentage of misshapen bulbs was also very high. This combination of treatments produced the highest plant population. While the combination of small plugs, 3 seedlings/cell, and 6-inch spacing had the same target population, the actual number of bulbs recovered was higher for the large plugs due to greater seedling survival.

The greatest yield of large bulbs was with the combination of large plugs, 3 seedlings/plug, and 12-inch spacing, followed closely by small plugs, 2 seedlings/plug, 6-inch spacing, and large plugs, 1 seedling/plug, and 6-inch spacing. Generally, when the in-row plant population exceeded 4/foot, the yield of large bulbs was reduced dramatically.

Picking the best treatment depends on the total yield, the desired bulb size, and the economics of establishing the stand. No one combination of treatments can be considered the best, since the treatments producing the highest total yields were not necessarily those producing the greatest yields of large bulbs and acceptably low percentages of bolters and misshapen bulbs, nor were they necessarily the least expensive in terms of stand establishment.

Stand establishment costs are reduced with smaller plugs, greater numbers of seedlings/plug, and greater in-row spacing. The combination of small plug, 2 seedlings/plug, and 6-inch spacing, requiring 52,272 plugs/acre, and the combination of large plug, 3 seedlings/plug, and 12-inch spacing, requiring only 26,136 plugs/acre, produced essentially the same yield of both large bulbs and total bulbs and only a moderate amount of misshapen bulbs. This may appear to favor the latter treatment, since only half as many plugs are required. However, the large plugs require much more greenhouse bench space, 726 square feet for 26,136 plugs, versus only 204 square feet for the 52,272 small plugs.

This trial does establish the feasibility of using multiple seedlings per plug, as acceptable yields of well-formed, large bulbs could be obtained, as long as the population of bulbs did not exceed 4/foot. Several treatments produced yields of high quality, large bulbs that were greater than the yield obtained with the check treatment of 1 seedling/cell and 3 bulbs/row foot.


  Table 1. Main effects of plug size, number of seedlings/plug, and   in-row spacing between plugs on seedling survival, bolting, and   maturity of transplanted onions, 1988                                                  Seedling              %           % tops down  Treatment         survival (%)          bolted         on 19 Aug.  Plug size  Small                  89                3.6               13  Large                  99               12.4               24                         **                 **                *  No./plug          1                      97                4.1                5  2                      98                8.6               25  3                      92               11.2               26                          *                 **               **  Spacing           6-inch                 94               10.9               26  12-inch                96                5.1               11                         NSz               **                **  Checky                 92                7.3               15       zNS,*,**: Non-significant, significant at the 5% and 1% levels,   respectively.  ySmall plug, 1 seedling/plug, 4-inch spacing.      Table 2. Main effects of plug size, number of seedlings/plug, and in-row   spacing on yield and size distribution of transplanted onion, 1988                             % bulbs by number       Total yield  Mean bulb    Misshapen 	  	     1-2"   2-3"   3-4"   4-5"   cwt/acre     wt. (g)   (% by number)  Plug size								  Small	     6.5    27.8   59.7   6.1      437          333         7.6	  Large	     2.6    19.5   60.7  17.4      573          382         3.8	  	      NSz     *     NS    **        **           **          *	  No./plug								  1	     0.0    4.2    67.5	 28.3      404          485         0.0	  2	     2.0   26.8    66.2	  5.3      521          325         2.6	  3	    11.6   39.9    46.9	  1.6      590          263        14.3	  	      *     **	  **	  **        **           **         **	  Spacing								  6 inches     8.7   32.3    51.9   7.2      610          325         7.1	  12 inches    0.4   14.9    68.5  16.3      401          390         4.3	  	      **    **      **    **        **           **          *	    Check	     1.8   28.9    68.1   1.2      546          343         0.9      	  zNS,*,**: Nonsignificant, significant at the 5% and at the 1%   level, respectively.      Table 3. Interaction of number of seedlings per plug and in-row   spacing on yield by weight of large onions and average bulb weight  of all bulbs, 1988                                                  		    Yield of large bulbsZ      Mean bulb   No./cell   Spacing       (cwt./acre)            wt. (g)             1          6 inches        524                   487	  2          6               441                   280	  3          6               288                   209	  1         12 inches        268                   483	  2         12               380                   370	  3         12               426                   317	              LSD (0.05)     115                    64              	  Z3-5 inch diameter      Table 4. Interaction of plug size, number of seedlings/plug, and in-row   spacing on yield and quality of transplanted onions, 1988                      Plug  No./   Spacing     Plant      Bulb yield   Mean bulb    %        %   size  plug             population   (cwt/acre)    wt. (g)   bolted  misshapen                          per acre    Large  All                                 Small   1     6 inch     52,272      500   516      489      3.4      0.0               12 inch     26,136      246   251      463      3.5      0.0	          2     6 inch    104,544      553   577      267      5.0      5.4	               12 inch     52,272      307   349      319      4.4      2.8	          3     6 inch    156,816      135   509      177      3.3     21.9	               12 inch     78,408      296   424      281      2.3     15.6	  Large   1     6 inch     52,272      549   553      485      9.2      0.0	               12 inch     26,136      290   296      503      0.7      0.0	          2     6 inch    104,544      520   688      292     19.0      2.1	               12 inch     52,272      452   471      420      6.0      0.9	          3     6 inch    156,816      440   817      240     25.6     13.2	               12 inch     78,408      557   612      351     13.9      6.5	  SmallZ  1     4 inch     78,408      440   546      343      7.3      0.9	                        LSD (0.05)     163   131       90      5.8      4.7      zCheck