NEWSLETTER ARTICLES

Small Farms and Specialty Crops

EFFECTS OF TRANSPLANT DATE AND TRANSPLANT SIZE ON PRODUCTION, QUALITY AND PUNGENCY OF SWEET ONIONS

A combination of daylength, growing season temperatures, and winter low temperatures limit the adaptability of these cultivars to somewhat specific growing conditions. The short-day nature of these cultivars means that they get a bulbing signal when daylength is around 12 or more hours. This typically happens in mid-March with the specific day and time depending upon latitude. Temperature interacts with daylength to affect bulbing and subsequent growth and bulb development.

We have conducted trials over the past three seasons in Santa Barbara and San Luis Obispo Counties which have shown that transplanting too early (November /December) contributes to a high percentage of bolting - particularly with larger transplant sizes. Previous studies have shown that transplanting in January and February would avoid the threat of bolting in coastal growing areas and still allow adequate time for normal foliage and bulb development. The size of the transplant is also important, however, as larger transplants often produce larger bulbs but are also most likely to be induced to bolt by late temperatures.

We sought to answer the following questions with four representative "sweet" type short-day onion cultivars:

• Does production, post harvest quality and pungency vary among a range of planting dates in January and February?
• Is production, post harvest quality, or pungency affected by transplant size?
• How might production and post harvest quality of "sweet" type onion cultivars be affected by latitude when the same sized transplants are planted on three dates in January and February?

EXPERIMENTAL
Onion transplants were seeded at the Speedling Transplant nursery in Nipomo on three successive dates in November and December 1997. Four short-day onion cultivars, which had previously shown harvest quality acceptable for sweet onions, were selected. These cultivars were Granex 33, Linda Vista, Primavera and Texas 1015. Onions were seeded into three transplant cell sizes at each seeding date, corresponding to 280, 392, and 595 cells per tray. The higher the number of cells, the smaller the cell (and transplant) size. All cultivar and transplant size treatments were transplanted approximately 10 weeks from seeding in Buellton (Santa Barbara County). The approximate transplant dates were January 27, February 13, and February 27, 1998. Plots were simultaneously transplanted within 2-3 days of these dates in Ventura County and in Hollister and Watsonville.

RESULTS
Delaying transplanting from January 27 to February 15 and then to February 27 reduced bulb size across all cultivars and cell sizes. Bolting did not occur at the Buellton site (Santa Barbara County) in any cultivar or transplant size in our earliest (January 27) transplanting. From the accumulation of data over the past three years, it appears that optimum transplant date for sweet onion cultivars in Santa Barbara County lies somewhere between December 15 and February 1. This time period will likely vary some from year-to-year and also depending upon the specific location and microclimate conditions. It is possible to produce sweet onions outside of this range of dates, but earlier plantings will have a higher percentage of bolting and bulb size will fall off as transplanting is delayed.

At the first two transplant dates (January 27 and February 15), small transplants (595 cell per tray) produced smaller onions in three of the four cultivars. The role of transplant size is not related to harvested bulb sizes with the last transplanting date (February 27) and larger transplants may actually produce smaller bulbs when transplanted late. Clearly, larger transplants will produce larger onions as long as bolting is avoided by controlling transplant date. The larger transplants had more foliar development prior to induction, and more leaves mean larger onions.

It is unclear why larger transplants may produce smaller onions with the latest transplanting. It may be that the larger transplants get induced to bulb earlier than smaller transplants at a given daylength and temperature regime. This may cause bulbing to take place in competition with the development of new leaf tissues. When transplanted late, these onions simply don’t have adequate foliage when induced to bulb.

The Primavera cultivar (Petoseed) generally outperformed the other cultivars across transplant sizes and transplant dates. Primavera is a globe type (round) onion which produces a high percentage of jumbo sizes when managed well. Primavera has been grown in diverse environments as a sweet onion and pungency is typically low. Linda Vista and Granex 33 will also produce a high percentage of jumbo sizes if managed well and are typically high quality sweet onion cultivars.

The Texas 1015 onions are unique and seem to act differently than the other three cultivars. They don’t show an effect from transplant date or transplant size consistent with the other cultivars. They have a narrower range of optimum transplanting date and optimum transplant cell size. The Texas 1015 onion was actually bred for sweet onion characteristics and for October 15 (1015) seeding in South Texas. It may be that the breeding inadvertently narrowed the range of adaptability.

We used laboratory analyses for pyruvic acid development as our indicator of pungency. Pyruvic acid development or PAD is widely used now in the produce industry and is accepted as an index of pungency for sweet onions. To be acceptable as a sweet onion, the marketplace generally will accept a PAD of 5.5 or below, and the lower the PAD, the milder the onion.

Overall, transplant date did not affect pungency, dry weight, or sugar concentration, with average values of 4.2 (range = 2.9-5.6) µmole pyruvate/g fr.wt., 7.9 (range = 6.6-10.2) % dry wt, and 42 (range = 29-49) milligram (mg)/gram fresh weight (g fr. wt.), respectively (Table 1.) Location, however, resulted in important differences, with pungency values at 2 locations of 4.7-5.0 and at the other two of 3.3-3.6 µmole/g fr.wt. The most dramatic differences in pungency were between the Hollister and Watsonville sites. Despite their close proximity, average PAD values were 3.3 at Watsonville and 5.0 at Hollister. These values represented the high and low values of the four sites evaluated. Soil sulfur level has been shown to elevate PAD because onions will continue to accumulate sulfur beyond their metabolic needs and pyruvate builds up. Soil sulfur levels are generally higher on higher organic matter soils, and this may be a factor contributing to the differences observed between the Hollister and the Watsonville site.

It is clear from these and previous trials over the past three years that high quality sweet onions can be produced in several coastal growing areas. Such factors as cultivar, transplant date, transplant size, and soil sulfur levels are all special considerations for sweet onions beyond other more general aspects of sound onion management.

Table 1. Dry weight, sugar concentration and pungency (pyruvic acid development) of four cultivars of sweet onions grown at four locations in California. Harvested June 1998.