Onions are becoming one of the more popular crops on area vegetable farms, and growers throughout the state are clamoring for information on how to produce high-quality uniform onion transplants for their operations.
High-quality onion seed is the most critical component in growing high-quality onion transplants. To grow quality transplants, growers should select a soilless sterile potting media for their plug trays. Most growers prefer sowing onion seed in plug trays using 288s or 200s (if you want larger plugs). When filling the plug trays make sure that you do not over-fill the tray and allow enough room for the placement of the seed. Also press down or firm the potting media in the plug cells prior to sowing the seed. If a grower is sowing Spanish onion seeds, they typically will sow one seed per cell. If the grower is sowing yellow or red onions, they frequently will sow 3 seeds per cell. Extra plants can be removed later and used to fill in missing cells.
Onions typically require 55 to 65 days from sowing to transplanting in the field under ideal growing conditions. Immediately after sowing the onion seed, the grower should uniformly water the plug trays. Onion seed will germinate in 7-8 days if the air temperatures are kept in the 65-75 F range. Cool temperatures can lead to delayed emergence and poor or uneven stands.
Onion transplants are subject to stretching unless a grower is using DIF to control growth. Growers wishing to keep their onion transplants compact should maintain their night temperature about 10 F warmer than the daytime temperature. So, if a grower is maintaining their nighttime temperature at 65 F, then they should be keeping the temperature in their greenhouse at 55 F during the day. By using DIF a grower can prevent their onions from becoming spindly.
Water quality should be assessed prior to growing any transplants, but onions grow best when the water source has a pH range of 5.5 to 7.0 with a low EC or salt level. Moisture management is critical in growing high-quality onion transplants. Overwatering can lead to damping off, poor root growth, or disease development. The edges of onion plug trays can also dry out before the center of the tray so a grower should monitor moisture levels regularly. While we do not like to see moisture stressed plants, most successful onion transplant growers try to confine their watering to the morning hours. Late afternoon watering under cloudy gray skies leads to longer drying times which could promote foliar diseases in the onion plugs.
Onion transplants are considered to be light feeders. The starter fertilizer charge in most potting media will last 2-3 weeks. About 2-3 weeks after onion germination and after the first true leaf has developed growers can begin to apply nutrients via fertigation. Growers should be applying fertilizer precisely using a calibrated fertilizer injector. Growers can opt to fertilize the crop once a week at 100 ppm nitrogen or use 50 ppm nitrogen via constant liquid feed. While many different water-soluble fertilizers can be used, I would opt for a 15-5-15 fertilizer. This fertilizer has a lower phosphorus level which will reduce the likelihood of stretching.
Trimming onion transplants can be a controversial topic since the physical removal of onion foliage would seemingly encourage the risk of disease because of the wounding process. While this is a valid concern, growers find that trimming onion foliage in the transplant house yields a stronger more compact plant that is easier to mechanically transplant. Growers that advocate for trimming recommend trimming the onion plants to 4″ in height after the first true-leaf develops and repeating the procedure if necessary.
Some growers have modified lawnmowers and other equipment to trim onion foliage. It is critical however that all blades be kept sharp and that growers only trim onion foliage on bright sunny days. After onion transplants have been trimmed all “clippings” (leaf blades) should be removed from the plug trays.
As growers move closer to transplanting their onions in the field, they should consider hardening them off. Approximately one week prior to transplanting in the field consider moving the onions outdoors. Reduce watering and fertilization to slow growth and encourage hardening. Some growers like to make a single insecticide application to the onions outdoors while in the plug trays to discourage onion maggots from laying their eggs on the onion foliage.
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