Onion Transplant Planting Guide
Best practices for growing onions from transplants
When your onion transplants arrive, REMOVE THEM FROM THE BOX IMMEDIATELY and keep them in a well-ventilated area, WITHOUT WATER OR SOIL until you are ready to plant.
Your onions are dormant and may appear dry. As a member of the lily family, onions can live off of the bulb for approximately three weeks. We recommend planting within one week.
Sun Requirement: Onions require full sun - the more the better! Avoid growing onions next to crops that might shade your onions.
Soil Fertility: Onions prefer soil with a pH between 6.2 and 6.8. If your soil is too acid, add ground limestone; if it is too alkaline, add peat moss. Added compost or a balanced organic fertilizer is needed to grow big, mild onions if your soil fertility is low. For the best growth and yield, onions need fertilizer at planting time and additional side dressing.(see "Planting" and "Fertilizing" below).
Planting: Onions grow best on raised beds or raised rows at least 4" high and 20" wide. Plant your onions 4 to 6 weeks before the last estimated spring freeze.
Fertilizer Trench: Dig a trench 4" deep and 4" wide and incorporate 1/2 cup fertilizer per 10 linear feet of row. Cover the fertilizer with 2" of soil. DO NOT plant onions in the fertilizer trench.
Plant the onions 6" from the edge of the trench on both sides of the trench. Plant the onions 1" deep and no deeper. Onions planted deeper than 1" may not bulb to full size.
If you want the onions to grow to maturity, space them 4" apart. If you prefer to harvest some earlier as green onions, space them 2" apart and pull every other onion during the growing season, leaving the rest to grow to maturity.
When planting several rows of onions, leave 16" between beds. The spacing from the center of one fertilizer trench to the center of the next should be 36".
Watering: For the largest bulbs, onions require 1" of water (rain or irrigation) weekly. Onions have shallow roots and need to be watered immediately after planting, and regularly thereafter. Drip irrigation is preferred to overhead irrigation, which may contribute to foliage diseases. If leaves develop a yellow tinge, cut back on watering. The greatest need for water is closest to harvest time.
Storage Onion Varieties: When the onion tops start to fall over, stop watering and let the soil dry out before harvesting.
Fertilizing: Every 2 to 3 weeks after planting, sprinkle a balanced organic fertilizer or compost on top of the original fertilizer trench at the rate of 1/2 cup per 10 feet of row and water. Stop fertilizing when the onions start to bulb (see Bulbing below).
Weeding: Onions to not like weed competition! Keep onions well weeded and the soil loose with shallow cultivation. Avoid allowing soil to cover the onion plants or their expanding bulbs so that a natural bulb will form. If mulching with a light layer of straw to help control weeds and preserve moisture, be sure to push the straw back when the plants start to bulb so they will cure properly (see "Bulbing" below).
Insects: The onion thrip, a light-brown insect that causes deformed plants with slivery blotches, is the most damaging to onions. Thrips overwinter in weeds so it is important to keep the garden or field weed free. Neem Oil can be used to combat the onion thrip.
Bulbing: When the ground starts to crack as the onions push the soil away, the bulbing process has begun. Stop fertilizing at this point.
Harvesting & Curing Storage Varieties: it's time to harvest when the tops of the onions turn brown or yellow and fall over. Bending the tops over early will decrease bulb size. Proper treatment at harvest maximizes storage time.
Pull the onions early in the morning on a sunny day and dry ("cure") the onions in the sun for two to three days. To prevent sunscald, lay the tops of one row over the bulbs of another. Onions must be cured thoroughly to avoid rot problems. When properly cured, the entire neck (where the leaves meet the bulb) should be dry all the way to the surface of the onion, and the skin will have a uniform texture and color. If the weather does not permit outdoor curing, you can dry your onions indoors by laying them out in a well-ventilated area. Drying indoors may take longer than outdoors.
Once the onions are thoroughly dry, clip the roots and cut back the tops to one inch or braid uncut tops together and hang onions in a well-ventilated area (see "Storing" below).
Storing: Store onions in a cool, dry, well-ventilated location, such as a garage or cellar. The best way to store onions is at near freezing temperatures with 65-70% humidity and in mesh bags or netting to permit better airflow. Periodically check for any soft onions and remove any that may have begun to decay.