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How to Grow Onions- What is a Sweet Onion?

“Sweet Onion is a legal term”

Among produce crops, onions and members of the Allium family (onions, garlic, shallots, leeks and chives) are somewhat unique in their growth habits and fertility requirements. Onions are the third largest fresh vegetable industry in the United States, with an annual per capita consumption of about 20 pounds and growing. ISP has worked with more acres of onions than any other produce crop except melons.

As a crop, onions require rather large amounts of nitrogen, yet it needs to be applied in the front half of the season, as late season applications can create serious bulb quality issues. Another significant challenge can be the variation in final bulb size resulting from mixed seedling vigor, nutrient availability or other growth factors. Finally, there is the chore of properly curing (drying) the onion bulb so that it has adequate storage capability.

Currently, we work with a high percentage of Candy onions. This season we will study more than 20 other varieties to provide growers with secondary variety choices, additional fertility information, and even exploring other market potentials in the arena of gourmet onions. Onions can be a good crop to grow when starting a vegetable garden, a gardener looking for plants to grow that don’t require a lot of maintenance would enjoy growing onion in their garden.

 “Onions Can Make You Cry ” :’ (

When you cut into an onion gas is released, Pyruvic Acid in Sulfur is what makes your eyes burn.  It is recommended to grow onions on grounds with lower levels of sulfur for sweet onions. When selecting a sweet onion it is expected to have at least 6% sugar and will hold more water than a storage onion, diluting the kick onion harness and indigestion problems. Locally grown fresh market onions taste very different than most store bought onions, they are to be taken advantage of for cooking during season because they will not store long. Taking the right management approach for growing onions will yield a quality with great flavor. Resulting in a onions with higher sugar content and lower Pyruvic Acid levels.


General Recommendations for Growing High Yield & Quality Onions

isp large onion feild
ISP Technologies Onion Trials

1. Ensure that your soil has high levels of available nutrients for optimum yield potential. Apply necessary fertilizer for optimum nutrient efficiency and plant uptake.

2. Prior to Planting/Transplanting – Incorporate into the bed 32 ounces of SB. There is also evidence that humates can be beneficial, especially in sandy, high magnesium or sodic soils. If desired, apply 64 ounces – 2 gallons of PhytoGro, or Pow’r Pak.

3. Flag Leaf Foliar Nutrients: If direct seeding into the beds, or starting transplants in a greenhouse, benefits have been observed through research that a high phosphorus plant food such as 10-45-10 or 15-30-15 can be beneficial to encourage more aggressive rooting. Under weather stress, seedling survival may also be increased with this foliar application.

4. Early Season Drip: Onions require rather high amounts of nitrogen. With a potential of N loss through leaching and/or denitrification from one or two large applications, we recommend adding N to all irrigations just prior to initial bulb swell continuing through the next 4 – 5 weeks. NC237 is a good choice, as the nitrogen and calcium combination can often enhance overall N efficiency.

5. Early bulb swell: 8 – 10 pounds 5-25-25 Onion Special (OS), and 16 ounces MetaCal applied as a foliar. Watch leaf tips; if any yellowing is occurring this is an indication of deficient nitrogen. All nitrogen should be applied within 5 weeks following early bulb swell. Late season N applications can be very detrimental toward bulb quality.

6. Mid Season Vegetative: Following early bulb swell the onion will continue to grow proportionately, with new leaves emerging as the entire plant enlarges. Although soil nutrients will play the most important role in growth, alternating drip applications of MetaCal (32 ounces/week) and PhytoGro (32 ounces/week) can be of benefit. Foliar applications of 6 – 8 pounds of 5-25-25 OS have shown positive results for yield and sweetness.

7. Primary Bulb Swell (Last Third of Season): Foliar up to 15 pounds per week of 5-25-25 OS, with 4 ounces Seed Boost, and 16 ounces MetaCal. Research observation has shown that this will tend to “tighten” the neck area of the onion, thus reducing susceptibility to disease.