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2021 Considerations: Optimizing Onions

Authors: Natalie Hoidal and Marissa Schuh, Extension Educators, University of Minnesota Extension

Our biweekly crop by crop growing season prep series continues. This week we are focusing on onions. The more you know about these likely issues, the better prepared you will be to deal with them.

Seeding time: weighing the risks of a wet spring

Many Minnesota gardening charts tell you to seed onions as early as February, but plenty of growers hold off until mid March. As a general rule of thumb, you should start your onions in Minnesota 10-12 weeks before planting, and transplant a month to a few weeks before the frost-free date. Some growers seed onions with a nurse crop of oats or another cereal to protect them from wind in early spring (which, mixed with cold weather, can cause significant plant stress), but the cereal nurse crop must be terminated with an herbicide, so this is not a viable option for organic growers. As such, organic growers may want to plant a bit more conservatively. Due to our unpredictable spring weather, some growers opt for starting a bit later in case they need to hold transplants longer than expected in the spring.

The main concern about keeping onion seedlings too long is that they tend to become spindly. There are a couple of strategies you can use to deal with stretching and spindliness in onions:

  • Trimming: While there is limited research about whether or not to trim onions, many growers utilize this practice. When onions develop their first true leaf, trimming to around 4’’ seems to encourage plants to invest in stronger root systems. Shorter onion seedlings may also be easier to transplant mechanically. Some farming blogs recommend trimming to 1-1.5’’, but this is likely overkill. To prevent disease issues, take sure to clean and sanitize your scissors before trimming, remove the clippings from your trays, and trim on sunny days so that the plants can dry and heal any damaged tissue quickly.
  •  DIF method: DIF is a temperature control method developed at Michigan State, and commonly used by larger-scale commercial growers, especially on the East Coast. DIF is the difference between daytime and nighttime temperatures. Stem elongation is exacerbated when daytime temperatures are warmer than nighttime temperatures, and so keeping daytime temperatures 4-10 degrees F lower than nighttime temperatures can help to slow down stem elongation. This method is effective not just on onions, but also on tomatoes, green beans, melons, and sweet corn. More info about the DIF method. 

Untrimmed onion seedlings. Photo author: cheeses, Flickr

Moisture maintenance

Since onions have shallow root systems, they require frequent and consistent moisture. If you’ve not done so already, consider investing in moisture sensors to help you plan your irrigation management more efficiently. Mulches of all kinds - plastic, straw, etc. can also help to retain moisture in the top few inches of soil.

Onion thrips

Onion thrips populations can grow quickly, look for them in the neck of onion leaves. Photo: Whitney Cranshaw, Colorado State University,

The most troubling insect pest of onions are onion thrips. Thrips are small (~2mm), yellow insects with multiple generations a year that are most likely to become an issue in hot, dry weather. Their feeding reduces yield as well as creates opportunities for disease to attack wounded plants.

Because thrips are small and reproduce quickly, they can be very difficult to control. Studies focused on reflective mulches found it’s effects to be inconsistent year to year.

In both organic and conventional systems, insecticides are a common tool. Growers in both systems should scout and choose products carefully, as the chances of thrips developing insecticide resistance is high. It is important to scout for thrips, and initiate spray programs while thrips numbers are low, especially when hot, dry weather is in the forecast. Rain and overhead irrigation can knock thrips off the plants and slow their lifecycle and feeding. Recommended thresholds thrips treatment range anywhere from 1 to 3 thrips per leaf (again, because their populations can grow so quickly).

Cornell provides detailed spray programs for conventional growers, which are important to consider as research has shown that the sequence in which products are used impacts how effective they are.

In organic production, research at Cornell has shown Entrust (spinosad) and neem oil reduced thrips numbers.

Soft rot

A frustrating issue often dealt with during the storage season is soft rot. This disease is caused by bacteria commonly found in the soil and plant debris. It takes advantage of wounds caused by insects, weather (hale, freezes, sunscald), and rough handling (bruises, cutting tops during harvest) to enter the plant and break down tissue. Watch the video below for more information about diagnosing soft rot.

Work in the Eastern US has suggested that tighter plant spacing and alternatives to black plastic mulches can reduce the incidence of bacterial soft rots. It is suspected that the alternatively colored plastics reduce the soil temperature, making it less hospitable for bacterial diseases. The tighter row spacing changed plant structure (smaller leaves allowing for quicker plant maturity and die down, less places for puddling water in leaf crevices) in ways that seemingly reduced bacterial disease. In these trials, silver mulch, biodegradable plastic, and even bare ground had lower incidences of bacterial rots and higher yield compared to black plastic mulch. In trials using silver plastic, reducing plant spacing from 48 inches² down to 32 or 24 inches² increased overall yield (though it did change the sizes of onions harvested). See Cornell’s research report for more information.

Another aspect of bacterial soft rot management is to manage the insects and diseases that wound plants. An insect to pay special attention to is onion maggot. This insect is able to host bacterial pathogens in it’s gut and spreads them as it feeds.

In weather events that damage onion plants, a quick application of copper can reduce the chance that bacteria move into the wounds.

Proper maturation, curing, and storage is also critical. Allow the tops of plants to dry out as much as possible before harvest. Curing that allows the outer layers of the onion and the neck will prevent soft rot from entering and taking hold. Existing soft rot infections will be hastened by warm, humid storage areas. Another key aspect is making sure any onions with hints of soft rot do not make it into storage, so onions that show early signs of disease, knicks, or bruises should not be stored.

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