Onions may be one of the most difficult vegetable crops to grow. The cultural practices for growing and harvesting onions varies dramatically from one region to the next and onions are more sensitive to pests, weeds and diseases than other vegetable crops.
Although onions are commercially grown in more than 20 states across the U.S., the National Onion Association (NOA) estimates that fewer than 1,000 growers produce onions commercially in the U.S. This includes traditional and organic growers. According to NOA’s website, U.S. farmers plant approximately 125,000 acres of onions each year, which yields about 6.2 billion pounds of onions annually.
“Onions will grow in nearly any well-drained soil so long as the soil has a moderate pH level,” said Tim Waters, Ph.D., an associate professor and regional vegetable specialist with Washington State University Extension.
California, Idaho-Eastern Oregon and Washington are the top three onion-producing areas. Each year 23,828 acres, 19,800 acres and 18,150 acres are planted respectively in these regions. Georgia, New York and Texas round out the top six with 12,700 acres, 10,250 acres and 9,500 acres, respectively.
“The crop is a little unusual in that varieties and cultural practices are considerably different across the country,” said George Boyhan, Ph.D., a professor and Extension vegetable specialist in the Department of Horticulture at the University of Georgia.
1. Long day vs. short day
The type of onion planted varies geographically. Onions are categorized by the amount of daylight needed to germinate and produce a bulb. “Short-day onions require 11 to 12 hours, intermediate day onions need 12 to 14 hours and long-day onions need 16 to 17 hours of daylight to create a bulb for harvest,” Boyhan said.
In Georgia, short-day varieties, specifically the Vidalia onion, are the onion of choice. These onions are high in water content, require regular irrigation and are known for their mild flavor. The seeds are sown into dense planting beds where 60 to 70 plants-per-linear-foot sprout. Six to eight weeks after planting, the seedlings are pulled and transplanted to specific spacing, allowing more room for the bulbs to develop.
“Not many other areas transplant seedlings,” he noted.
A thousand miles to the west, producers in the Rio Grande Valley and Winter Garden region of southern Texas also use short-day onions. In contrast to the excessive water content needed for the varieties in Georgia, the varieties in Texas respond to arid conditions. “We hope that it is dry at least 30 days before harvest,” said Juan R. Anciso, Ph.D., a professor and Extension specialist at Texas A&M’s AgriLife Extension. “Rain spells big trouble and can lead to decay and disease.”
Growers in Georgia and Texas begin planting in the fall and anticipate a harvest in early spring. In contrast, producers in the Northeast and Pacific Northwest plant long-day varieties in late winter or early spring with harvest arriving by fall.
2. Planting tips
Healthy stands equal bountiful harvests. Depending on geography, ideal plantings achieve 60,000 to 140,000 plants per acre. In Georgia 60,000 to 80,000 plants per acre is optimum while about 100,000 is on target in Texas.
“A good stand of onions starts with good weed control,” Anciso said. “Onions are not good competitors and so a preemergent herbicide is important to good weed control.”
At the same time the soil is prepped to control weeds, the soils need to be loose to allow room for the shallow-rooted vegetable to grow. Roots and other debris should be removed from the seed bed and the dirt loosened. Debris makes it difficult for the small seeds, about the size of the tip of a ballpoint pen, to reach the surface.
“Shallow compaction is not good for growing onions,” Waters said.
Seed selection (see sidebar) significantly impacts crop yield. “Choose a good quality seed that has higher than a 90 percent germination rate,” Anciso said. “We use seed that has a 95 percent germination rate.”
Growers who find the greatest success are diligent in maintaining and monitoring the equipment used to seed the fields. Planters should be cleaned and tuned so that proper and uniform spacing is achieved throughout the field. “If two onion seeds fall into the same spot, they combat one another or the two bulbs grow together,” he added.
Well-tended seed beds produce onions that germinate and develop at nearly the same rate. Significant developmental differences not only affect harvest yield, but can also interrupt herbicide applications. “If the plants in one field are not all at the same stage then it really throws off the herbicide schedule,” Waters said.
A less common alternative to direct seeding is planting sets. Sets are a small onion bulb that is planted instead of seed to yield a mature bulb. Approximately 15 years ago, commercial growers experimented with using sets and found direct seeding to be more effective.
“Nearly all commercial growers are planting from seed,” Boyhan said.
Waters said that a small percentage of crops in Washington are planted as sets. “Sets shorten the growing season,” he said, “but seeds are much less expensive and offer growers a more diverse selection of varieties to choose from.”
3. Shallow root system
A shallow root system means that onions require a lot of water and regular fertilizer applications. Fertilizer applications should be spaced throughout the season to ensure the onions have enough nutrients available during all stages of development. “You can’t put all of the fertilizer on the crop at the beginning of the season,” Waters said. “The shallow root system means the nitrogen can easily be flushed out of the rooting zone.”
Depending on the region and natural rainfall levels, irrigation may be a necessity to keep the shallow roots moist. “We have ready access to irrigation, and overhead systems are used in the fields,” Boyhan said.
Controlling weeds in onion fields is challenging. Other crops develop dense canopies naturally designed to suppress weeds. In contrast, onions grow thin shoots that lack the ability to overpower weeds.
Strategically timed applications can be effective in managing weeds even though farmers have limited options. “There are not a lot of herbicides developed specifically for onions because the total acreage is so low, so it can be challenging,” Waters said.
Onions respond to herbicides more like a grass than a broadleaf plant, exacerbating the challenge.
There are only a handful of chemistries available for effective control, which include oxyfluorfen, bromoxynil, ethofumesate, pendimethalin and clethodim for weed control.
Anciso said that herbicides applied at planting for pre-emergent control do not last all season long. “Applications have to continue throughout the season,” he said.
Weeds aren’t the only challenge to growing healthy onions – insects plague onion crops, too. “Onions are most sensitive to pests compared with other vegetables,” Anciso said.
Problematic pests vary from one region to the next. Seed corn maggots can be troublesome in the Pacific Northwest. The adult flies lay eggs on the soil and the larvae develop in the germinating seedlings causing plant mortality. This region also has to contend with wireworms, the larvae of click beetles that feed on the shallow root systems of onions.
In southern Texas, leafminers and Armyworms can quickly damage a crop. Leafminers primarily cause cosmetic damage in onion crops; contamination by pupae and larvae, however, is a problem. Natural enemies, especially parasitic wasps, can reduce leafminer numbers. These parasitic wasps are susceptible to insecticide sprays and may not be effective in fields where insecticides have been used.
By far the most devastating pest in every region except Georgia is thrips. The two species growers are most concerned with are onion thrips and western flower thrips. These insects feed on the leaf of the onion, stifle leaf growth and reduce the surface area for photosynthetic output.
“You don’t lose bulbs, but the size of each onion is significantly smaller,” Waters said. “Fields infested with thrips can record 10 percent to 40 percent less yield by volume.”
Only five insecticides, which include spinetoram, abamectin, spirotetramat, methomyl and cyazypyr, combat thrips. “It’s important to rotate between different effective chemistries and not to use the same insecticide more than two times,” Waters said.
Fortunately for Georgia growers, thrips are unable to survive in the damp environment created by regular rains and overhead irrigation.
“Because of the amount of rain and overhead irrigation we use here, we don’t have as much of a problem with thrips,” Boyhan said. “They don’t like the naturally damp conditions and regular irrigation.” (More about pests in Critter Control on page 24).
Viruses, as well as fungal and bacterial diseases, can thwart growers’ efforts to cultivate healthy crops. Iris yellow spot virus is perhaps the most threatening disease. “This can have an even greater impact on yield than thrips,” Waters said.
The disease, which is transmitted from one plant to the next through thrips, can devastate an onion field by destroying the photosynthetic area of the leaves of infected plants, reducing the ability of the plant to develop bulb size and directly affecting yield and grade of onions at harvest.
Although iris yellow spot virus is not as common in Georgia where thrips populations are lower, bacterial diseases like sour skin and slippery skin are concerns.
“Sour skin primarily affects onion bulbs, but foliar symptoms may also be observed from time to time. This disease usually manifests itself during harvest when temperatures above 85 degrees are common,” Boyhan said.
A three- to four-year crop rotation with small grains or sugarbeet can help reduce the chances of the bacterial disease. Avoiding the reuse of irrigation water and overhead irrigation, especially after bulb initiation, also limits the pathogen’s opportunity to spread.
7. Harvest tips
After months of preparation and care, the onions are ready for harvesting. Readiness is determined by the percentage of “tops down” in a field. “Tops down” refers to the point in time when the onion leaves break over at the bulb neck and lay on the ground. This process allows the neck of the bulb to “cure” or dry out before packing and storing or shipping.
“Wait for 80 percent to 90 percent of the tops in the field to fall over before going in and undercutting,” Waters said.
Georgia growers harvest onions on the green side. “We’re looking for 20 percent to 50 percent of the tops down,” he said. Then workers move in to undercut the roots and lift the onions to surface so they can dry for a day or two.
Undercutting separates the roots from the soil and brings the onions to the surface for curing. At the same time the neck dries out, the outside skin dries into a thin, papery-like layer. This process protects the onions from pathogens and prepares them for storage.
Onions are susceptible to sunburn so some farmers send laborers into the field to hand clip the roots and tops and pack the onions into burlap bags. The bags remain in the field until the onions are cured.
Others collect the onions from the fields with equipment. At the warehouse they are put under hot-air blowers to further promote the drying process.
In many areas of the country, harvesting is done with equipment, but in Georgia, the onions are harvested by hand. “We have a fairly narrow window of opportunity to harvest, usually only three to four weeks,” Boyhan said.
Laborers move in to hand clip the tops and roots from each onion. Post-harvest the onions are heat cured with hot forced air to help dry down the neck and the outer part of the bulb.
“Then they go through the grading process,” Boyhan said.
At the end of the day, the weather is a big factor in a producer’s outcome.
“A grower can manage weeds, treat for pests and disease and follow all of the correct cultural practices, but if the weather throws a curve ball an entire crop can be lost,” Anciso said.
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