Site Selection for Planting Onions
Climate is the most critical factor in onion cultivation. The plant has particular demands when it comes mainly to bulb formation. Things are much easier if you plan to grow onions for their leaves (fresh onions). While onions are cool-season plants with good frost tolerance and can be grown in USDA hardiness zones 4-10, they require specific temperature and sunlight conditions to thrive. Ideal growth temperatures range from 13-25 °C (55.4-77 °F), and onions need full sun exposure. Onions can be grown from sea level up to altitudes of 1,900 meters (6234 feet). Areas with low humidity are preferred, as these reduce the risk of fungal diseases that can compromise crop health.
Depending on local conditions and onion variety, growers can either sow seeds directly in the field or start them in a nursery before transplanting. The optimal temperature for seed germination is 20-27 °C (68 -80.6 °F), where seeds typically sprout within 4-5 days. Lower temperatures, such as 10°C (50 °F), extend germination time to around 13 days, while high temperatures close to 40 °C (104 °F) may lead to complete sprouting failure.
However, temperature is one of many factors that affect onions' growth. Photoperiod is, in fact, the most critical factor, especially when it comes to bulbing.
Photoperiod (day length) plays a crucial role in bulbing:
- Long-Day Onions: Require 14-16+ hours of light per day.
- Intermediate-Day Onions: Need 13-14+ hours of sunlight.
- Short-Day Onions: Require 11-13+ hours of light daily.
If day length is shorter than the onion variety’s requirement, plants focus on vegetative growth, which may be ideal if onions are grown for their leaves but detrimental for bulb production.
Soil Requirements for Onion Cultivation
As for their soil requirements, onions are not very demanding. They grow well in a wide variety of soils as long as they are not too heavy, such as clay (over 30% clay), since there is a high risk of forming crust and curds leading to deformation or multiple bulbs. The plant thrives best in sandy to silty loams and fertile soils with proper aeration and drainage. Onions do not grow well in soils with high salinity (>4 mmhos) and prefer pH levels between 6-6.8.
Soil Preparation for Onion Cultivation
Soil preparation for onions begins several weeks before planting. Farmers start by plowing to improve soil aeration and drainage, removing rocks and debris to create a loose structure that allows bulbs to grow without any (space) limitation (e.g., soil compaction).
One week later, many farmers apply a pre-planting fertilizer such as well-rotted manure or synthetic commercial fertilizer after consulting a local licensed agronomist. Usually, farmers apply and incorporate (using a hoe) to the soil 24-40 tons per hectare (10-16 tons per acre) of well-rotted-manure/compost at least 1-2 weeks before transplanting the onion seedlings.
Onions grown for bulb production are often planted on raised beds formed by tractors a few days before planting. If furrow irrigation is applied, furrows with 45 cm (18 in) spacing should be formed. On the other hand, if drip or sprinkler irrigation is used, the onion grower must also install the system before planting. Following this step, some may apply soil disinfection substances through the irrigation system if soil analysis has revealed soil infection problems (ask a licensed agronomist in your area).
In regions where soil temperatures are non-ideal during planting, growers may apply linear polyethylene films like black or green Infrared-Transmitting (IRT) film over the rows. These films help maintain optimal root zone temperatures and suppress weed growth.
Onion Planting and Plant Distances
There are 3 main ways to start an onion farm :
- By seed (directly sown/seeding on the field)
- By transplanting seedlings (after they have been grown indoors)
- By planting small bulbs ( sets)
Each method offers distinct advantages and is chosen based on cost, labor, and production goals.
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Starting an Onion Farm by Direct Seed
Direct onion seed sowing has become more popular lately, mainly because of its significantly lower cost than the other two methods.
Onion growers start their crop in early spring by mixing onion seeds with sand to achieve a better spreading rate. It is important to select healthy certified seeds from a legitimate seller to ensure variety purity, good health, and germination rate of the propagation material. Remember that onion seeds generally have a low germination rate, close to 60-70% (max 90%). Producers in large commercial farms mainly use mechanical harvesters that either spread the seed randomly on the field or, in most cases, sow the seeds in rows on flat surfaces or raised beds. In the case of precision sowing, the most commonly used distances are 25-30 cm (10-12 in), max 45 cm (17.7 in) space between rows, and 7-10 cm (2.7-4 in) space between the plants. Finally, the seeds should be placed at a depth of 0.6-1.3 cm (0.25-0.5 in).
In some cases, they create wide raised beds on top of which they plant 3-4 rows, keeping the same distances.
The amount of the seed sowed depends on the variety of the onions, planting distances, the germination rate, the seed size, and, of course, the nature of the final product and the producer's yield goals. Broadly speaking, onion producers have reported using approximately:
- Dry Bulb Production: 1.8-3.2 kg of seed per hectare (1.6-2.85 lbs/acre)
- Set Production: 60-80 kg per hectare (53.5-71.4 lbs/acre)
- Fresh Leaf Production: 12-20 kg per hectare (10.7-17.8 lbs/acre)
On average, 1 gram of onion seeds contains 220-290 seeds, which can maintain good germination rates for up to a year when stored properly.
2. Starting Onion Farm by Seedlings
Despite the good final yields obtained, onion farmers do not generally prefer this method due to its higher cost. Seedlings are suitable mainly for dry bulb production.
In the case of seedling transplanting, producers begin by sowing onion seed in an indoors-protected environment. They start 6-12 weeks before the day of transplanting by sowing 80-100 gr of onion seed per ㎡. They sow the seeds at close distances to exploit the available space as much as possible.
During late spring or early summer, it is time for transplanting. One day before transplanting, irrigation should be applied if the soil moisture is not at the desirable levels. Special equipment is required for transplanting the onion seedlings when using sets. Producers plant the young onion seedlings, keeping on average 35-60 cm (13.7 in-24 in) between rows and 7-10 cm (2.7-4 in) space between plants in a row. Following this pattern, they approximately plant 270,000 plants per hectare (75,000-120,000 per acre). You are advised to maintain good soil moisture during the first week after transplanting to help plants establish and reduce transplanting shock.
However, in the case of the production of large Spanish onions, the space between plants in a row should be 10-15cm (4-6 in). In some cases, they create wide raised beds on top of which they plant 3-4 rows, keeping the same distances. Before planting, seedlings, in most cases, are trimmed to 10 cm (4 in) or at 50% of their height. Some growers trim the roots as well. Finally, the planting depth is usually 2.5 cm (1 in).
3. Starting an Onion Farm By Sets
This method is the most commonly used one by onion producers globally due to its numerous benefits, but similar to transplanting, it has a high cost, and specific equipment, like a precision seed drill, is needed. However, despite the initial cost, these last 2 methods can shorten the time until harvest by up to 2 months (4-6 months required), reducing the need for weed and disease/pest control practices and maximizing yields. Sets are nothing more than small onion bulbs produced the previous year. Sets are grouped into 2 major categories depending on their diameter and 2 more depending on their shape. Thus, we have:
- Small Sets (<1.8 cm or 0.7 in): Suitable for bulbs or fresh onion production.
- Large Sets (>1.8 cm or 0.7 in): Primarily for fresh onions.
- Spherical Sets: Produce wide bulbs.
- Elongated/Conical Sets: Big elongated sets are ideal for fresh onion production as they produce many leaves and smaller bulbs.
There are two periods to start planting the sets: early spring and autumn. Although growers have reported greater yields from autumn planting, the climate conditions do not allow such action in some areas.
Traditionally, sets are planted manually. However, producers in high commercial onion farms nowadays use mechanical seeders that sow sets on rows. Beck, vacuum, and mono-air-planters are successfully used. Some commonly used planting distances are 25-30 cm (10-12 in) distance between rows and 8-12 cm (3-5 in) space between sets on the row. In some cases, the farmers may create wide raised beds on top of which they plant 3-4 rows, keeping the same distances. Depending on the size of the sets, the purpose, and the producer's goals, growers use, on average, 1-1.5 tons of sets per hectare (892-1338 lbs/acre). In the case of fresh onion production, producers, in most cases, perform multiple sowings from spring until autumn to maximize their yields and meet the market demands.
References
https://agsci.oregonstate.edu/mes/sustainable-onion-production/planting-methods
https://www.jica.go.jp/project/english/kenya/015/materials/c8h0vm0000f7o8cj-att/materials_20.pdf
https://extension.usu.edu/yardandgarden/research/onions-in-the-garden
https://agritech.tnau.ac.in/horticulture/horti_vegetables_small%20onion.html
https://ohioline.osu.edu/factsheet/hyg-1616
https://www.fao.org/land-water/databases-and-software/crop-information/onion/en/
https://pfaf.org/user/plant.aspx?LatinName=Allium+cepa
Further information
- Interesting Facts about Onions
- Onion: Info, Nutritional Value, and Health Benefits
- Onion Plant Information and Variety Selection
- How to Grow Onions At Home: Onion Growing Guide for Beginners
- Commercial Onion Farming - How to Grow Onions for Profit
- Onion Soil Requirements, Soil Preparation, and Planting
- Which Factors Can Affect Bulbing in Onions?
- Onion Water Requirements and Irrigation Systems
- Onion Nutrient Needs and Fertilization Requirements
- Major Onion Pests and Diseases and Sustainable Management Practices
- Onion Weed Management: How to Control Weeds in an Onion Field
- Onion Harvest, Yields, and Storage