Table of Contents
Onion Transplants
Growing onions from transplants have the added benefit of being earlier to harvest, and the product will be more uniform in size and stand.
Seed waste is mitigated with onion transplants and it can help reduce pressure from pests and disease.
On the downside transplanting onions is a larger cost investment and requires specialized equipment to do at larger scales.
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What to Do When You Get Your Onion Sets or Transplants
Onion Transplants are shipped dormant. Do not be alarmed if they appear dry upon arrival. Dry roots and slightly yellowing plant tips are normal. The onion plant can live off of its bulb for approximately three weeks.
When should onion transplants be planted?
Onions can be planted 4-6 weeks before the last average frost date for your area. The most common issues for onion plants are related to fungal growth, which occurs during periods of high moisture and humidity.
As soon as onion sets arrive you will want to remove them from the boxing to avoid fungal issues. They will need to be placed in a well-ventilated area free from sunlight, soil or moisture.
Also keep in mind that you should plant as soon as possible after receiving your onions. They can be stored for up to three weeks in ideal conditions although the quicker you can plant the better. It’s also important to keep them away from soil and water until you are ready to plant the sets/transplants.
Soil Conditions
Onions require full sun and good soil drainage. Choose a location that gets plenty of direct sun. Onions grow best on raised beds or raised rows at least 4" high and 20" wide.
The soil should be loose and crumbly. If it's compacted, work in compost to improve aeration and drainage.
To stop weeds for up to six weeks, rake a pre-emergent herbicide, such as Treflan or corn gluten meal, into the top inch of soil before you plant. Don’t worry, this type of herbicide will not affect the onion plant roots.
What's the best soil pH for onions?
It’s helpful to know whether your soil is acid (pH below 7.0) or alkaline (pH above 7.0). Onions prefer soil with a pH between 6.2 and 6.8. Your agricultural extension service can test your soil for you, or you can buy a home test kit at your local garden center. If your soil is too acidic, mix in ground limestone, available at your garden center. If it’s too alkaline, add peat moss.
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Where to Grow Onion Sets or Transplants
Site conditions are very important to successfully growing onion sets. Onions require full sun and won’t tolerate partial shade.
While onions need a steady supply of water, they don’t like to be damp either. Exposure to the sun will help dry them off and keep diseases from being a problem.
Onions will have to be watered immediately after transplanting and will require 1” of water per week.
Growing Healthy Onions
Onion Planting Tips
Plant your onions 4 to 6 weeks before the last estimated spring freeze. (Your agricultural extension service can tell you when that is.) For the best growth and yield, onions need fertilizer right from the start. Use a fertilizer with the middle number higher than the other two, such as 10-20-10.
Fertilizing Onions
Nutritional needs are different during the growing season. While the onions are growing you will want to continue fertilizing them. Every 2 to 3 weeks after planting, fertilize with ammonium sulfate (21-0-0) in alkaline soils, or calcium nitrate (15.5-0-0) in acidic soils. Sprinkle it on top of the original fertilizer strip at the rate of ½ cup per 10 feet of row. A light and balanced fertilizer will be ideal, but you could apply some compost as well if you prefer. Water the onions after every application. Eventually, as the onion plants begin to form bulbs the soil will crack, at this time fertilizing should stop.
Watering Onions
Water thoroughly after planting, and regularly thereafter. Onions have shallow roots, so don't let the soil at the base of the plants become dry and cracked. Overwatering is equally problematic. If leaves develop a yellow tinge, cut back on watering. The closer to harvest time, the greater the need for water. However, when the onion tops start falling over, stop watering and let the soil dry out before harvesting.
How to Space Your Onions
Onion spacing is also very important and what your desired end goal is, will determine your spacing requirements. If you’re growing to harvest green onions then you will want them to be closer, 2” apart. However, if your plan is to harvest full-sized onions than you want to spread them out to 4”.
If you wish to have both, space them 2" apart and pull every other onion during the growing season, leaving the rest to grow to maturity. As far as the spacing between rows, 16” should give your onions plenty of room to grow. If you want to fertilize your onions, it’s ideal to dig a companion ditch next to your onion row 6” away. The spacing from the center of one fertilizer trench to the center of the next should be 36".
The fertilizer trench should be 4” by 4” so you can spread .5 cup of fertilizer for every 10 feet of trench. After you’ve applied the fertilizer you can backfill with 2” of soil.
Directions to follow when spacing your onions:
- Dig a trench that's 4" deep and 4" wide. Sprinkle ½ cup fertilizer per 10 linear feet of row. Cover the fertilizer with 2" of soil.
- Plant the onions 6" from the edge of the trench on both sides of the trench. DO NOT plant the onions in the trench! Leave a 2" margin between the onions and the outside edge of the bed.
- Plant the onions 1" deep and no deeper, as this will inhibit their ability to bulb.
Controlling Weeds
You may be surprised to hear that onions aren’t any different than most vegetables, they don’t care for weeds as they are growing. Controlling weeds is critical to prevent competition for nutrients.
An application of Treflan or corn gluten meal raked into the top inch of soil every six weeks during the growing season will prevent annual weeds from germinating. Mulching with a light layer of straw will help control weeds and preserve moisture. Be sure to push the straw back when the plants start to bulb so they'll cure properly.
Insects
The biggest insect issues that you typically see with onions is Onion Thrips, so if you begin to see silvery blotches on your onions, you will want to apply Neem Oil as directed.
Part of the reason why weeding is so important is that Thrips will overwinter in the weeds.
When It’s Time to Harvest Onions
Eventually, the top of your onion plant will begin to turn a brownish-yellow. When your onions are ready to be harvested pick a dry early morning to do the pulling.
Curing Onions
The onions will need to be dried out before they are ready for storage or you will risk them rotting away.
Outdoors
The sun can help you dry your onions out, if you lay the onions in two rows and allow the tops of one row over the other this will prevent the onions from being scalded.
Indoors
If the weather isn’t cooperating and you need to get your onions out of the ground, curing indoors is possible but takes longer. If curing indoors be sure to choose an area with good ventilation and air movement.
After your onions are cured the neck of the onion to the skin will be dry and the entire skin will have a consistent texture to it. Once the onions are thoroughly dry, clip the roots and cut back the tops to one inch. Now they are ready to eat.
When your onions are dried enough you can cut the bottom roots off and braid the tops together. If you prefer not to braid the tops, you can snip those off as well, but be sure to leave 2-3” of the top on to allow the stems to dry and cure completely.
How to Store Onions
Store onions in a cool, dry, well-ventilated location, such as a garage or cellar. Place them in mesh bags or netting to permit airflow. Periodically check for any soft onions, and remove them to avoid deterioration of the others. As a general rule, sweeter onions don’t store as long as more pungent ones, so use the sweeter onions first.
What Temperature Do You Store Onions?
When you store onions the best results will be had when they are kept at NEAR freezing temperatures.
After temperature, the humidity will be your biggest concern. The ideal humidity for onion storage is 65-70% in a bag or container that will allow air movement.
Troubleshooting Tips for Growing Onions
The most common problems found in growing onions are blight, purple blotch, and thrips. Both blight and purple blotch are caused by fungus, and are more common during periods of high moisture.
Blight appears as small white spots surrounded by a greenish halo. Purple blotch causes a purplish discoloration of leaves. Proper plant spacing helps increase air flow and reduces both blight and purple blotch.
The best preventative measure, however, is the use of a fungicide every two weeks after planting. Thrips are insects that sometimes attack onion plants, causing the leaves to turn grey. Thrips are barely visible as tiny yellow or dark specks. Treat thrips infestations with an application of insecticide.
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