Grow Onions from Transplant

A Guide to Growing Onions from Transplants


Correct variety selection is the key to growing a successful onion crop. There are three daylength types of onion varieties – long day, intermediate day, and short day. Varieties are somewhat interchangeable given where they are grown in relation to the map. If a grower’s location is on the border between long day and intermediate day both will work. Intermediate will be harvested a little earlier than a long day. Below is a general guide for the different daylengths, but keep in mind there is flexibility as some areas can successfully grow multiple daylength varieties.

Long Day varieties grow best in the northern states. For these onions to be ready for bulbing when daylength reaches 14-16 hours in May to July, these varieties should be planted March to May. It’s important for these to get planted at the right time so that your tops have enough carbohydrates to transfer and grow big bulbs! Harvest for long day varieties range from July to October.


Intermediate Day varieties have the most success in the middle section of the U.S. Intermediate day varieties start bulbing between mid-March and May, or around the time that day length ranges from 12-14 hours. The best planting times for intermediate varieties are January – March. Harvest for intermediate day varieties ranges from June to August.

Short day varieties have the most success in the southern portion of the U.S. Southern California across to Georgia. Sunbelt is located in the short day area; we can grow onion plants of all varieties due to our heat units and daylength. Short day areas have days 10-12 hours long. The bulbing process is triggered for this day length between February and mid-March, or around the time that day length reaches 10-12 hours. Harvest for these short day varieties ranges from April to June.


Planting Window

Tying things back to that appropriate variety mentioned above, your proper planting window is critical! The general rule of thumb is to plant about 6 weeks before your last average frost. We plant the seed at the proper time on our farm; we want our customers’ fields and beds to be ready to plant as soon as your plants arrive.

You want to be sure to receive those plants in time to plant, establish their new root system (about 2 weeks after they take off), start growing tops (also known as storing carbohydrates), and get several fertilizer feedings under their belts before that bulbing process is triggered. Mother Nature dictates the daylength hours so we as growers must do our job to ensure those onions have enough top growing time before daylength triggers bulbing.

For your proper planting window, see our recommendation by USDA Hardiness Zone!


Onions need the appropriate amount of water at the appropriate times in the appropriate manner. Too much water can lead to disease in your onion crop. Too little water will only get you small bulbs. Onions like to take a big, long, slow drink. What does that mean? They like a nice soaking where the ground is fully saturated, and they can absorb slowly. They are ready for their next watering as soon as that ground is dry about ½ inch down from the topsoil.

Drip irrigation > Flood or furrow irrigation > Overhead irrigation


Drip irrigation gets the moisture right to the root in the most efficient manner. Flood and furrow irrigation are beneficial as they make that root system go chase the water in the furrows, resulting in a stronger, hardier plant. Both previously mentioned methods keep your tops dry. Overhead irrigation can promote disease in the onion tops are they are wet. All bacterial and fungal disease are typically caught via air-borne spores. Weak, wet tops are much more susceptible to attracting those. While sprinkler and overhead irrigation aren’t our favorite watering methods, you can certainly use them, but we HIGHLY recommend a bi-weekly fungicide routine during your growing period.

You want the onion plants to have enough time to dry out properly for storage. Your last irrigation should be 2-3 weeks before you harvest your crop.

Feed and Protect

A routine fertilization program is highly recommended on a bi-weekly basis. Use a heavy nitrogen fertilizer, such as ammonium sulfate (21-0-0). If you have more acidic soil, we recommend ammonium nitrate (34-0-0). Your last fertilizer application should be about the time you start seeing bulbing take place. If you continue to fertilize after bulbing, you increase the risk of thick necks which don’t dry down well in storage and the centers will rot in storage.

Protect your onions with a routine fungicide to keep disease at bay. Onions are quite susceptible to diseases such as blight, mildew, botrytis, etc. A fungicide on a bi-weekly basis can help your onions defend themselves against whatever bacteria or fungus comes their way. Fungicides also promote longer storage and shelf-lives of onions. Fungicide is a win-win!

Store to Last

While storage has been mentioned above, cutting the water and fertilizer off at the appropriate times, protecting with a fungicide, allowing the tops to dry long enough, and storing in a temperature controlled, low-humidity environment will allow your onions to last for months. We mean, you didn’t do all of the hard work growing onions to not get to enjoy them well after 6-8 months after harvest, right?

Let the tops dry until they can be yanked off. Don’t clip the tops too short or disease can enter the bulb. Store in mesh bags or containers that have plenty of ventilation. Check them periodically. We all know the saying, “one bad onion, apple, whatever”. You get our point!

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