Onions are grown from seed, sets/bulbs, or onion starter plants and can take as little as 60 days and as much as 215 days to grow. The quickest way to grow onions is from a starter plant and it takes the longest to grow onions from seed planted in the fall and harvested the following spring/summer.
How Long Does It Take To Grow Onions From Seed?
Onions grown from seed take the longest to grow but are my favorite option to grow onions. You get to choose from a wide variety of onions and have control over the health of the plant and inputs throughout the plants life.
In Michigan where I live it’s necessary most years to start onions indoors for 8-12 weeks before transplanting them outside. They need this time to grow decent sized root and vegetative growth before daytime lengths trigger bulbs to start growing.
Use this sunrise/sunset calendar to easily see the day lengths in your area at different times of the year. Day length is an option in the settings you can choose and it will do the math for you and display day length along with time of sunrise and sunset.
Check out “Long Day Onions vs. Short Day Onions” for a more detailed explanation on how daytime lengths affect onion growth.
In this research study, Georgia Onion Research – Extension Report, 46 different onion varieties were grown and their planting date and harvest date were recorded.
The onions were seeded indoors in September and transplanted outside in the field in November, roughly two months after being planted.
The average length of time from the onion seed being planted to the onions being harvested was right around 7-7 1/2 months. Scroll to Table 1 in the link above to see the exact dates.
How Long Does It Take Onion Seeds To Sprout?
Onion seeds will germinate with soil temperatures at 40 degrees or above but will germinate optimally with soil temperatures at 75 degrees. Onion seeds will sprout in 5-28 days depending on soil temperature. They will sprout faster with soil temperatures closer to 75 degrees and slower with cooler soil temperatures.
Soaking seeds in water before planting and using heating mats are two ways that gardeners can speed up the germination time of onion seeds.
I don’t soak any seeds before planting in my garden. If you do soak seeds soak them overnight or for a few hours, but never more than 24 hours.
If you use a heating mat make sure to remove the heating mat once 50% or so of the seeds have sprouted. Most seeds like warm temperatures to germinate but seedlings like cooler temperatures to grow, in the 55-75 degree range.
How Long Does It Take To Grow Onions From Starter Plants?
This is the fastest way to grow onions. Whether you’re growing from starter plants that you got from a nursery or preferably transplanting onion seedlings that you started indoors onions can be ready for harvest as quick as 60 days after planting them outside.
If daytime lengths are at a point that they will trigger onion bulb growth when transplanting outside, 10-12 hours for short day onions, 12-14 for day neutral onions, and 14-16 hours for long day onions then the onions will be ready for harvest in 60-80 days.
Onion transplants already have the size they need to grow a quality and good size onion bulb.
How Long Does It Take To Grow Onions From Sets?
By growing an onion from a bulb or set you are putting the plant at about a 4-6 week head start compared to planting from seed.
If you’re sowing onions in the fall to harvest in the spring you can plant the sets about a month later than sowing seeds.
If you’re in the north, above the 35th parallel, and planting in the early spring in the hopes of harvesting in summer the same challenges remain as growing from seed. You need the plant to grow to a decent size, 4 or more leaves that are the size of a pencil, before daytime lengths trigger the onion bulb to start growing.
If you don’t want to start the onion plants inside try planting the sets/bulbs as early as 6 weeks before the last frost date in the spring. Onion plants should be able to handle temperatures in the 20’s.
How Long Does It Take To Grow an Onion From an Onion?
There are some misconceptions out there on what’s possible with growing an onion form an onion.
If you take the bottom of an onion and plant it it will likely grow new roots out of the bottom and grow green shoots that can be harvested as green onions out of the top. However, it will not grow a new onion.
If you have an onion on hand that’s sprouted you can use these sprouts to grow new onions. The new onions will be smaller and more cylindrical than onions grown from other methods. Also there’s a good chance that on onion grown this way will flower and produce seed since it is in its second year of growth.
On the left is a sprouted onion. It can be divided into different onion starter plants and be planted and grow into smaller onions, or used for green onions, or to produce onion seeds.
On the right is the bottom of an onion and the beginning of its growth of roots and green shoots. It can be used to harvest green onions!
How Long Do Red Onions Take To Grow?
Most red onion varieties available are long day onions and will take a similar amount of time to grow as other long day onions, about 120-150 days.
Please comment below with your own thoughts on how long onions take to grow!