How to Start Onion Seeds Indoors in 7 Steps
If you dream of using homegrown onions in soups, pizzas, and mirepoix, starting with seeds is a simple and cost-effective way to grow these alliums. Join farmer Briana Yablonski to learn how to get a head start on your onion patch by starting onion seeds indoors.
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Onions are some of my favorite vegetables to grow at home. Not only are they versatile in the kitchen, but fresh onions’ vibrant colors and patterns make them some of the most beautiful crops.
While you can plant onions from seeds, sets, or transplants, starting these crops from seed is the most economical option. Plus, it allows you to grow varieties you may not be able to find available as sets or transplants.
While you can directly sow seeds in your raised bed or garden, the tiny and slow-growing seedlings are susceptible to weed pressure. Plus, they won’t germinate until the soil warms. Starting onion seeds indoors allows for reliable germination, healthy plants, and earlier harvests.
Choose the Proper Time of the Year
Most onions take at least 100 days to grow from a seed to a mature plant that’s ready to harvest. Therefore, they need to be in the ground for multiple months! Starting the seeds at the right time of year ensures the plants have time to mature while the weather is suitable.
Spring is the most popular time of year to plant onions outdoors. These allium seedlings can withstand light frosts so that you can transplant them outdoors before your last frost date. However, you should wait until the ground thaws and the danger of heavy freeze has passed.
If you want to start onion seeds indoors, plan on sowing them 10-14 weeks before your predicted last frost date. Sowing at this time means you’ll have ready-to-plant seedlings a few weeks before your final spring frost arrives.
If you live in zone 8 and above, you can successfully overwinter onions. Planting them in the early fall allows the plants to put on growth before the days turn cold and short. Although they won’t grow much in the winter, they’ll resume growth in the spring and be ready to harvest before spring-planted onions.
Starting the seeds in August allows you to transplant them in your garden in October. This timeline ensures the plants have time to acclimate to chilly temperatures before frigid winter days arrive.
Choose the Right Variety
Regardless of when you plan to sow your onion seeds, aim to purchase them early in the year. Shopping in January or February will give you the largest selection of varieties, so you can purchase exactly what you want. Since onion seeds remain viable for one year, you should buy new seeds each season.
When you’re shopping, pay attention to more than just the color and size of the bulbs. These alliums use day length as a signal that it’s time to form bulbs, so you must select the proper type for your latitude. Choosing the wrong type will result in plants that grow into healthy sticks but never develop hearty bulbs.
There are three main onion categories: short-day, intermediate-day, and long-day.
- Short-day onions mature only at latitudes between 20-35°. Therefore, they’re best for the southernmost areas of the United States, including Florida, Texas, and southern California.
- Intermediate-day onions will form healthy bulbs at latitudes between 33-40°. They’re well-suited for areas including North Carolina, Colorado, and northern California.
- Long-day onions will mature at latitudes between 38-50°. They’re best suited for states including Pennsylvania, Washington, and Minnesota.
If you’re not sure which variety to choose, look up your latitude and pick a variety that will grow well in your location.
Fill Trays with Potting Mix
There are a few different ways to start onions from seeds. You can fill small individual cells with a single seed or plant multiple seeds in a larger tray. Since you can easily separate onion seedlings without damaging them, the second option works best for most growers.
Plant two to six seeds per square inch if you want to pack multiple seedlings in a single-cell tray or pot. This planting density is more important than the size of the container, so use whatever you have available. For example, if you want to grow 50 onions, you can seed them in a 4-cell tray or a larger square pot.
Once you determine which trays to use, fill them with a well-draining potting mix designed for seed starting. Gently tamp the potting mix so the top half-inch of the trays is empty.
Sow the Seeds
Sprinkle the onion seeds evenly across the surface of the potting soil, leaving a gap of one-third to one inch between each seed. You don’t have to be overly precise, especially if you’re sowing hundreds or thousands of onions. Just ensure they’re not piled directly on top of each other.
Cover them with about an eighth of an inch of potting soil, then lightly tamp the soil. Gently water with a watering can or hose.
Once the soil is moist, cover the trays with a humidity dome. Although this step is optional, it will help trap moisture and encourage germination.
Place Trays in a Suitable Location
After the seeds are tucked under the soil, place them somewhere warm. Onion seeds germinate best when the soil is between 75-85°F (24-29°C), so choose the warmest room in your home.
While onions can germinate at lower temperatures, the germination will be slower and less reliable. Consider using a heat mat if you keep your home cool like I do. These mats heat the soil without using an exorbitant amount of energy.
Since the mats heat the soil without warming the air, they can be placed in any area of your home. I recommend having one on hand to germinate onions and other heat-loving crops like tomatoes, peppers, and eggplant.
Watch for Germination
Viable onions should germinate within 5 to 15 days. It’s crucial to keep the soil moist until they germinate since a lapse in moisture can stall or halt the sprouting process. If you aren’t using a humidity dome, be prepared to water or mist the top of the soil daily.
Don’t panic if you don’t see your seedlings sprout within a week. The temperature impacts germination time, and cooler soil means slower germination. However, if you don’t see any seedlings within two weeks, there’s likely a problem with the seed or the environment.
Some common causes of poor germination include old seeds, cold soil temperatures, dry soil, and too deep of a planting depth.
Provide Lots of Bright Light
Once you see the first seedling emerge, remove the humidity dome and place the tray in a place that receives bright light. It’s okay if you take the seedlings off the heat mat—the warm temperatures are more important for seed germination than seedling growth.
Unless you have a bright south-facing window or a greenhouse, I recommend using a grow light to provide supplemental lighting. Position the lights 6 to 12 inches above your plants and keep them on for 12 to 14 hours each day. Setting the light on an automatic timer makes it easy to ensure your seedlings with the proper amount of light.
As the seedlings grow, keep the potting soil slightly moist. Allow it to dry out slightly, but never become completely dry.
Planting Onions Outdoors
After your onions are eight to ten weeks old, they’re ready to transplant outdoors. You can plant seedlings that are only a tenth of an inch wide, but pencil-sized plants are better able to tolerate stress.
Properly prepare your soil before planting by adding any necessary compost and fertilizer. Space each plant four inches apart to allow for good airflow and room for the plants to expand. Expect to regularly water the soil to ensure the short-rooted plants can access the proper moisture.
If you want to learn more about caring for onions in the garden, explore details about how to plant onions as well as how to harvest and cure them.
