Starting Onion Sets Indoors
It’s important to start onion sets indoors in the early spring, as soon as the soil can be worked. Plant them after your last frost date and about eight weeks before the first expected frost date for your area. Onion bulbs form best when nighttime temperatures are between 40°F and 50°F (5°C and 10°C).
Select a set.
Selecting the right onion sets is important. You will want to select a variety that you like, but it is also important to make sure the set are firm and healthy. A good set should be at least 2 inches in diameter and from the same variety. If you have more than one variety of onion growing in your garden, you may want to plant sets from different seed lots so that when you harvest them later on, they look similar and taste similar as well.
Prep your planting mix.
To prepare your potting mix, you can use one of the following combinations:
- Soil (50 percent) + compost (25 percent) + fertilizer (25 percent). This is my favorite method for starting onions.
- Soil (50 percent) + sand (25 percent) + peat moss (25 percent). This will keep your plants from becoming too compacted or waterlogged.
- Vermiculite (50 percent), perlite (30 percent), and coconut coir fiber chunks or pieces. This has a light feel and is great for container growing because it absorbs water well and provides good drainage.
Plant your sets.
When it comes to planting, there are two ways you can go. If you want to avoid the mess and hassle of transplanting, you can plant your onion sets right in the ground. The other option is to start them indoors as seeds or seedlings before transplanting them outside later on. Whichever way you choose, make sure that the soil is well-drained and kept moist but not soggy (we’ll talk about this more later).
Once your plants have been planted at least 3 inches apart from each other (the wider spacing will allow for bigger onions), water thoroughly but do not over-water; keep an eye out for signs of dryness (like wilting) and make sure that the soil remains moist but not soggy throughout their growth cycle.
In addition to watering during dry spells or periods of high temperatures, fertilizing once a month with organic fertilizer will help ensure healthy growth with large bulbs.
Water and fertilize.
Once you set your sets in the soil, give them a good watering. Watering thoroughly is crucial because it helps get the onion roots growing fast and helps break up any air pockets in the soil that may otherwise suffocate the onion sets. After you’ve watered your set, cover them with mulch to keep them further protected from wind and drying out.
For fertilizer, use a balanced fertilizer (such as 10-10-10) at one-eighth strength every two weeks. You can also go for something with a higher nitrogen content (like 20-20-20), which will encourage fast growth of the tops of your onions but won’t make much difference when it comes to their roots—just be sure not to apply too much or else you could burn or stunt their growth. Conversely, if you want your onions to grow slowly but have thick white bulbs, consider using a lower nitrogen fertilizer like 5-5-5 instead of 10 or 20; just make sure not to go below 5 on either end!
Onions can be started indoors, but don’t wait too long to plant them out.
You can start onions indoors, but don’t wait too long to plant them out:
- Start your four-week-old onion sets in clean and damp seed starting mix. Be sure to follow the package directions for soil and temperature.
- Onion sets are really small, so you’ll need to plant them deeply—the top of the set should be just below the surface of the soil in its container.
- You can sow seeds directly outdoors as well as indoors; either way will produce good results if you give them plenty of sunlight, water and fertilizer once they’re planted outside
In conclusion, starting onion sets indoors before transplanting them outside can be a great way to increase the success of your harvest. By starting the onions in small pots and keeping the soil evenly moist, you’ll give them a good start in life. And by all means, experiment with different varieties of onion sets to see what works best for you