Days to germination: 6 to 12 days
Days to harvest: 90 to 110 days
Light requirements: Full sun
Water requirements: Regular watering
Soil: Loose, fertile and high in nitrogen
Container: Large pots only
A multiplier onion is one variation of the more common green onions (or spring onion, or shallot), that produces more than one bulb per plant. They are also sometimes referred to as potato onions or underground onions. These are an older type of onion that are not as wide-spread in home gardens as they once were.
The only difference between multiplier onions and “regular” onions is the way they grow in bunches. The flavor is much the same, though usually on the milder side.
Onions are usually grown as a vegetable to be used for seasoning and flavor, but you shouldn’t ignore the nutrition in an onion either. They are quite high in chromium, fiber and even vitamin C. You may not think of chromium as an important mineral, but many people are actually deficient in it.
Starting from Seed
Unlike other onions, the multiplier onion doesn’t produce any seed and can only be reproduced by planting bulbs or sets. While you can start your sets indoors, it’s not common practice with multiplier onions.
You’ll want to have your sets planted out into the garden about 3 weeks before you expect to have your last winter frost. Dig your garden area well, so there are no rocks or hard pieces of soil to block the onion bulb growth. Add some fertilizer, compost or aged manure to the soil before planting to make sure there are plenty of nutrients (particularly nitrogen).
Onion sets are just small bulbs, rather than seeds. You don’t need to plant them very deeply, in fact the top of each one can be pointing out of the soil. Keep them spaced further than with traditional onions, about 10 inches apart.
You should plant multiplier onions where there will be full sun during the day. If you have the space, you may want to plant them around your other vegetables so that the strong smell can help keep insects away from your other plants.
They can tolerate hot weather, so you can also plant a second crop a month or so later than your spring planting. As long as you have a long enough growing season to allow them to mature when the first frosts arrive.
While your onions are first growing, keep them well-watered. Once they have several leaves sprouted, you can just give them regular waterings when the soil starts to dry out around them.
The growing bulbs won’t be very deep, so you don’t want to be digging around your onions to get rid of weeds. Either do your weeding by hand (no hoes or cultivators), or add a thick layer of straw or mulch around your plants to keep out the weeds.
You can definitely grow multipliers in containers, but you will need much larger pots that you would with any other kind of onion due to the extra bulbs being produced. Each plant should be in a pot at least 18 to 20 inches across.
Pests and Diseases
Just like with garlic, the pungent smell of onions can repel many insects so they are often a bug-free kind of vegetable to grow. But even without insect pests, you still need to watch for a few possible threats to your onions. Various fungus and mold problems can crop up.
If your onion leaves start to wilt and turn yellow, you may have an infestation of white rot. You won’t be able to tell for sure until you dig up one of your onions and check the bulbs and roots. At the base of at least one of your onions there will be fibrous white fungus instead of actual plant roots. Dig up the onions that are showing symptoms, but once it starts to spread in the soil, there is no treatment.
Another untreatable disease is Botyrtis blight, that will first show up as pale spots on the leaves which quickly spread until the entire leaf looks bleached. Your onions will soon die once it strikes. Again, dig up the effected plants to slow its spread in your garden.
Lastly, be on the lookout for onion smut. Black streaks will start to show on the leaves, and is more likely to be a concern during wet weather.
With any of these fungus infections, you should rotate your onion crops to another part of the garden for at least 3 or 4 years because the spores can survive that long.
Harvest and Storage
Each multiplier onion can produce up to 7 or 8 bulbs that will be about 2 inches across.
You can pull your onions at any time, but you should wait at least 2 to 3 months in order to get decent size bulbs. If you pull them much sooner, either to thin or just to get a headstart on having fresh onions, you will lose the benefit of planting multipliers in the first place. They need to be left to mature so that you get the full number of bulbs.
Being underground, they can tolerate some frost so any onions you planted later in the season should be fine even once the cold weather hits. No rush to harvest them at once.
After harvest, keep your bulbs somewhere dry and warm for a few days. Laying out in the sun works, as long as the weather is dry. This helps to dry the outer layers of skin on the bulbs, and they will store much longer. A cool and dark location is best to store your crop of onions, and they should last for months. Don’t keep them in a closed container or they will certainly get moldy.
At the end of the season, keep out a handful of small onions and store them somewhere dark and dry over the winter. You can plant them as your new sets for the next season, rather than buy new bulbs again.