If you’re wondering how to grow bigger, stronger onions, you’re in the right place. After years of trial and error and weighing in the most important factors, I’ve managed to grow onions that I’m proud of, and so can you. You just have to get familiarized with a few tricks – and one of them I’m sure you’ve heard of is topping onions.
Topping onions is a counterintuitive step for most gardeners and means actively trimming young growth on seedlings and onion sets before they reach maturity. Topping onions provides bigger bulbs, but more importantly, stronger root stocks to support the plants through cold winters and dry summers.
In this article, I’m going to sum up some of the best, and worst, options for getting bigger onions, wherever you are and whether you’re growing from onion seed or sets.
Understanding the basics of onion bulb formation
Onions grow in three stages:
Onion roots can be strengthened by trimming young leaves in the root growth stage in winter, and by adding nitrogen in the leaf growth stage in spring you increase leaf health. By controlling leaves in summer, you can focus energy on the bulb too, so the three stages have direct correlations. The most notable correlation between growth stages in onions is that the number of leaves in spring will always correlate to the number of layers in the harvested onion bulb.
Onion bulbs can go to seed, or flower very quickly in hot weather, particularly when grown from onion sets, and some of these tricks for growing bigger onion bulbs can help prevent that too.
Tricks for bigger onions
1. Topping onions for bigger bulbs
Cutting tops off seedlings when you first buy them in spring, or regularly pruning the stems from onions grown from seed over winter, produces stronger roots. The more you cut the stems, the harder your roots need to work, just think of it like a little haircut.
Start topping onion seedlings when they are around 5-6 inches tall, but don’t worry if they go over that. Use fine trimming scissors to avoid damaging your small plants. Each time you trim onion seedlings, leave around 3 inches of the original stem. You can top onions up to seven times before planting them in their final position.
While it is debatable whether this leads to bigger bulbs, it certainly produces a stronger rootstock, giving your onions a greater chance of success
By pruning leaves, you also slow down the second stage of growth (the leaf stage), which means when leaves begin to grow in spring, they grow from stronger roots, giving more and healthier leaves which in turn, provides more energy for the layers of the onion bulb below.
- Tip: In the spirit of thrift, every time you cut the onion tops off, you can eat them, either sautéed down, or fresh in salads similar to bunching onions.
2. Grow from seed for bigger onion bulbs
Onion sets are easier to grow because they are in their second year. The downside of this is that they are already trying to produce flowers, which can a) make them more likely to bolt, and b) put less energy into the bulb.
Growing onions from seed started in autumn or winter indoors gives the onion a full growing cycle, so when they begin to focus on leaf growth in spring, it follows more naturally by putting all their energy into bulb growth through summer. Therefore, you get a bigger bulb in autumn, as the plant has focused everything on its first-year bulb growth.
3. Leaf restriction for bigger onion bulbs
One trick that, honestly, I have never understood the value of, is trimming onion leaves in spring so you have 13 leaves. Supposedly, the perfect onion has 13 rings, which is meant to be the optimal number of rings to get the perfect bulb for every variety.
The idea behind this is that an onion with 13 leaves, by default, has 13 rings in the bulb, and each ring will have the perfect amount of water and energy by doing this, rather than spreading its energy across extra rings, or creating fewer more watery rings.
For most homegrown veg though, I think we can all agree that unless it’s being grown for competition, it’s the flavour that counts, so leaf restriction might be a great way to optimize your space and create onions that store better, but I’ve never found it to have any impact on onions bulb size.
4. Fertilizing for bigger onion bulbs
Gardeners can forget about fertilizing onions because we assume they are easy to grow plants that need little to no intervention in the summer months, but they really benefit from a boost in some very specific nutrients that can be in scarce supply in vegetable beds.
Phosphorus and potassium during the first 2-3 weeks after planting out will help rejuvenate your soil. Banana skins are a good source of both, and if you have houseplants to take care of, a banana skin tea can be a useful source of phosphorus for the garden, we well as indoor plants.
If you grow onions as part of a crop rotation, ensure you properly fertilize them throughout the season, including adding nitrogen in spring, and potassium and phosphorus through summer. The nutrients onions needs are not replaced by other crops, and as such need a little more help than other plants in a crop rotation.
- Phosphorus for bigger onion bulbs
Phosphorus is good for cell division, which speeds up the growth of plants. It’s also a supplement for healthy bones in humans too for the same reason. By adding phosphorus you’re essentially speeding up the production of new cells in the onion bulb that can then fill with water increasing the size of the bulb.
The best source of phosphorus that doesn’t cost a penny, is to spend a penny. Human urine contains high amounts of phosphorus, but obviously, there are many, many, reasons why taking a tinkle on your vegetable patch isn’t great idea. Other more palatable sources of phosphorus for onions are bone meal (here’s an organic option), crab and shrimp shells, burned cucumber skins, pet or human hair, and mushroom compost.
- Potassium for bigger onion bulbs
Kitchen scraps are the cheapest source of potassium, and fruit and vegetable peelings (particularly banana) are high in potassium, and can either be added to compost mixes to increase potassium levels through mulching, or turned into a tea with boiling water, and left to cool before straining and using directly to water crops.
Coffee grounds sprinkled on the soil help in three ways, as they increase potassium levels, but also increase nitrogen in the soil, which is essential for growing onion leaves in spring. Your onions will also benefit from the acidity of coffee grounds as they prefer slightly acidic soils.
5. Folding onion stems for bigger bulbs
Folding onion stems in late summer speeds up the ripening of bulbs, but doesn’t necessarily produce bigger bulbs. By folding the stems, it reduces the water that can travel upwards out of the bulb, which causes water to build up, as the roots continue to feed the plant.
As the water builds it fills the cells in the bulb, expanding them faster than when they are left to their own devices.
While technically this won’t increase the overall size of the bulbs, it will speed up their growth once bulbs have fully formed, and the plant will often do this by itself in autumn in high winds as the leaves start to grow weaker anyway. The other benefit of folding leaves is that it creates bulbs that dry more effectively and therefore store more reliably for longer.
When I started growing onions I was desperately trying all these tricks because I found it really hard to grow the bulbs at all. For so many different reasons, which I now know were all just rookie mistakes, my onions either rotted, or bolted, and learning to use some of the more basic tips and tricks for bigger bulbs in this article means I now grow predictably regular onions every year.
For some gardeners, you know who you are, you might be searching for these tips and tricks for bigger onions to show off, and that’s fine too. If you try topping onions this year and get bigger bulbs as a result, just remember you’ve got Tiny Garden Habit to thank!