Plant onions in an area that gets direct sun (at least 6-8 hours). The top foot of the soil must be loose and have good drainage. Add compost to compacted soil before planting to improve drainage and friability. Onions grow best in soil with a pH between 6.2 and 6.8.
If onions are planted too close to one another they will compete with one another, resulting in smaller bulbs. Whether direct seeding, transplanting seedlings or planting onion sets, onions should be spaced 3-4 inches apart to maximize growth.
Plant onions in early spring once the ground is workable. In-ground gardens and raised beds are both excellent options for growing onions. Space onion plants 6 inches apart in rows that are 12 inches apart. Grow them in a sunny spot that has fertile, well-drained soil with a pH of 6.0 to 6.8.
Feed your onions lots of nitrogen to increase bulb size. Because they are a root vegetable, many people believe that onions require lots of phosphorus, however, for increased onion size, nitrogen is your key nutrient. Each leaf that an onion plant produces means an additional ring on the onion.
Onions require a high source of nitrogen. A nitrogen-based fertilizer (ammonium sulfate or ammonium nitrate) should be applied at the rate of one cup per twenty feet of row.
Plants will grow best when you use Miracle-Gro® soil and plant food together to create the ideal nutrition-filled growing environment. A month after planting, begin feeding growing onions by mixing Miracle-Gro® Performance Organic® Edibles Plant Nutrition Granules in with the soil.
One bulb will produce one onion; sets may be planted 2” apart if harvested as scallions or 'green onions', or 4”-6” apart if allowed to mature into a full-sized cooking onion. You don't need to plant in rows; a 1'x1' square can grow 24 scallions!
When you notice the leaves on the first few plants start to fall over, stop watering and leave the onions in the ground for 7 to 14 days (depending on how dry or humid your climate is) to allow them to finish maturing. Withholding water at this stage helps keep the onions from rotting.
Dig up any onions that produce a flower stem. Once they start to set seed they won't grow any more, and they don't store well either. But onions that have bolted are still edible and will taste fine, so dig them up first and eat them, leaving the others to mature in the ground.
Stick your finger into the ground near the plants; if you can't feel moisture up to your first knuckle, it's time to water. In a typical 12 week growing season, we recommend irrigating with one inch of water once or twice a week depending on the amount of rainfall received.
Onions need about 1 inch of water per week, so if the weather is dry, you'll need to water.
It's a little-known fact that many seasoned gardeners aren't aware of: you can grow onions (and shallots) in the winter. These super-hardy plants can survive incredibly cold temperatures with a little protection, and provide quality bulbs even after they bolt in the spring.
When you have a large onion, it's going to be stronger because it's spent more time beneath the ground. The longer an onion sits in the soil, the stronger the sulfur will be," explains Palak Patel, a chef and educator at the Institute of Culinary Education.
Plant onions on a mound because they need extra good drainage, and leeks in a trench so you can backfill them as they grow to encourage long white stems.
Again, don't bury them more than 1 inch under the soil. It's important that onions aren't planted too deep, as this can affect bulb development.
Overwatering is equally problematic. If leaves develop a yellow tinge, cut back on watering. The closer to harvest time, the greater the need for water, but watch the plants carefully—when the onion tops start falling over, stop watering and let the soil dry out before harvesting.
As maturation proceeds, necks soften and the weight of the leaves causes the tops to fall over. "Tops down" is the physiological response that results from compounds shuttled from the onion's leaves to its scales; consequently, the bulb swells and the tops dry down.
So, why are your onions so small? Planting a variety of onion that is incompatible with your climate will result in smaller onions. Also, early flowering due to fluctuating temperatures causes small onions. Finally, a lack of nutrients due to competition or poor soil also causes small onions.
Sets are readily available in early spring and late summer in garden centres and from online suppliers. They are usually planted in spring, from mid-March to mid-April. Some cultivars are suitable for planting in October to mid March – these are less sensitive to cold, which would otherwise cause bolting.
Sets are dormant bulbs that can be planted directly into the ground. Onion sets should be planted 1 to 2 inches deep and 2 to 3 inches apart. Then, once the plants begin to grow, thin them to 3 to 4 inches apart.
Baking soda helps the plants become less acidic and prevents fungal growth.
Adding Epsom salt is a simple way to increase the health of their blooms, and is something that you can include easily as a part of a normal routine. For potted plants, simply dissolve two tablespoons of Epsom salt per gallon of water, and substitute this solution for normal watering once a month.
Carnivorous plants — Pitcher plants, venus flytraps, and sundews are some insect-eating plants that should not be applied with Epsom salts. Because they are adapted to grow in mineral-poor and depleted soil, supplementing fertilizers with even a tiny dosage could mean death to the bug-trapping ornamentals.