Growing big bulbs starts with good site selection. Ensure the planting site is not shaded, onions need the maximum number of hours of daylight to grow into big bulbs. Choose a site that gets plenty of direct sun. Prepare the soil so that it is loose and crumby.
Dig a shallow trench that is approximately 4″inches deep and 4 inches ″wide. Add in compost or fertiliser and cover it loosely with 2 inches ″of soil. Plant the onions 6 inches ″ from the edge of the trench on both sides of the trench. Don’t plant the onions in the trench! Allow a 2 inch margin between the onions and the outside edge of the bed. Plant the onions an inch deep 4inches ″ apart.
Planting the onions too deep will inhibit their ability to bulb. When planting several rows of onions, leave 16 inches ″ between the outside edge of one bed, and the outside edge of the next. The spacing from the centre of one fertiliser trench to the centre of the next should be 36 inches.
For big bulbs you need to feed the crop well. Onions are heavy feeders, apply phosphorus and potassium for root development during the first two to three weeks after planting. This should be followed by a biweekly application of nitrogen fertilizer to keep the foliage growing.
The ideal way to provide the fertiliser without burning the bulbs is to lay it in a 3-inch-deep trench that runs the length of the onion row but 2 inches away from it. Cover the fertiliser trench with soil and water slowly. Frequent and adequate fertiliser encourages the growth of large onion bulbs. Once the bulbing starts (characterised by cracks forming around the crop) stop the application of fertiliser.
Always water your plants thoroughly. Onions need at least 1-2 inches of water per week until the bulbs starts to form. This is because they have a shallow root system and dry soil will cause bulbs to split.
Avoid overhead irrigation because, wet leaves increases susceptibility of the crop to fungal and bacterial diseases. Onions do well with consistent moisture but are prone to rot if the soil is too wet. If you see your onion tops developing a yellow tinge, cut on the water. Drip irrigation is ideal. Reduce the amount of water once the bulbs have formed, and the tops start to die back.
This will help the onion crop cure properly. Mulching can be used to maintain moisture and to control weeds, but it should be removed immediately bulbing begins. This is also meant to help in curing the bulbs.
Control fungal and bacterial diseases especially during the rainy season. Be proactive by applying a biweekly protective fungicidal spray to the foliage throughout the onion’s growing cycle. If you don’t want to use synthetic fungicides, consider organic options. If you don’t spray and disease takes hold, it will move to the rings on the inside of the onion and cause decay in storage.
Keep the farm weed free; otherwise, the onions won’t grow big. Given their weak root structure, onions do not compete well with weeds. Mulch can be used to control weeds. Plastic papers have been used effectively. I prefer use of straw for mulching.
However, should you use straw you must pull it back once formation of onions bulbs starts. This will allow the onions to dry out naturally and will help you preserve them when they are mature. Once these tops have completely faded and fallen over, onion plants are ready for harvesting and can be pulled and left in the sun to dry for several days before storing in a cool, dry area.