What's the secret to growing large onions?
Written by
Olivia Mitchell
Reviewed by
Prof. Martin Thorne, Ph.D.To grow truly large onions, it is essential to start with day-length appropriate varieties. 'Texas SuperSweet' is a short-day type for the southern zone, while 'Kelsae' long-day types develop well in the northern summer. I test three different varieties each year, looking for superior performers in my garden. Your latitude determines how large your onions can grow!
Water your bulbs consistently during their critical growth phases, aiming for 1 to 1.5 inches of water per week when the leaves are developing, applied via drip irrigation to the soil. Once the bulbs start to swell, cut them back to half an inch, ceasing altogether about a week before harvest. My device prevents watering too much and causing splitting. However, your timing allows them to avoid rotting while they are still growing.
Prepare loose soil 12 inches deep, enriched with compost. Onions require space to grow unhindered. I double-dig each spring into soil that has been built in this way to a volume of 30% compost. No dense strata to hamper expansion. Your type of soil can grow the best-sized bulbs.
Establishment Phase
- Apply high-nitrogen fertilizer at 3-leaf stage
- Maintain consistent soil moisture
- Thin seedlings to 4-inch spacing
Bulbing Phase
- Switch to low-nitrogen fertilizer
- Reduce watering frequency
- Remove soil from bulb shoulders
Pre-Harvest
- Stop watering 14 days before harvest
- Monitor for onion fly larvae
- Support leaning plants with soil mounds
Strategic thinning will help to limit overcrowding. When your seedlings are approximately 6 in. tall, remove the weaker ones. I use scissors so that the roots are not disturbed when I remove any seedlings from the garden bed. This method of spacing enables greater penetration of sunlight and improves air circulation between adjacent seedlings, reducing disease exposure.
Use a nitrogen rich fertilizer only when growth is early. Fish emulsion or blood meal at planting time will make the tops grow more at the expence of the bulb. Do not feed at all when the necks start to soften and become hollow. My rule: fertilize before the 15th of June, and thereafter strictly to give water.
Check for thrips and maggots weekly; inspect the underside of leaves with a magnifying glass if needed. Use floating row covers at the height of the pest season. I spray neem oil at the first sign of trouble. Your diligence prevents plants from being damaged throughout the season, thereby reducing their size.
Read the full article: How to Grow Onions Successfully at Home
