Guide to Cultivating Onions in Your Backyard Plot

In the world of vegetables, onions (Allium cepa) are a staple in many kitchens. Here's a comprehensive guide on how to grow big, delicious onions using three different methods: seeds, sets, and transplants.

Growing from Seeds

Start onion seeds 8 to 10 weeks before your last frost date to give seedlings a strong start. Sow seeds in a well-drained seed-starting mix about 4 inches deep in containers. Keep seeds warm (~70-75°F) and moist until they sprout (7-10 days). After sprouting, reduce humidity and move them to a cooler spot. Transplant seedlings outdoors once they are pencil-thick and spaced about 3-4 inches apart. Onions require frequent watering to keep soil moist and benefit from nitrogen fertilizer early on; however, stop nitrogen-heavy feeding by mid-July to encourage bulb formation rather than leaf growth. Seeds offer the greatest variety, including day-length types, better disease resistance, and longer storage life. However, they need a longer growing season and are more challenging to start compared to sets or transplants.

Growing from Sets

Sets are best planted early spring or fall depending on your growing zone. Sets are small immature onion bulbs that can be directly planted into the garden. Plant them about 1-2 inches deep, spaced 4 inches apart in well-prepared soil. Water regularly, ensuring soil does not dry out. Onions grown from sets may bolt more often and typically produce smaller bulbs than from seed. Sets establish quickly and are easier for gardeners seeking faster or more convenient crops. However, they offer limited variety options, prone to bolting, and often better for green onions or scallions rather than large bulbs.

Growing from Transplants

Purchase or start transplants indoors and plant out after last frost when seedlings are pencil-thick and sturdy. Transplants are young seedlings started under controlled conditions, making them easier than seeds but offering more variety than sets. Transplant seedlings carefully to avoid root disturbance, spacing 3-4 inches apart. Provide consistent moisture and fertilize with diluted fish emulsion or compost tea early on, stopping nitrogen feeding by mid-summer. Transplants provide a middle ground with earlier harvest and variety compared to seeds and sets. However, they come at a higher cost.

Additional Tips for Growing Big Onions

Preserve healthy leaves by pruning above the newest leaf tips during growth to encourage size increase — this prolongs leaf production and sends more energy to the bulb.
Ensure well-drained, fertile soil rich in organic matter, with consistent moisture but not waterlogged conditions.
Stop heavy nitrogen feeding by mid-July to promote bulb maturation rather than leafy growth.
Space plants adequately (around 3-4 inches apart) to allow ample room for bulb enlargement.
Harvest onions when tops begin to yellow and fall over, then cure bulbs in a dry, airy place for storage.

In summary, seeds offer the largest, most diverse onions but need an early start and longer care; sets are fast and easy but yield smaller bulbs; transplants provide a middle ground with earlier harvest and variety. Timing, soil preparation, consistent watering, and managing nitrogen carefully are key to maximizing onion size regardless of method. Happy gardening!

[1] Bloomfield, C. (Manager of Marketing Communications). Personal communication, 2022. [2] Rhoades, H. (Founder). (2007). Gardening Know How. Retrieved from https://www.gardeningknowhow.com [3] Onion Growers Council. (n.d.). Onion Growing Tips. Retrieved from https://www.onion-info.org [4] National Gardening Association. (n.d.). Onion Growing Guide. Retrieved from https://www.garden.org [5] University of Illinois Extension. (n.d.). Growing Onions in the Home Garden. Retrieved from https://web.extension.illinois.edu