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Onion Growing Guide🧅: Planting, Spacing, and Watering for Perfect Bulbs

Onion Growing Guide🧅: Planting, Spacing, and Watering for Perfect Bulbs

Introduction

Onions are a kitchen staple and a rewarding addition to any garden. They thrive in raised beds, SIPs, and wicking beds, where steady hydration leads to larger, sweeter bulbs and longer storage life. Using GardenWells inserts converts any raised bed into a high-efficiency wicking bed, ensuring onions stay consistently watered without over-saturation.

When to Plant Onions

  • Spring: Plant onion sets, seedlings, or seeds March through April once soil is workable. See the March Gardening Guide for timing tips.

  • Summer Successions: Planting through June provides late-season green onion harvests. Visit the June Gardening Guide for watering strategies.

  • Fall Overwintering: In mild regions, plant onion sets in September for early spring bulbs; see the September Gardening Guide.

Square Foot Gardening Spacing

  • Spacing: 9–16 onions per sq ft depending on variety size

  • Depth: Plant sets 1” deep, tips just beneath the soil surface

  • Companions: Works beautifully with carrots, beets, and lettuce.

How to Plant Onions

Using Onion Sets (Easiest)

  • Plant small bulbs pointy-side up for faster growth and better yields.

  • Space 3–4” apart for full-sized bulbs, closer for scallions.

Starting From Seed

  • Start seeds indoors 10–12 weeks before your last frost.

  • Transplant seedlings outdoors once they reach 6” tall and soil temperatures are stable.

Transplanting

  • Space transplants 4” apart and handle roots gently.

  • Water thoroughly after planting to settle soil around bulbs.

Watering Your Onions

Consistent moisture is the secret to plump, sweet bulbs:

  • Use your WaterStem: when the Hummingbird rises, the reservoir’s full; when it drops, refill.

  • Before establishment: Top-water gently for the first 7–10 days.

  • After establishment: Refill reservoirs every 1–2 weeks depending on bed size and weather.

  • Mulch lightly to stabilize soil temps and reduce weed pressure.

  • See the June Gardening Guide for detailed SIP watering techniques.

Harvesting Onions

  • Green Onions: Ready to harvest 30–45 days after planting.

  • Bulb Onions: Harvest when tops yellow and fall over naturally, usually 90–120 days.

  • Curing: Lay bulbs in a dry, shaded area for 7–10 days to harden skins for storage.

  • Visit the September Gardening Guide for fall curing and storage tips.

Common Issues & Fixes

Issue Likely Cause Solution
Small Bulbs Crowding or insufficient sunlight Thin seedlings and ensure full sun exposure
Splitting Bulbs Inconsistent watering Use SIPs or wicking beds for stable hydration
Thrips / Pests Warm, dry weather Spray neem oil or interplant basil for deterrence

Companion Plants for Onions

Best companions (with cross-links):

  • Carrots → Mutual pest protection; carrots deter onion maggots, onions deter carrot flies.

  • Beets → Compatible root structures, perfect for SIP interplanting.

  • Lettuce → Uses shallow soil, leaving space for deeper onion bulbs.

  • Spinach → Shares similar watering needs in spring SIP setups.

  • Strawberries → Natural companions in compact SIP beds.

Avoid planting with:

  • Peas and beans → Onions release compounds that inhibit legume growth.

  • Sage & Asparagus → Compete for nutrients and growing space.

Layout Tip:

  • Plant 9–16 onions per sq ft in SIPs or wicking beds.

  • Interplant quick greens like lettuce between onion rows for layered productivity.

  • Use onions strategically as natural pest deterrents at SIP bed edges.

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