How to Plant a Sprouted Onion: Complete Growing Guide

You can successfully grow a new onion plant from a sprouted kitchen onion by planting the sprouted bulb 1 inch deep in well-draining soil with the green shoots facing upward. Water lightly after planting and place in a sunny location. Within 2-3 weeks, you'll see vigorous green growth, and you can harvest green onion tops in 30-45 days or wait 90-120 days for a full bulb.

Why Sprouted Onions Make Perfect Garden Starters

That onion you forgot in your pantry isn't wasted—it's actually ready to grow! When onions sprout, they're entering their natural reproductive cycle. The green shoots emerging from the top are the plant's way of reaching for sunlight to continue growth. According to research from the University of Minnesota Extension, this sprouting process indicates the onion has broken dormancy and is primed for planting.

Unlike starting from seeds which takes 100-175 days to mature, sprouted onions give you a significant head start. You're essentially bypassing the germination phase, saving 3-4 weeks of growing time. This biological shortcut makes sprouted onions one of the easiest kitchen waste items to transform into fresh produce.

What You'll Need: Simple Supplies Checklist

Don't worry about special equipment—most gardeners already have these basic items:

Essential Items	Optional But Helpful
Sprouted onion bulb	Organic fertilizer (10-10-10)
Container with drainage holes (6" deep minimum)	Moisture meter
Potting mix or garden soil	Small trellis for support
Watering can	PH testing kit

Step-by-Step Planting Process

Preparing Your Sprouted Onion

Carefully separate the green shoots from the bulb if they've grown longer than 3 inches. This prevents damage during planting. The Oregon State University Extension recommends trimming excessively long shoots to 2-3 inches to reduce transplant shock while preserving the plant's energy.

Choosing the Right Container

Select a container at least 6 inches deep with proper drainage. Onions develop shallow root systems but need room for bulb expansion. For best results, use a pot with a diameter of 8-12 inches—this gives enough space for 1-3 onions without overcrowding.

Planting Technique That Works

Fill your container with moist potting mix, leaving 1 inch of space below the rim. Place the sprouted onion with the root end down and green shoots facing up. Plant at a depth where the top of the bulb sits just below the soil surface (about 1 inch deep). Gently firm the soil around the bulb without compacting it.

Watering and Light Requirements

Water thoroughly after planting until moisture drains from the bottom, then maintain consistent moisture—about 1 inch of water per week. Onions prefer full sun (6-8 hours daily), but will tolerate partial shade. The ideal temperature range is 55-75°F (13-24°C), making spring and fall the best planting seasons in most climates.

Growth Timeline: What to Expect Week by Week

Understanding the growth stages helps you provide proper care at each phase:

Time Period	Visible Changes	Care Requirements
Days 1-7	New green shoots emerge	Maintain consistent moisture
Weeks 2-4	Rapid leaf growth, roots establish	Fertilize lightly every 2 weeks
Month 2	Bulb begins forming underground	Reduce watering slightly
Months 3-4	Bulb reaches full size	Stop watering 1 week before harvest

Troubleshooting Common Growing Problems

Yellowing Leaves

If leaves turn yellow from the tips downward, this typically indicates overwatering. The Utah State University Extension notes that onions prefer drier conditions as bulbs mature. Reduce watering frequency and ensure proper drainage.

Stunted Growth

Limited growth often results from poor soil nutrition or overcrowding. Onions need phosphorus for bulb development—add bone meal or a balanced fertilizer if growth stalls after 4 weeks. Remember that each onion needs at least 3-4 inches of space in all directions.

Pest Prevention

Onions naturally repel many pests, but thrips and onion maggots can be problematic. The most effective organic solution is companion planting with carrots or lettuce, which creates a natural barrier. If pests appear, a mild soap spray (1 tsp dish soap per quart of water) applied in the evening works effectively.

Harvesting Your Homegrown Onions

You have two harvesting options depending on your needs:

Green Onion Stage (30-45 days)

When green shoots reach 6-8 inches tall, you can harvest individual leaves as needed. Cut only the outer leaves, leaving the inner shoots to continue growing. This provides fresh scallions while allowing the bulb to keep developing.

Full Bulb Harvest (90-120 days)

Watch for the natural signal: when about half the leaves fall over and turn brown. Stop watering completely for 7-10 days before harvesting. Carefully dig up bulbs with a garden fork, then cure them in a warm, dry, well-ventilated area for 2-3 weeks before storing.

Maximizing Your Success Rate

Based on field trials conducted by the University of Illinois Extension, these three practices significantly improve results:

1. Soil pH matters: Onions prefer slightly acidic to neutral soil (6.0-7.0 pH). Test your soil and amend with lime if below 6.0.
2. Rotation is key: Don't plant onions in the same spot two years in a row to prevent disease buildup.
3. Trimming technique: When harvesting green onions, cut at a 45-degree angle 1 inch above the bulb to encourage faster regrowth.

Frequently Asked Questions

Can I plant a sprouted onion that's started to rot?

No, avoid planting onions with soft spots, mold, or significant rot. The University of California Extension recommends only using sprouted onions that feel firm throughout. Rotting bulbs will likely fail to establish and could introduce pathogens to your soil.

How deep should I plant a sprouted onion bulb?

Plant sprouted onions with the top of the bulb just below the soil surface—about 1 inch deep. Planting too deep (more than 2 inches) can cause rot, while planting too shallow may expose the developing bulb to sunlight, causing greening and bitterness.

Can I grow onions from sprouted grocery store onions year-round?

Yes, with proper indoor conditions. Maintain temperatures between 60-75°F (15-24°C) and provide 12-14 hours of artificial light daily using LED grow lights placed 6-12 inches above the plants. Indoor-grown onions typically produce green tops rather than full bulbs but still provide fresh flavor.

Why are my planted sprouted onions not growing larger?

Onions need cool temperatures (50-75°F) to form bulbs. If temperatures exceed 80°F consistently, the plant focuses on leaf growth instead of bulb development. The University of Georgia Extension notes that day length also matters—onions require 12-14 hours of daylight to trigger bulb formation.