Do Onions Grow from Bulbs? - Plant Care Guide

Yes, mature onions are botanically bulbs, and these bulbs are the primary edible part we cultivate and consume. However, when it comes to growing onions, you don't always start directly from a fully formed bulb. Instead, onions can be grown from seeds, small immature bulbs called "sets," or even from transplant seedlings. The growth cycle culminates in the formation of the large, fleshy storage bulb.

What is an Onion (Botanically Speaking)?

An onion (Allium cepa) is a staple vegetable cultivated and consumed worldwide. Botanically, the part of the onion we typically eat is a fascinating and complex structure designed for food storage and the plant's survival.

Here's what an onion is, botanically speaking:

• A True Bulb: The mature onion that we slice, dice, and cook is a true bulb.
  • Definition: A bulb is a specialized underground storage organ composed of a short, flattened stem (called the basal plate) from which roots grow downwards and fleshy leaves (scales) grow upwards, tightly layered around a central bud.
  • Function: These fleshy scales store food (sugars) that the plant produced during the growing season. This stored energy allows the plant to survive dormancy (e.g., winter) and then produce new growth (leaves and a flower stalk) in the next growing season.
• Monocotyledon: Onions are monocots, a group of flowering plants characterized by having a single cotyledon (seed leaf) in their embryo, parallel venation in their leaves, and flower parts typically in multiples of three.
• Biennial Nature: Most cultivated onions are biennials.
  • First Year: They grow from seed (or set) and produce the edible storage bulb.
  • Second Year: If left in the ground (and they survive winter), they will use the stored energy in the bulb to produce a flower stalk (scapes) and then seeds. After flowering and seeding, the parent bulb usually dies.
• The Basal Plate: At the very bottom of the bulb is the basal plate, a small, disc-like stem from which the roots emerge. If you cut off the root end of an onion and plant it, sometimes new growth can emerge from this basal plate.
• The Leaves: The edible, fleshy layers of the bulb are actually modified, swollen leaves. The green tops of the onion are also leaves.

So, the edible part of an onion is fundamentally a bulb, a clever and efficient way for the plant to store energy and ensure its survival and reproduction.

How Do Onions Grow? From Seed, Set, or Transplant?

While mature onions are bulbs, gardeners have three primary methods to grow them, each starting from a different stage of the plant's life cycle. Understanding these options helps in planning your onion patch.

Here's how onions grow, from seed, set, or transplant:

1. Growing Onions from Seed:

   • Method: This involves planting tiny onion seeds directly into the soil or starting them indoors.
   • Process: Seeds germinate into small seedlings with thin, grass-like leaves. These seedlings then grow throughout the season, gradually forming a single, large storage bulb underground.
   • Pros:
     • Widest Variety: Offers the largest selection of onion varieties (colors, sizes, shapes, flavors).
     • Long-Day/Short-Day Specifics: Allows you to choose varieties specifically adapted to your region's day length.
     • Cheapest Option: Seeds are very inexpensive.
     • Less Bolting Risk: Lower risk of the plant "bolting" (going to seed prematurely) compared to sets.
   • Cons:
     • Longest Time to Harvest: Takes the longest time to mature (typically 100-175 days).
     • Requires Patience: Seeds are tiny, and initial growth is slow.
   • Best For: Gardeners who want specific varieties, have a long growing season, and are willing to invest time. You can buy onion seeds from most seed suppliers.

2. Growing Onions from Sets:

   • Method: Onion sets are small, immature onion bulbs (usually 1/2 to 3/4 inch in diameter) that were grown from seed the previous year, then harvested prematurely and stored over winter.
   • Process: You plant these dormant small bulbs directly into the garden. They quickly sprout leaves and then begin to swell into full-sized bulbs.
   • Pros:
     • Fastest to Harvest: Much quicker to mature than seeds.
     • Easiest to Plant: Simply push into the soil.
     • Good for Beginners: More forgiving than seeds or transplants.
   • Cons:
     • Limited Variety: Fewer varieties available compared to seeds.
     • Higher Bolting Risk: More prone to bolting if exposed to cold temperatures after planting, especially if the sets are too large.
     • More Expensive: Cost more than seeds.
   • Best For: Gardeners who prioritize speed, ease, and don't need a specific heirloom variety. You can find onion sets at garden centers in spring.

3. Growing Onions from Transplants:

   • Method: Onion transplants are young, immature onion plants (often sold in bundles of 50-100) that were started from seed indoors or in a greenhouse. They look like grass-like seedlings.
   • Process: You plant these pre-grown seedlings into your garden.
   • Pros:
     • Faster Than Seeds: Offers a good head start over direct-sown seeds.
     • Good Variety: Many varieties available.
     • Less Bolting Risk: Less prone to bolting than sets, especially smaller, pencil-thin transplants.
   • Cons:
     • More Expensive: More costly than seeds.
     • More Fragile: Requires careful handling during transplanting.
     • Limited Availability: Must be purchased at the correct time of year.
   • Best For: Gardeners who want a balance of variety and a head start, particularly in areas with shorter growing seasons.

Each method has its place in the gardener's toolkit, allowing you to choose the approach that best suits your goals and local climate.

What is a Bulb (Botanical Definition)?

To truly understand how onions grow from bulbs, it's helpful to clarify the botanical definition of a bulb. In botany, a bulb is a very specific type of underground storage organ, and not all swollen, underground plant parts are true bulbs.

Here's the botanical definition of a bulb:

• A Modified Underground Stem: A true bulb is essentially a short, vertical, compressed underground stem (often called a basal plate or basal stem).
• Fleshy Storage Leaves (Scales): Growing from this basal plate are tightly packed, fleshy, modified leaves (called scales or tunics). These scales are where the plant stores its food reserves, primarily carbohydrates (sugars), produced during photosynthesis.
• Apical Bud: At the center of the basal plate, surrounded by the fleshy scales, is an apical bud (or multiple buds). This bud contains the embryonic shoot that will develop into new leaves and a flower stalk in the next growing season.
• Roots: Fibrous roots grow downwards from the underside of the basal plate.
• Protective Outer Layer (Tunic): Many bulbs (like onions, daffodils, tulips) have a dry, papery, protective outer layer called a tunic, which helps prevent desiccation (drying out) and physical damage. These are called tunicate bulbs.
• Function: The primary function of a bulb is to serve as a storage organ for nutrients, allowing the plant to survive adverse conditions (like winter dormancy or dry spells) and then regenerate in favorable conditions.

Examples of True Bulbs:

• Onions (Allium cepa)
• Garlic (Allium sativum)
• *Tulips (Tulipa spp.)*
• *Daffodils (Narcissus spp.)*
• *Hyacinths (Hyacinthus spp.)*
• *Lilies (Lilium spp.)*

Distinguishing from Other Underground Storage Organs:

It's important to differentiate true bulbs from other types of underground storage organs:

• Corms: (e.g., gladiolus, crocus) - Solid, swollen underground stems, not layered leaves.
• Tubers: (e.g., potato, dahlia) - Swollen underground stems with "eyes" (buds) but not layered.
• Rhizomes: (e.g., ginger, iris) - Horizontal underground stems that send up shoots.

So, when you hold a mature onion, you are indeed holding a precisely engineered botanical bulb, perfectly adapted for storing energy.

What is the Difference Between Long-Day and Short-Day Onions?

The distinction between long-day and short-day onions is a critical factor for successful onion cultivation, particularly when growing from seed. This refers to the amount of daylight (photoperiod) a particular onion variety needs to trigger the formation of its bulb.

Here's the difference between long-day and short-day onions:

1. Long-Day Onions:

   • Requirement: These varieties require 14-16 hours of daylight per day to initiate bulb formation.
   • Where to Grow: Best suited for northern latitudes (roughly above 36°N latitude, which includes most of the northern US, Canada, and Northern Europe). In these regions, the summer days provide the necessary long periods of sunlight.
   • Planting Time: Plant in early spring. The plants grow foliage during the gradually lengthening days of spring, and then the long days of summer trigger bulb development.
   • Characteristics: Tend to produce larger, often pungent, bulbs that store well.
   • Examples: 'Walla Walla', 'Sweet Spanish', 'Red Zeppelin'. You can find long-day onion seeds from various suppliers.

2. Short-Day Onions:

   • Requirement: These varieties require 10-12 hours of daylight per day to initiate bulb formation.
   • Where to Grow: Best suited for southern latitudes (roughly below 36°N latitude, which includes the southern US, Mexico, and other tropical/subtropical regions). In these regions, the longest days of summer are not long enough to trigger long-day varieties, but the milder winters allow for earlier planting and the shorter days of spring/early summer trigger bulbing.
   • Planting Time: Plant in fall (for a spring/early summer harvest) or very early spring. They grow foliage through the shorter days and then bulb up as days lengthen slightly but remain relatively short.
   • Characteristics: Tend to produce milder, sweeter bulbs that generally don't store as long as long-day varieties.
   • Examples: 'Vidalia', 'Granex', 'Red Creole'.

3. Intermediate-Day Onions (Day-Neutral):

   • Requirement: These varieties require 12-14 hours of daylight per day to initiate bulbing.
   • Where to Grow: Best suited for middle latitudes (transitional zones, roughly between 32°N and 42°N).
   • Characteristics: Offer a compromise in flavor and storage.
   • Examples: 'Candy', 'Red Candy Apple'.

Why is This Important?

• If you plant a long-day onion in a short-day region, it won't get enough sunlight hours to trigger bulbing, and you'll end up with only green tops (scallions) or very small bulbs.
• If you plant a short-day onion in a long-day region, it will start bulbing too early in the spring while the plant is still small, resulting in small, underdeveloped bulbs.

Knowing your region's latitude and choosing the correct onion day-length type is paramount for a successful onion harvest.

How to Harvest and Cure Onions for Storage

Successfully harvesting and curing onions is crucial for ensuring they store well through the winter, preventing rot and extending their shelf life. The curing process hardens the outer skin and dries the neck, making the bulb resistant to moisture and disease.

Here's how to harvest and cure onions for storage:

Step 1: When to Harvest

• Foliage Falling Over (Primary Sign): The main sign that your onions are ready for harvest is when their green tops begin to yellow, soften, and fall over naturally. This indicates that the plant has stopped putting energy into leaf growth and is directing it into the bulb for storage.
• Timing: This usually occurs in mid to late summer, depending on your climate and the onion variety (typically 100-175 days from seed, or less from sets/transplants).
• Don't Rush: Let at least half to two-thirds of the tops fall over naturally before harvesting.
• Emergency Harvest: If a hard frost is imminent, or if a disease takes hold, you may need to harvest slightly early, but their storage life will be reduced.

Step 2: Harvesting

• Loosen Soil (If Needed): If your soil is very hard or compacted, gently loosen the soil around the bulbs with a garden fork or trowel to avoid damaging the bulbs when pulling.
• Pull Gently: Carefully pull each onion bulb from the ground. Be careful not to bruise or damage the bulbs.

Step 3: Curing (The Most Important Step for Storage)

• Definition: Curing onions is the process of drying the outer skin and the neck of the bulb, creating a protective barrier against moisture and rot.
• Method:
  1. Leave in the Garden (If Dry): If the weather is dry and warm, you can leave the pulled onions right in the garden bed for a few days to a week. Spread them in a single layer. The tops provide shade to prevent sunscald.
  2. Move to Sheltered, Well-Ventilated Area: For reliable curing, especially in humid or unpredictable weather, move the onions to a warm (24-30°C / 75-85°F), dry, and well-ventilated location out of direct sunlight and rain. Examples:
     • A covered porch or patio.
     • An open shed or garage.
     • A greenhouse with good airflow.
  3. Spread Evenly: Spread the onions in a single layer on screens, slatted shelves, or clean newspaper. Ensure good air circulation around each bulb.
  4. Duration: Cure for 2-4 weeks, or until:
     • The neck is completely dry and tight.
     • The outer skins are papery and fully dry.
     • The tops are completely shriveled and dry.

Step 4: Prepare for Storage

• Trim Tops: Once fully cured, use clean scissors or pruning shears to trim the dry tops to about 1 inch (2.5 cm) above the bulb.
• Trim Roots: Trim the dried roots cleanly, but don't cut into the basal plate.
• Inspect: Discard any onions that show signs of damage, disease, or soft spots. These will not store well and can spread issues to other bulbs.

Step 5: Storage

• Ideal Conditions: Store cured onions in a cool (4-10°C / 40-50°F), dark, and dry place with good air circulation.
  • Avoid Humidity: Humidity encourages sprouting and rot.
• Storage Methods:
  • Mesh Bags: Old pantyhose, mesh onion bags, or braided ropes (for onions with strong necks) allow for excellent airflow. You can find onion mesh storage bags online.
  • Slatted Crates/Shelves: Allows air circulation.
• Longevity: Well-cured storage varieties (often pungent long-day types) can last for many months, even up to a year. Sweet short-day varieties generally store for only a few weeks to 2-3 months.

By carefully harvesting and curing your onions, you maximize their storage potential, allowing you to enjoy your homegrown bounty long after the growing season has ended.

Common Problems When Growing Onions

Growing onions can be a rewarding experience, but like any crop, they can encounter common problems that affect their growth and yield. Recognizing these issues early is key to successful cultivation.

Here are some common problems when growing onions:

1. Bolting (Premature Flowering):

   • Problem: The onion plant sends up a flower stalk (scape) prematurely instead of focusing energy on bulb development. This makes the bulb tough, woody, and reduces storage quality.
   • Causes:
     • Temperature Stress: Often caused by onion sets or young plants experiencing a cold snap (below 7°C / 45°F) for several weeks, then warming up. Large onion sets are more prone.
     • Incorrect Day Length: Planting a short-day variety in a long-day region, or vice versa, can trigger bolting.
   • Solution: Use appropriate day-length varieties. Plant sets that are less than 3/4 inch in diameter. Avoid planting too early. If a plant bolts, cut off the flower stalk immediately to redirect some energy back to the bulb, but eat that onion first, as it won't store well.

2. Small or Undeveloped Bulbs:

   • Problem: Onions only produce green tops or very small bulbs.
   • Causes:
     • Incorrect Day Length: The most common reason. Wrong long-day/short-day type for your region.
     • Overcrowding: Plants too close together compete for resources.
     • Lack of Sunlight: Not enough hours of direct sun.
     • Nutrient Deficiency: Especially phosphorus or potassium.
     • Water Stress: Inconsistent watering, especially during bulbing.
   • Solution: Use correct day-length varieties. Thin plants to 4-6 inches apart. Ensure full sun. Fertilize appropriately. Water consistently.

3. Pests:

   • Onion Maggots:
     • Problem: Larvae tunnel into bulbs, causing rot and plant collapse.
     • Signs: Wilting, yellowing leaves; soft, rotting bulbs; small white maggots in the bulb.
     • Solution: No easy chemical solution for home gardeners. Practice crop rotation (don't plant onions in the same spot for 3-4 years). Cover young seedlings with row covers for pest protection. Plant resistant varieties.
   • Thrips:
     • Problem: Tiny, slender insects that feed on leaves, causing silvery streaks and stunted growth.
     • Signs: Silvery streaking on leaves, distorted growth, tiny black specks (frass).
     • Solution: Blast with water. Use insecticidal soap or Neem oil. Ensure good air circulation.

4. Diseases:

   • Downy Mildew:
     • Problem: Fungal disease, especially in cool, wet conditions.
     • Signs: Pale green to yellow spots, often fuzzy, on leaves; eventually, leaves turn yellow and die back.
     • Solution: Ensure good air circulation. Avoid overhead watering. Plant resistant varieties. Remove infected plant debris.
   • Neck Rot:
     • Problem: Fungal disease that infects the neck of the bulb, causing it to rot. Often develops during curing or storage.
     • Signs: Soft, watery rot around the neck, often with gray mold; can spread throughout the bulb.
     • Solution: Proper curing is paramount. Avoid bruising bulbs. Don't over-fertilize with nitrogen. Discard infected bulbs.

5. Soft or Rotting Bulbs (After Harvest):

   • Problem: Onions spoil quickly in storage.
   • Cause: Improper curing (not fully dried), damage during harvest, too warm/humid storage conditions, storing varieties not meant for long storage.
   • Solution: Ensure thorough curing. Handle bulbs gently. Store in cool, dark, dry, well-ventilated conditions.

By being vigilant and providing appropriate care, you can overcome these common problems when growing onions and enjoy a successful, flavorful harvest.

Companion Planting for Healthier Onions

Companion planting with onions is a beneficial strategy that leverages the plant's strong aroma and its ability to deter pests, while also attracting beneficial insects and sometimes improving growth. Integrating onions strategically into your garden can enhance the health of other plants and reduce pest pressure naturally.

Here are some good companion plants for onions:

1. Carrots (Excellent Companion):
   • Benefit: Onions help deter carrot rust flies, whose larvae damage carrot roots. Carrots, in turn, may help repel onion flies. This is a classic mutual beneficial pairing.
   • Placement: Plant rows of onions alternating with rows of carrots.
2. Lettuce and Other Leafy Greens:
   • Benefit: Onions deter various pests that attack leafy greens, such as aphids. Their upright, sparse growth doesn't shade out low-growing greens.
   • Placement: Interplant onions around lettuce, spinach, or kale.
3. Cabbage and Other Brassicas (Broccoli, Brussels Sprouts, Kale):
   • Benefit: Onions help repel several pests that target brassicas, including cabbage loopers and cabbage worms.
   • Placement: Plant onions in rows or clumps among your brassica crops.
4. Chamomile:
   • Benefit: This herb is believed to improve the growth and flavor of onions and other plants when planted nearby. It also attracts beneficial pollinators.
   • Placement: Plant small patches of chamomile near your onion beds.
5. Dill and Parsley:
   • Benefit: These herbs attract beneficial insects like hoverflies and predatory wasps, which prey on aphids and other pests that might affect onions.
   • Placement: Plant in adjacent beds or rows.
6. Tomatoes:
   • Benefit: Onions can help repel some general garden pests, and tomatoes often appreciate their presence.
   • Placement: Plant onions around the perimeter of your tomato patch.
7. Beets:
   • Benefit: Onions are considered good companions for beets, with some suggestions of improved growth.
   • Placement: Plant in adjacent rows.

What to Avoid (Potential Bad Companions):

• Beans and Peas (Legumes): Onions can inhibit the growth of beans and peas. This is a common negative interaction.
• Asparagus: Some gardeners report that onions can stunt asparagus growth.
• Sage: The strong aroma of sage can sometimes interfere with onions.

By strategically using companion planting, you can create a more resilient and balanced ecosystem in your garden, reducing pest pressure on your onions and promoting overall plant health, which ultimately supports a better harvest.