Introduction: Why Grow Onions from Sets?
Onions (Allium cepa) are a kitchen staple—versatile, shelf-stable, and easy to grow. While you can start onions from seeds or transplants, onion sets (small, dormant onion bulbs sold in bags) are the fastest and most foolproof option for home gardeners. They skip the slow seedling stage, resist pests better than seeds, and produce mature bulbs in a fraction of the time.
But how long exactly do onion sets take to grow? The answer depends on your climate, the type of onion (long-day vs. short-day), and when you plant—but most sets mature in 90–120 days. This guide breaks down the full onion set growth timeline, key factors that affect speed, and tips to ensure a bountiful harvest of crisp, sweet onions.
Part 1: Key Factors That Impact Onion Set Growth Time
Before diving into timelines, it’s critical to understand the variables that determine how fast your onion sets will mature. These factors can shorten or lengthen the growing period by 2–4 weeks.
1.1 Onion Type: Long-Day vs. Short-Day vs. Day-Neutral
Onions are categorized by how much daylight they need to form bulbs—a trait that directly impacts growth time and where they thrive:
- Long-Day Onions: Need 14–16 hours of daylight to bulb (e.g., ‘Yellow Globe’, ‘Red Wing’). They grow best in northern climates (USDA Zones 3–6) and take 100–120 days from set to harvest.
- Short-Day Onions: Need 10–12 hours of daylight to bulb (e.g., ‘Texas Early White’, ‘Red Burgundy’). They thrive in southern climates (Zones 7–10) and mature faster—80–90 days from set to harvest.
- Day-Neutral Onions: Adapt to 12–14 hours of daylight (e.g., ‘Walla Walla Sweet’, ‘Candy’). They grow in most zones (3–10) and take 90–100 days to mature, making them a flexible choice for beginners.
1.2 Planting Time: Spring vs. Fall
When you plant onion sets depends on your climate, and it’s one of the biggest drivers of growth time:
- Spring Planting (Zones 3–7): Plant sets 2–3 weeks before the last spring frost (when soil temperature reaches 45–50°F/7–10°C). Bulbs mature by late summer/early fall (90–120 days later).
- Fall Planting (Zones 8–10): Plant sets in late fall (6–8 weeks before the first frost). Sets establish roots but don’t bulb before winter, then resume growth in spring and mature by early summer (total 120–150 days from planting, but most growth happens in spring).
1.3 Climate & Growing Conditions
Onions grow fastest in mild temperatures (60–75°F/15–24°C) with consistent moisture and full sun. Extreme conditions slow growth:
- Cold Soil: Soil below 40°F/4°C stops root growth—sets sit dormant until temperatures rise.
- Heatwaves: Temperatures above 85°F/29°C cause bolting (premature flowering), which diverts energy from bulb growth and shortens storage life.
- Shade: Onions need 6–8 hours of direct sunlight daily. Less light means smaller bulbs and slower maturation.
Part 2: The Complete Onion Set Growth Timeline (Spring Planting)
Spring planting is the most common method for most gardeners. Below is a week-by-week breakdown of how onion sets grow from planting to harvest, using a long-day variety (100 days) as an example.
| Growth Stage | Timeline (After Spring Planting) | What Happens |
|---|---|---|
| 1. Root Establishment | Weeks 1–2 | Dormant onion sets send out thin, white feeder roots into the soil. No above-ground growth yet—roots anchor the set and absorb water. |
| 2. Green Top Growth | Weeks 2–4 | A green shoot emerges from the top of the set and grows into long, hollow leaves (called “scapes”). Leaves grow 1–2 inches per week—this stage is critical for photosynthesis (energy stored in bulbs). |
| 3. Leaf Expansion & Bulb Initiation | Weeks 4–8 | Leaves continue to grow (reaching 8–12 inches tall) as daylight hours increase. For long-day onions, when days hit 14+ hours, the plant starts shifting energy from leaves to bulb formation. You may notice the soil around the set start to bulge slightly (early bulb growth). |
| 4. Bulb Swelling | Weeks 8–16 | Bulbs grow rapidly, expanding in size and pushing the soil upward. The outer layers (papery skins) start to form. Water consistently during this stage—dry soil leads to small, tough bulbs. By Week 12, bulbs are the size of a golf ball; by Week 16, they reach mature size (tennis ball or larger, depending on variety). |
| 5. Maturation & Drying | Weeks 16–20 | Leaves start to yellow and fall over (a key sign of maturity). The plant stops watering the bulb, and the outer skins dry and harden. Bulbs shrink slightly as they lose moisture—this helps them store longer. |
| 6. Harvest | Weeks 20–22 (90–100 days total) | Once 70–80% of the leaves have fallen over and dried, onions are ready to harvest. Gently pull or dig them up—avoid bruising the bulbs. |
Fall-Planted Onion Set Timeline (Zones 8–10)
For warmer climates, fall planting extends the growing season and produces earlier harvests:
- Weeks 1–4 (Fall): Sets plant roots but grow only 2–3 inches of green top (no bulb formation).
- Weeks 4–12 (Winter): Growth slows to a stop as temperatures drop. Sets go dormant but stay alive (mulch heavily to protect from light freezes).
- Weeks 12–24 (Spring): As temperatures rise, green tops regrow rapidly. Bulb initiation and swelling happen from Week 16–24.
- Harvest (Weeks 24–26): Bulbs mature by early summer (120–150 days from fall planting), 3–4 weeks earlier than spring-planted sets.
Part 3: Speeding Up Onion Set Growth (Without Harming Bulbs)
Onions are relatively low-maintenance, but you can shorten maturation time by 1–2 weeks with these simple steps. The goal is to help sets grow strong roots and leaves early—so they can bulb faster when daylight triggers it.
3.1 Prepare the Soil for Fast Root Growth
Onion roots are shallow (6–8 inches deep), so they need loose, fertile soil to spread quickly. Prepare your bed 2–3 weeks before planting:
- Till the soil to 8–10 inches deep to break up clumps—compact soil slows root growth.
- Mix in 2–3 inches of aged compost or well-rotted manure—this adds nutrients and improves drainage.
- Add 1 tablespoon of balanced fertilizer (10-10-10) per square foot—avoid high-nitrogen fertilizer (it causes leafy growth but small bulbs).
- Test soil pH: Onions prefer 6.0–6.8. If too acidic, add lime; if too alkaline, add sulfur.
3.2 Plant Sets Correctly (Depth & Spacing Matter)
Planting sets too deep or too close together slows growth and leads to small bulbs:
- Depth: Plant sets 1–2 inches deep, with the pointed end up. The top of the set should be just below the soil surface (if planted too deep, bulbs take longer to emerge; too shallow, they dry out).
- Spacing: Space sets 4–6 inches apart in rows, with 12–18 inches between rows. This gives bulbs room to expand and lets air circulate (preventing disease).
3.3 Water & Feed for Steady Growth
Consistent moisture and nutrients keep onions growing at a steady pace:
- Watering: Give onions 1 inch of water per week (rain or irrigation). Water deeply once a week instead of light daily watering—this encourages deep root growth. Stop watering 2 weeks before harvest (drying bulbs store better).
- Fertilizing: Side-dress with a phosphorus-rich fertilizer (5-10-10) 4 weeks after planting—phosphorus boosts bulb growth. Avoid fertilizing after bulb initiation (Weeks 8–10) —it can delay maturation.
3.4 Mulch to Regulate Temperature & Moisture
Apply 2–3 inches of straw, shredded leaves, or grass clippings around sets after planting. Mulch:
- Keeps soil cool in spring (speeding up root growth) and warm in fall (extending growth in southern zones).
- Retains moisture, so you water less often.
- Suppresses weeds—weeds compete with onions for nutrients and slow growth.
Part 4: Common Mistakes That Slow Onion Set Growth (And How to Fix Them)
Even small errors can delay your onion harvest by weeks. Avoid these pitfalls to keep your sets on track for a timely, bountiful crop.
4.1 Planting the Wrong Onion Type for Your Zone
Planting long-day onions in the South (where days are shorter) or short-day onions in the North means bulbs will never form—or take months longer.
- Fix: Check the seed packet or set bag for “long-day,” “short-day,” or “day-neutral” labels. Match the type to your zone (long-day for Zones 3–6, short-day for 7–10, day-neutral for all).
4.2 Allowing Bolting (Premature Flowering)
Bolting happens when onions are stressed (cold snaps, heatwaves, or drought). The plant flowers to produce seeds, which stops bulb growth entirely.
- Fix: Plant sets at the right time (avoid late spring frosts). Water consistently to prevent drought stress. If a set bolts, harvest it immediately—bolted onions are edible but don’t store well.
4.3 Overwatering or Underwatering
- Overwatering: Soggy soil rots roots, so sets stop growing and may die.
- Underwatering: Dry soil causes leaves to wilt and bulbs to shrink.
- Fix: Stick your finger 1 inch into the soil—water if it feels dry. Use drip irrigation or a soaker hose to water at the base (avoid wetting leaves, which causes fungal disease).
4.4 Ignoring Pests & Diseases
Onion flies, thrips, and white rot can slow growth or kill sets. These pests feed on roots or leaves, robbing the plant of energy.
- Fix:
- Cover sets with row covers after planting to block onion flies.
- Spray thrips (tiny white bugs on leaves) with neem oil or soapy water.
- Avoid planting onions in the same spot every year (rotate crops every 3 years) to prevent white rot (a soil-borne fungus).
Part 5: How to Know When Onion Sets Are Ready to Harvest
Harvesting at the right time ensures onions are crisp, flavorful, and store well. Wait for these key signs—don’t rush it (unripe onions are soft and don’t keep):
- Leaves Fall Over: 70–80% of the green tops have turned yellow and collapsed onto the soil. This means the plant has stopped feeding the bulb.
- Outer Skins Dry: Bulbs have tight, papery outer skins (no soft or green areas).
- Necks Shrivel: The neck (where leaves meet the bulb) is dry and thin—squeeze it gently; it should feel firm, not soft.
Harvest Tips for Best Results
- Time of Day: Harvest in the morning when soil is moist—bulbs pull up easier.
- Method: Gently loosen soil around the bulb with a garden fork, then pull it up by the leaves. Avoid yanking—this can bruise the bulb.
- Curing: Lay onions in a single layer in a cool, dry, well-ventilated area (60–70°F/15–21°C) for 2–3 weeks. Trim the leaves to 1 inch and the roots to ½ inch after curing.
Conclusion: Fast, Foolproof Onions from Sets
Onion sets are the quickest way to grow onions at home—maturing in 80–120 days for spring planting, or 120–150 days for fall planting. With the right variety, planting time, and care, you’ll be harvesting crisp, sweet onions in just 3–4 months.
The key to success is matching the onion type to your climate, planting sets at the correct depth, and keeping soil consistently moist (but not soggy). Patience pays off—wait for leaves to fall over before harvesting, and you’ll get onions that taste better than store-bought and store for months.
Now that you know the timeline, grab a bag of onion sets, prepare your bed, and start planting—your next batch of homemade onion rings or roasted veggies is just a few months away!
If you want a custom onion set planting calendar tailored to your hardiness zone (e.g., exact planting dates, when to stop watering), just let me know—I can create one that maps out every step for your area.