When to Plant Onion Sets by Zone

November 06, 2026

Disclosure: This post contains affiliate links. As an Amazon Associate I earn from qualifying purchases. See our disclosure policy for details.

Onion sets are small dormant bulbs, usually dime- to nickel-sized, sold for direct planting. Compared with seed, sets skip the slow germination stage and reach harvest size weeks earlier. Compared with bare-root transplants, they store longer on the shelf and handle colder soil with less shock. If your goal is full storage bulbs rather than a few scallions, sets are the middle path: faster than seed, sturdier than many transplants, and timed to your USDA zone’s last spring frost.

The planting window is set by soil workability and frost risk, not by the calendar alone. Sets go in once the bed can be opened without smearing clay into hard pans and once hard freezes that heave shallow bulbs are past their peak. Month-level timing is enough to plan the bed; exact local windows live on the onions plant page, which pulls from state extension guides. For a full vegetable calendar matched to your frost dates and zone, start with the regional gardening books.

How to read your onion set window by zone

Onions tolerate cool soil better than tomatoes or peppers, so the set window opens before most warm-season crops. In practice you plant when daytime soil is consistently above the mid-40s °F and overnight air is no longer dropping into hard freezes for stretches of nights. That usually means two to six weeks before your zone’s average last spring frost, or earlier still where winters stay mild enough for fall planting.

Fall planting is not a second thought in the warm half of the map. In Zones 8 through 10, sets planted in fall establish roots through winter and push hard as day length climbs, often outperforming a late-winter spring planting of the same variety. Cold-zone gardeners reverse that: spring is the main shot, and fall sets need heavy mulch or a cold frame to survive.

Month-level planting overview by zone band

Zone band	Primary set window	Strong secondary option
3–4	April–May	none outdoors without protection
5	late March–April	protected fall trial only
6–7	March–April	mild fall plantings in 7b
8	late February–early March	October–November fall sets
9	January–February	October–December fall/winter sets
10	December–January	almost any cool month

No cell above carries a day-of-month target. Local frost history and soil drainage still decide the week you open the bed. The frost dates in the zone sections below are the figures each zone page publishes, drawn from NOAA’s 1991–2020 U.S. Climate Normals; a garden in a frost pocket or near the coast can run earlier or later.

Zones 8a and 8b

Zone 8b averages a last spring frost around April 7 and a first fall frost around October 29, with roughly 205 frost-free days. Zone 8a runs slightly cooler: last spring frost around April 16, first fall frost around October 23, and about 190 frost-free days. Both zones support a late-February into early-March spring set planting once beds drain.

The stronger play in much of Zone 8 is fall. Sets planted in October or November root before cold weather, sit through winter, and bulb as days lengthen in spring. Choose intermediate- or short-day types for fall work here; long-day storage onions from northern catalogs often fail to bulb on Zone 8 day lengths even when the calendar timing looks perfect. Keep crowns just covered, water in once, and mulch lightly so freeze-thaw cycles do not lift the sets.

Zones 9a and 9b

Zone 9 flips the script further. Zone 9b averages a last spring frost around March 8 and a first fall frost around November 22, with about 259 frost-free days. Zone 9a is a step cooler: last spring frost around March 26, first fall frost around November 6, and roughly 225 frost-free days. Spring set planting still works in January and February, but fall and early-winter planting is the crop that defines the region.

Short-day onions are the default for both 9a and 9b. They start bulbing while days are still relatively short, which matches Gulf Coast, Deep South, and low-desert light patterns. Intermediate mixes can work on the cooler edges of 9a; pure long-day storage types usually produce thick necks and small bulbs. Plant fall sets into loose, well-drained soil, keep nitrogen modest after midwinter, and harvest when tops fall naturally rather than waiting on a northern storage schedule.

Cold band: Zones 3 through 5b

Short seasons set the rules north of Zone 6. Zone 3b averages a last spring frost around June 1 and only about 99 frost-free days. Zone 4a and 4b sit near late May for last frost (May 29 and May 28) with roughly 107 to 110 frost-free days. Zone 5a last-frost averages land around May 26; Zone 5b around May 21, with about 115 and 126 frost-free days.

Plant sets in April into May across Zones 3 and 4, as soon as the soil can be worked and standing water is gone. In Zone 5, late March into April is realistic in lighter soils. Long-day varieties belong here: they need the long midsummer days of northern latitudes to bulb. Fall outdoor planting without heavy protection is a gamble in this band; overwintered sets often rot or heave. If you want an earlier green-onion harvest, start a few sets in a cold frame in late winter and move them out after hard freezes ease.

Zones 6, 7, and 10

Zone 6a and 6b average last spring frosts around May 17 and May 11, with first fall frosts near September 29 and October 5. Sets go in through March and April. Long-day and intermediate types both appear in this band; match the label to your latitude rather than buying by color alone.

Zone 7a and 7b open earlier. Last spring frosts average around May 6 and April 24, with first fall frosts near October 9 and October 17. March planting is standard, and Zone 7b can trial fall sets in sheltered beds. Intermediate-day onions fit most of Zone 7; short-day types gain ground on the warm edge.

Zone 10a still sees a last spring frost around February 16 and a first fall frost around December 10. Zones 10b and warmer are effectively frost-free. Plant sets in the cool months, December through January for a spring bulb crop, and lean hard on short-day varieties. Heat, not frost, ends the useful season here, so bulbing must finish before late-spring temperatures stall growth.

Day length is the trap, not timing

A correctly timed set still fails if the variety’s photoperiod does not match your latitude. Onions bulb in response to day length once plants reach a minimum size:

• Long-day types need roughly 14 to 16 hours of light and suit Zones 3 through 6.
• Intermediate-day types respond around 12 to 14 hours and cover much of Zones 5 through 7, with some reach into cooler 8a sites.
• Short-day types bulk at about 10 to 12 hours and belong in Zones 7 through 10.

Plant a northern long-day set on a perfect February date in Zone 9b and you will often get lush tops and marble-sized bulbs. Plant a short-day set in Zone 4 in May and the plant may try to bulb before it has enough leaf area to feed a storage onion. Read the day-length class on the bag before you read the color. Tricolor mixes aimed at Zones 3 through 10 usually blend intermediate, day-neutral, and long-day bulbs; they are a practical hedge when one bed serves a household that wants red, white, and yellow without three separate orders.

Spacing, depth, and set size

Plant sets pointed end up, one to two inches deep, so the tip sits at or just below the soil line after watering. Space three to six inches apart in the row, with twelve inches between rows, or run a staggered grid in a wide bed at the same in-row spacing. Closer spacing yields more scallions and smaller bulbs; wider spacing favors storage size.

Set diameter matters. Pieces about half an inch across tend to bulb cleanly. Oversized sets bolt more often, sending up a flower stalk instead of packing rings. If a bag arrives with mixed sizes, plant the largest ones for early green pullings and save the medium bulbs for full sizing. Keep soil evenly moist through establishment, then water deeply and less often as roots take hold. Side-dress lightly once with nitrogen when tops are six to eight inches tall if the bed was not amended at planting; heavy late nitrogen softens storage life.

Weed early. Onions compete poorly with broadleaf weeds, and shallow cultivation is safer than deep hoeing around the bulbs. A thin mulch after the soil warms holds moisture without keeping necks wet into harvest.

Which onion sets to buy

Start with a mixed bag when you want red, white, and yellow onions from one bed. TomorrowSeeds - 3 Colors Mix Onion Sets (Bulbs) - 1/2 Pound (~50 Sets) - Fresh Crop, Stored Cold, for 2026 Planting Yellow, Red, White, Excellent Storage Noon Intermediate Intermediate Day ($12.97) is a tricolor mix labeled intermediate-day, which puts it in the Zones 5 through 7 lane described above.

Northern and high-elevation gardeners who want a single storage onion should stay inside the long-day or intermediate lane rather than grabbing a generic mixed bag aimed at the South. A yellow storage set labeled long-day or intermediate-day belongs in that northern slot, not in a short-day Zone 9 or 10 bed.

Skip any bag that cannot state a day-length class, and skip seed-department bins that have sat warm and dry for weeks. Firm, dormant sets with tight skins root faster than soft or sprouted stock. Plant soon after purchase; long indoor storage at room temperature invites sprouting and rot before the bed is ready.

Match the bag to your zone’s frost dates, plant on the month window above, and let day length finish the job the calendar starts.

---