When to plant scallions (green onions) in Boise, Idaho
April 19 – May 3
Direct-sow outdoors · typical Boise yearClimate basis: NOAA station Boise 7 N, ID (USC00101017), 6 mi (9 km) from downtown Boise (ZCTA 83702 centroid), elevation 3885 ft, on the 1991–2020 climate normals. Direct-sow scallions (green onions) from approximately the dates above, anchored to the median last spring 32°F freeze (May 17). One year in ten still freezes after June 10.
The Boise year
- indoors under lights
- in the garden
- frost-free season
- 1-in-10 years still freeze to Jun 10
Every method, with its sources
Direct-sow outdoors
Apr 19 – May 3Colorado State University Extension's CMG GardenNotes #720 groups onions with the Hardy Vegetables that may go in "As early as 2-4 weeks before the date of the average last spring frost," and the same publication's Table 1 Cool Season Vegetable Planting Guide lists "Onions, green" among those cool-season crops, so the 2-to-4-week pre-frost span is used directly; no other extension source located gave a later-starting green-onion spring rule, so nothing needed reconciling. Anchored to the median last spring freeze (May 17). Colorado State University Extension (Colorado Master Gardener)
Fall planting
Aug 17 – Sep 14Purdue Extension HO-186-W Table 2 lists green onion among cool-season plants for late-summer or fall planting to be planted "at least 4-8 weeks before first fall frost," which encodes as a window running from eight weeks down to four weeks ahead of the first fall freeze; it was the only extension source found anchoring a green-onion fall sowing to frost, so it stands unreconciled. Anchored to the median first fall freeze (Oct 12). Purdue University Extension, Department of Horticulture and Landscape Architecture
Variety assumption: Assumes non-bulbing bunching types or true onions pulled young; Colorado State's Table 1 gives 60 typical days to harvest for "Onions, green" versus 110 for dry onions, and this entry encodes only the green-onion crop.
These windows describe typical years (1991–2020 climate normals), not this year's forecast. Freezes happen outside them — especially in frost pockets, valleys, and urban edges. Watch the forecast before committing tender plants.
Crop rule sources (2)
- Colorado State University Extension (Colorado Master Gardener): CMG GardenNotes #720: Vegetable Planting Guide (accessed 2026-07-25)"Hardy Vegetables ... Crops: broccoli, cabbage, kohlrabi, onions, lettuce, peas, radish, spinach, turnips"
- Purdue University Extension, Department of Horticulture and Landscape Architecture: Indiana Vegetable Planting Calendar (HO-186-W) (accessed 2026-07-25)"Table 2. Cool season plants for late-summer or fall planting (plant at least 4-8* weeks before first fall frost) (See Figure 2)"