Best Fertilizer for Onions
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GardenSays
Sidedress now: 0.25-0.5 lb per 25 ft of row, spread ~6 in from the plants and scratched in, then repeat every 28-35 days.
- Now
- 0.25-0.5 lb per 25 ft of row, spread ~6 in from the plants and scratched in
- Repeat
- Every 28-35 days
- Stop
- ~1 month before frost
Fruit set is the trigger: from here on, onions can use extra nitrogen productively. Use a nitrogen sidedress (e.g. urea 45-0-0): 0.25-0.5 lb per 25 ft of row, spread ~6 in from the plants and scratched in. Water it in well.
- Stop feeding nitrogen once bulbs start to swell (about mid-July in the north) — late nitrogen makes soft, large-necked bulbs that are hard to cure and don't store.
Do this this week
- 1.Sidedress each plant: 0.25-0.5 lb per 25 ft of row, spread ~6 in from the plants and scratched in, watered in well
- 2.Repeat every 28-35 days while fruit keeps setting
- 3.Keep watering steady while feeding — fertilizer without moisture stresses roots
Calculated result · 1 verified source · Checked 2026-07-18 · How we decide
Why
Keep watering steady while feeding — fertilizer without consistent moisture concentrates salts and stresses roots.
- Post-set feeding replaces the nutrients each fruit flush removes.
- Wants steady nitrogen early, but you must cut it off at bulbing — late N ruins storage.
When this doesn't apply
- Late season: stop feeding about a month before your first frost so remaining fruit ripens instead of the vine growing.
How this was calculated
- 1. Feeding profile
Onion: Wants steady nitrogen early, but you must cut it off at bulbing — late N ruins storage.
- 2. Sidedressper plant (our estimate)
after roots are well-developed (once or twice), and always BEFORE bulbing starts: a nitrogen sidedress (e.g. urea 45-0-0) — 0.25-0.5 lb per 25 ft of row, spread ~6 in from the plants and scratched in
- 3. Repeat28-35 days
Repeat every 28-35 days while the plant keeps setting fruit
Data sources
- University of Minnesota Extension — Growing onions in home gardens — Side-dress after root systems are well-developed, once or twice, with urea (45-0-0) at 0.25 to 0.5 pounds per 25 feet of row, about six inches from the plants. Too much nitrogen results in late maturity, large necks, soft bulbs and poor storage. (checked 2026-07-18)
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