Best Fertilizer for Leeks
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GardenSays
Sidedress now: about 1/4 lb of 21-0-0 per 100 sq ft, then repeat every 40-50 days.
- Now
- about 1/4 lb of 21-0-0 per 100 sq ft
- Repeat
- Every 40-50 days
- Stop
- ~1 month before frost
Fruit set is the trigger: from here on, leeks can use extra nitrogen productively. Use a nitrogen sidedress (e.g. 21-0-0): about 1/4 lb of 21-0-0 per 100 sq ft. Water it in well.
Do this this week
- 1.Sidedress each plant: about 1/4 lb of 21-0-0 per 100 sq ft, watered in well
- 2.Repeat every 40-50 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.
- Benefits from steady nitrogen all season for thick, long shafts.
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
Leek: Benefits from steady nitrogen all season for thick, long shafts.
- 2. Sidedressper plant (our estimate)
mid-spring once established, then again ~6 weeks later: a nitrogen sidedress (e.g. 21-0-0) — about 1/4 lb of 21-0-0 per 100 sq ft
- 3. Repeat40-50 days
Repeat every 40-50 days while the plant keeps setting fruit
Data sources
- University of Minnesota Extension — Growing leeks in home gardens — A half-strength 20-20-20 or 10-10-10 transplant solution gets plants started; leeks need additional nitrogen, with side dressing of 1/4 lb (21-0-0) per 100 square feet in mid-May and again in late June. (checked 2026-07-18)
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