Tradition
Good
avoidgood
1 chart entry: 1 recommend, 0 advise against.
0 cited, 1 uncited.
Onion
Allium cepa
Amaryllidaceae
Leek
Allium ampeloprasum var. porrum
AmaryllidaceaeCan you plant onions and leeks together?
Charts say yes. Nobody has tested onions with leeks, as far as we could find. Both are in the Amaryllidaceae family, so they share pests and diseases.
Why we can't give a firm answer
Garden charts recommend it. 1 garden chart entry recommends growing them together. It cites no source. Charts pass on gardening tradition; they are not tests.
List of companion plants (uncited entry) (Wikipedia)Same family, shared pests. Leeks and onions are both in the Amaryllidaceae family, so the same pests and diseases can move from one to the other.
Crop Rotation in the Vegetable Garden (Steil · 2025 · Iowa State University Extension and Outreach, Yard and Garden)
What to plant with onions and leeks
Good neighbours of both, best tested first.
What to keep away from onions and leeks
PeaWith onions: Garden charts advise against it, not tested- AsparagusWith onions: Garden charts advise against it, not tested
Swiss chardWith leeks: Garden charts advise against it, not tested
The evidence in detail
For readers who want to dig in: every chart entry, study and yield trial behind the answer above, with the numbers.
Tradition, research and yield trials
Same family, shared pests and diseases. Both in Amaryllidaceae: often susceptible to the same diseases and insects. Source: Iowa State University Extension and Outreach, Yard and Garden
Research
No papers found on this pair.
Yield trial
No intercropping yield trial.
Untested folklore
What happens above and below ground
Each animation illustrates a mechanism a study reported for this pair. Pick a card to focus on it.
Everything at once.
Companion plants for both onions and leeks
Plants that are good neighbours of each. Biggest harvest gain first; each cell links to the evidence for that pair.
| Plant | With onions | With leeks |
|---|---|---|
 Cabbage | Fewer pests and draws in pest-eating insects8% more harvest from the same spaceField trial, 8 studies. Charts: recommend.Evidence for onions and cabbage | 16% more harvest from the same spaceField trial, 1 study. Charts: not listed.Evidence for leeks and cabbage |
 Tomato | Better soil and nutrient uptake and bigger combined harvestMixed results on soil and nutrientsField trial, 7 studies. Charts: not listed.Evidence for onions and tomatoes | Less disease and better soil and nutrient uptakeGreenhouse or pot trial, 2 studies. Charts: recommend.Evidence for leeks and tomatoes |
Harvest from the same space compares a bed with both crops against the same area split into single-crop beds, counting the harvest of both crops (the land equivalent ratio in yield trials).
The evidence, with sources
- GoodLeek helps OnionLeek: helps onionsList of companion plants (uncited entry) (Wikipedia)
- Both in Amaryllidaceae: often susceptible to the same diseases and insectsCrop Rotation in the Vegetable Garden (Steil · 2025 · Iowa State University Extension and Outreach, Yard and Garden)
All neighbours of onion and of leek, or every pair at once in the companion planting chart.
Have you grown onions and leeks together?
Tell other gardeners what happened: spacing, pests, harvest, or ask a question. Reader notes are not checked by us and do not change the verdicts on this page.