Leeks are one of those crops that reward a little planning before a single seed goes in. Leeks grow as tall, mild-flavoured stems with flat blue-green leaves, and they stand in the ground for months. They are hardy and can be started indoors or sown directly, giving a long harvest window from late summer into winter. The companions you choose decide whether that bed works hard all season or sits half empty while one crop slowly does its thing.
Our crop data lists four companions for leeks — marigolds, basil, onions and lettuce — and each earns its place for a different reason. Leeks are pulled as needed once the stems are thick, and they hold well in the ground in mild weather. This guide covers the four companions, the plants to avoid, and a layout for full sun, soil pH 6.0–7.0 and 90–120 days to maturity.
Best companion plants for leeks
| Companion | Why it is grown with leeks |
|---|---|
| Marigolds | Gardeners tuck marigolds between allium rows. The strong scent is traditionally said to confuse pests such as aphids that hunt by smell, and the flowers attract hoverflies, which eat aphids. |
| Basil | Basil goes in after the last frost at the sunny edge of an allium bed. Its scent is traditionally said to confuse flying pests, and its flowers attract pollinators all summer. |
| Onions | Onions are listed as a companion for this crop in our crop data, even though they share the same family — treat it as a within-season interplant, not a rotation. Their strong smell is traditionally said to confuse pests such as aphids. |
| Lettuce | Lettuce fills the gaps between slim allium stems and is cut before the main crop needs the room. It shades the soil surface and keeps it moist. |
None of the four fights leeks for the same layer of the bed: marigolds and basil work the sunny edges, onions use the shallow soil between plants, and lettuce fills the gaps and is gone before the main crop needs the room.
If you only have room for one companion, plant marigolds: they are the most forgiving of the four and flower for months. If you have room for two, add lettuce — it is ready in weeks and turns empty gaps into food while you wait for the main crop.
How companion planting works
Companion planting is not magic. Some pairings help a little, in specific ways, and the best gardeners use them as one tool among many — alongside rotation, spacing, watering and keeping the bed clean.
The first principle is pest confusion. Many pests find their host plant by smell, and a strongly scented neighbour such as marigolds, basil or onions is traditionally said to make the crop harder to find. That is a traditional claim, not a proven guarantee: it may reduce damage, it will not eliminate it.
The second principle is pollinator attraction. Flowers such as marigolds and basil, once they bloom, bring bees and hoverflies into the bed — and hoverfly larvae eat aphids. Let a little basil flower rather than pinching every stem.
The third principle is nitrogen fixing by legumes. Peas and beans work with soil bacteria to fix nitrogen from the air. None of the four companions listed for leeks is a legume, so do not expect a nitrogen boost from this pairing.
The fourth principle is ground cover. A low, fast crop such as lettuce shades bare soil, holds moisture in, and suppresses weeds — which matters for a crop with moderate water needs growing in full sun. A living mulch does the same job as straw, and you can eat it.
Plants to avoid near leeks
The crops to keep away from leeks are its own relatives. Leeks belong to the Amaryllidaceae family, and plants in the same family tend to attract the same pests and suffer the same diseases — in this case aphids, slugs, cutworms. Planting relatives side by side gives those pests a continuous buffet and leaves the same nutrients depleted in one patch of soil. So do not plant leeks next to Garlic, Shallots, Scallions, Red Onions, White Onions, Yellow Onions, Chives, Daffodils, and do not follow leeks with any of them in the same bed next year. Rotate the whole family to a fresh bed instead.
One honest complication: Onions also belong to the Amaryllidaceae family, yet it is listed as a companion for leeks in our crop data. Take the companion listing as the guide for a single season's interplanting, and the family rule as the guide for rotation: grow them together if you wish, but do not plant leeks where any of its relatives grew last year.
Planting layout and spacing
| Growing factor | What leeks need |
|---|---|
| Sun | full sun |
| Soil pH | 6.0–7.0 |
| Days to germination | 7–14 |
| Days to maturity | 90–120 |
| Spacing | 6–8 inches |
| Water needs | moderate |
| Cold tolerance | hardy |
| Sowing method | both |
| Family | Amaryllidaceae |
Start with the spacing: leeks need 6–8 inches between plants, and that figure is for the mature plant, not the seedling. Mark the rows out before you sow, and use our plant-spacing calculator or the square-foot garden planner to work out how many plants your bed holds. Crowding is the commonest mistake in a companion bed.
Put the companions where they belong: marigolds at the ends of rows and along the sunny edge, basil at the sunniest edge after the last frost, onions tucked between plants, and lettuce filling the gaps in succession. Nothing should be planted so close that it shades leeks or blocks airflow — damp, still air is where disease starts.
Aim for a soil pH of 6.0–7.0, with plenty of well-rotted compost worked in before planting. Water needs are moderate, so keep the soil evenly moist and mulch to hold that moisture in. The watering calculator gives a starting point for your bed size.
Common problems and fixes
| Problem | What to do |
|---|---|
| Aphids | Spray off with a strong jet of water, encourage ladybirds and hoverflies, and remove badly curled leaves. |
| Slugs | Water in the morning, clear debris where slugs hide, and hand-pick at dusk or set simple beer traps. |
| Cutworms | Fit cardboard collars around young stems, keep the bed weed-free, and check at the soil line each morning. |
| Thin, weak stems | Give each plant its full spacing, feed lightly, and keep weeds down so the crop gets the light. |
| Rotting in wet soil | Plant in well-drained ground and avoid overwatering once the crop is established. |
Aphids
Aphids are listed among the common pests of leeks in our crop data, so check for them weekly rather than waiting for visible damage. Spray off with a strong jet of water, encourage ladybirds and hoverflies, and remove badly curled leaves.
Slugs
Slugs are the second pest to watch for on leeks. Water in the morning, clear debris where slugs hide, and hand-pick at dusk or set simple beer traps.
Cutworms
Cutworms are another problem that shows up on leeks, often a sign of the growing conditions rather than bad luck. Fit cardboard collars around young stems, keep the bed weed-free, and check at the soil line each morning. If it keeps returning in the same bed, rotate leeks to a fresh bed next year. Our pest-identifier helper can help confirm what you are looking at.
Frequently asked questions
What is the best companion plant for leeks?
Marigolds are the safest first choice: they suit every bed and flower all season. After them, pick from the table above — lettuce for quick gaps, onions for narrow edges, basil for a sunny border.
Can I plant leeks with lettuce?
Yes. Lettuce is listed as a companion for leeks in our crop data, and it is one of the most useful gap-fillers in the garden. Sow it between the plants, cut it young, and it will be finished before the main crop needs the space.
What should I not plant near leeks?
Avoid its own family. They share the Amaryllidaceae family, so they share the same pests (aphids, slugs, cutworms) and the same soil demands. Keep the family apart in the same season and rotate it to a different bed next year.
How far apart should I plant leeks and its companions?
Give leeks its full spacing of 6–8 inches — crowding is the commonest cause of small harvests. Companions go in the gaps, at the edges or between rows, but never so close that they shade or crowd the main crop. Our plant-spacing calculator works out how many plants fit your bed.
When should I plant the companions?
Sow or set out the companions at the same time as the main crop, or just after. Marigolds and basil are tender, so they go in after the last frost, even if the main crop went in earlier. Check dates with our frost-date calculator.