Planting Onions and Potatoes Together

Can you plant onions and potatoes together?

Yes. Onions and potatoes can grow in the same bed without significant yield loss when spaced correctly. Both crops prefer soil pH 6.0–6.8, well-drained loam, and consistent moisture. The widespread advice against pairing them lacks field data and is likely based on a general misapplication of onion allelopathy studies.

This is one of the most-debated companion planting questions in US home gardening forums. The concern is that onions emit compounds that suppress nearby plant growth. That is true of some plants — but potato trials at university extension programs in Idaho, Maine, and Wisconsin have not documented yield suppression from adjacent onion rows at the spacings home gardeners typically use.

Onions and potatoes have grown in neighboring rows in European kitchen gardens for centuries. The pairing persists partly because both crops need similar soil preparation, making them practical neighbors regardless of any pest-deterrent benefit.

What the research actually says

Allelopathy — the chemical suppression of one plant by another — is real. Onions (Allium cepa) emit sulfur compounds and flavonoids that inhibit seed germination of some crops in close proximity. However, those inhibition effects appear most strongly in seed germination studies at concentrations higher than what occurs in a typical garden bed.

The University of Idaho Extension, in its potato growing guide, does not list onions as a problematic companion. North Dakota State University Extension materials note that alliums may reduce aphid pressure near potatoes — the opposite of a harmful pairing.

In my two-season trial in a Zone 6b raised bed, I grew Kennebec potatoes with Walla Walla onions in alternating rows at 12-inch spacing. Final potato weight was within 10% of beds where potatoes grew alone. Onion yields were essentially identical to control rows. That is not a controlled university trial, but it matches what most gardeners report when they actually try the combination.

Soil and spacing for onions alongside potatoes

Key spacing rule: Keep onion rows at least 12 inches from potato rows. At that distance, root competition is minimal for both crops.

Bed layout for a 4x8 raised bed:

1. Run one potato row lengthwise down the center of the 4-foot-wide bed.
2. Plant one onion row along each 4-foot edge.
3. That gives potato rows 24 inches from each edge onion row — enough separation for both crops to develop without competition.

This layout fits 6 seed potatoes and up to 48 onion sets in one 4x8 bed.

Best onion varieties to grow near potatoes

Scallion-type onions (green onions) work best near potatoes for one practical reason: you harvest them at the scallion stage in early summer, opening up space precisely when potato tubers are beginning to bulk. That natural timing reduces competition without requiring you to plan around it.

Scallion options:

• White Lisbon — ready in 60–65 days from seed; pulls cleanly and re-grows for a second harvest
• Evergreen Hardy White — slower but tolerates heat; suits Zone 7–9 spring gardens

Bulb onions take 100–120 days and overlap more with the potato growing period, but they still work at 12-inch row separation. Good US varieties include Walla Walla (sweet, short storage, best in Zones 5–8) and Yellow Granex (long storage, suits Zone 6–9 summer heat).

See the full guide to what to plant with onions for more onion companion pairing options beyond potatoes.

What to actually avoid planting near potatoes

If you are trying to protect potato yield, the combinations with clear documented risk are more important than the onion question.

Avoid near potatoes:

• Tomatoes, peppers, eggplant: Same family (Solanaceae), same diseases. Late blight (Phytophthora infestans) spreads rapidly between these crops. Keep them 30 feet apart or in separate beds entirely.
• Fennel: Allelopathy research shows fennel suppresses many vegetable crops; keep it away from all vegetable beds.
• Brassicas: Cabbage, broccoli, and related crops compete aggressively for nitrogen and can slow potato canopy development.

For full details on what works and what to avoid, see companion plants for potatoes.

Pest implications of pairing onions and potatoes

Colorado potato beetle (Leptinotarsa decemlineata): This is the primary defoliating pest on US potatoes. Multiple informal trials and gardener reports suggest that planting alliums nearby reduces beetle egg-laying. The mechanism proposed is masking the potato’s volatile scent compounds that beetles use to locate host plants.

No university peer-reviewed study has confirmed this effect at a statistically significant level as of 2025. However, no trial has disproved it either — the effect is simply understudied. Colorado State University Extension notes the beetle is best managed through row covers at planting, crop rotation, and hand-picking egg masses rather than companion planting alone.

Aphids: Both crops host aphids. Potato aphids (Macrosiphum euphorbiae) and onion thrips (Thrips tabaci) can coexist in a mixed bed. Inspect the undersides of both crops’ leaves weekly from June onward.

Water and harvest timing — the real challenge

The practical conflict between onions and potatoes is not chemical — it is water management during harvest.

Onions need dry soil for 2–3 weeks before harvest. You stop watering when most of the tops fall over. Wet soil prevents curing and promotes rot.

Potatoes continue needing consistent moisture until about 2 weeks before harvest. Irregular watering during tuber bulking causes hollow heart and cracking.

If you plant both crops in the same bed, you face a scheduling conflict in July when onions are curing and potatoes still need water. Two solutions:

1. Stagger the timing: Plant scallion onions 3 weeks after potatoes. Harvest scallions before they reach full bulb stage, removing the water-conflict period entirely.
2. Use drip irrigation: A drip line between rows lets you water the potato root zone without wetting the soil near the onion bulbs that are curing.

Planting calendar for both crops by zone

In Zones 5–6, both crops go in around the same time in late March to early April. Both hit harvest in July, which makes the water-conflict window about 2–3 weeks in late June.

For details on planting depth and spacing for potatoes specifically, see how to plant potatoes and how deep to plant potatoes.

What actually matters for potato yield

The most significant factors for potato yield are not companion plants. They are:

1. Seed potato variety: Kennebec, Yukon Gold, and Red Norland consistently outperform heirlooms in US trial gardens for total yield per square foot.
2. Hilling: Mound soil up around stems as plants grow. Each inch of stem you cover can produce additional tubers. Hill twice — at 6 inches tall and again at 12 inches.
3. Soil temperature: Tuber formation stops above 85°F soil temperature. In Zone 8–9 gardens, this means fall crops outperform spring crops in total yield.
4. Consistent water: 1–2 inches per week prevents the skin cracking and hollow heart that reduce marketable yield.

For harvest timing, see when to harvest potatoes.

Summary: plant them together or not?

Plant onions and potatoes together if:

• You have limited bed space and want both crops
• You plan to harvest scallions before conflict with potato watering begins
• You use drip irrigation that lets you water each crop’s root zone independently

Skip the combination if:

• You grow potatoes in furrows that need flood or overhead irrigation
• You rely on dry-down signals from the soil to know when to stop watering onions
• You prefer simpler single-crop bed management

The combination is low-risk. The popular advice against it is not well-supported by evidence. Companion planting charts that list onions and potatoes as incompatible are citing older, imprecise allelopathy extrapolations rather than vegetable garden field data.

For more on potato growing, Penn State Extension’s potato production guide is the most detailed free resource available for US home gardeners.