Can Onions Be Planted with Tomatoes? Yes – Here's How

By Robert Taylor · August 11, 2025

Yes, onions can be planted with tomatoes. This companion planting combination is beneficial for pest control, space efficiency, and improved growth. Onions repel many common tomato pests, while tomatoes may help deter onion pests, creating a mutually supportive garden environment.

Why Onions and Tomatoes Make Good Companion Plants

Companion planting involves growing different crops in proximity to enhance growth, deter pests, and maximize garden space. Onions and tomatoes are excellent companions due to their complementary characteristics.

Pest Deterrence Benefits

Onions emit a strong scent that repels aphids, spider mites, and cabbage loopers—common pests of tomatoes.
Tomatoes produce solanine, which may discourage onion flies and other soil-dwelling pests.
The aromatic nature of onions masks the scent of tomatoes, reducing attraction to hornworms.

Space and Resource Efficiency

Onions have shallow root systems, while tomatoes develop deep taproots. This minimizes root competition and allows both plants to access water and nutrients from different soil layers.

Disease Prevention

Intercropping onions with tomatoes improves air circulation and reduces humidity around plants, lowering the risk of fungal diseases such as early blight and powdery mildew.

Best Practices for Planting Onions with Tomatoes

To maximize the benefits of this pairing, follow these expert-recommended practices:

Proper Spacing and Layout

Plant onion sets or seedlings 6–8 inches away from tomato stems.
Use a staggered row pattern or interplant in raised beds for optimal airflow.
Avoid overcrowding to prevent shading and moisture retention.

Soil and Sunlight Requirements

Plant	pH Range	Sunlight (hrs/day)	Water Needs (inches/week)
Tomatoes	6.0–6.8	6–8	1–2
Onions	6.0–7.0	6–8	1

This compatibility in environmental needs makes them ideal partners in the same growing area.

Fertilization Tips

Tomatoes require higher nitrogen early on but benefit from increased phosphorus and potassium during fruiting. Onions need consistent nitrogen for bulb development. Use a balanced organic fertilizer initially, then supplement tomatoes with potassium-rich amendments like composted banana peels or kelp meal.

Growing Conditions Comparison Table

Factor	Tomatoes	Onions	Compatibility Rating
Mature Height	3–8 ft	12–18 in	High
Root Depth	12–24 in	3–6 in	High
Drought Tolerance	Low	Medium	Medium
Harvest Time (Days)	60–100	90–120	Medium
Preferred Mulch	Straw or Black Plastic	Straw	High

Table data source:1, 2

The data shows high compatibility in root depth and mulching preferences, minimizing resource competition. While harvest times differ slightly, overlapping growing seasons allow for shared care routines. The height difference ensures onions won’t shade tomatoes, preserving sunlight access.

Common Challenges and Solutions

Despite their compatibility, some challenges may arise:

Overwatering risk: Tomatoes need more water than onions; ensure good drainage to prevent onion rot.
Nutrient imbalance: Monitor nitrogen levels—too much can reduce tomato fruiting.
Crop rotation: Avoid planting either in the same spot year after year to prevent disease buildup.

Frequently Asked Questions About Planting Onions with Tomatoes

Can I plant onions next to tomato seedlings?

Yes, you can plant onions near tomato seedlings once both have been hardened off and outdoor conditions are stable. Maintain at least 6 inches of spacing to prevent early competition for light and nutrients.

Do onions keep bugs away from tomatoes?

Yes, onions help repel several tomato pests including aphids, whiteflies, and spider mites due to their strong sulfur-containing compounds. Their scent disrupts pest navigation and host detection.

What should not be planted with onions?

Avoid planting onions near peas, beans, and sage. Legumes may have stunted growth when grown near alliums, and sage can inhibit onion development. Additionally, heavy feeders like squash may outcompete onions for nutrients.

Can I grow green onions with tomatoes?

Yes, green onions (scallions) are even better companions than bulb onions because they mature quickly, occupy less space, and provide continuous pest-repelling benefits without long-term soil occupancy.

How far apart should onions and tomatoes be planted?

Space onions 6 to 8 inches away from the base of tomato plants. This distance prevents root competition while allowing onions to exert their pest-deterrent effects effectively.

In conclusion, planting onions with tomatoes is not only possible but highly advantageous. With proper spacing, soil management, and attention to watering needs, this pairing supports healthier plants and higher yields. Incorporating this companion planting strategy enhances garden biodiversity and reduces reliance on chemical pesticides.