Why is it strongly advised against planting tomatoes and onions in the same place?

There are many tips and advice for a successful vegetable garden, especially with regard to the association of plants and crop rotation. Among these recommendations, we often find that of avoiding planting tomatoes and onions in the same place. What are the reasons for this precaution? This article invites you to discover the main arguments in favor of separating these two popular vegetables.

The problems related to the cohabitation between tomatoes and onions

The main problem with planting tomatoes and onions side by side is that these two plants have different needs in terms of soil, water and space.

Why shouldn't tomatoes and onions be planted in the same place?
Why shouldn’t tomatoes and onions be planted in the same place?

Differences in water needs

First of all, the tomatoes require a lot of water, especially during fruit formation. They therefore appreciate regular and abundant watering. Onions, on the other hand, need less water and fairly well-drained soil. Too much humidity can promote the development of fungal diseases in onions.

Space and light requirements

In addition, tomatoes form quite bulky plants and can create shade on neighboring crops. However, onions need a lot of light to grow properly. Proximity to tomatoes can therefore harm the growth of onions.

The problems of competition for nutrients

Finally, tomatoes and onions have different nutrient needs. Tomatoes are high in nitrogen, potassium and phosphorus, while onions mostly need potassium and phosphorus. There may therefore be competition between these two plants for the absorption of nutrients present in the soil.

tomato plant
tomato plant

The risk of disease transmission

Another argument in favor of separating tomato and onion crops concerns the risk of disease transmission between the two plants.

fungal diseases

One of the main fungal diseases affecting onions is onion fly. This fly lays its eggs at the foot of onion plants, and the larvae then feed on the bulbs. However, it has been observed that this fly is also attracted to tomato plants. Planting tomatoes close to onions therefore increases the risk of spreading this disease.

Bacterial diseases

Likewise, some bacterial diseases can be transmitted between tomatoes and onions. This is particularly the case of bacterial canker, which causes plants to wilt and die. By separating these two vegetables, the risk of cross-contamination is limited.

The benefits of crop rotation and companion planting

Beyond the specific problems linked to the tomato-onion association, it is generally recommended to practice the crop rotation and the companionship in the vegetable garden to promote plant health and productivity.

onion plant
onion plant

Crop rotation

Crop rotation consists of not replanting the same vegetables in the same place several years in a row. This practice has several advantages:

  • Reduce disease transmission : by avoiding installing plants sensitive to the same pathogens side by side, the risk of spreading diseases is reduced.
  • Limit the proliferation of parasites : regularly changing the location of crops disrupts the life cycle of pests that feed on plants.
  • Maintain soil fertility : by alternating the types of vegetables, we avoid always drawing the same nutrients from the soil, which helps preserve its richness and structure.


Companionship refers to the association of plants that present beneficial synergies between them. This practice allows in particular:

  • Improve plant health : certain associations promote resistance to diseases and parasites.
  • Optimize the use of space : by associating plants with complementary needs, we avoid the waste of space and resources.
  • Promote pollination : by mixing the species, we attract more pollinating insects, which improves the production of fruits and vegetables.

To sum up, it is best to avoid planting tomatoes and onions in the same place because of the differences in the needs of these two vegetables and the risk of disease transmission. It is also recommended to practice crop rotation and companion planting to promote the health and productivity of the vegetable garden. For example, combine carrots with onions is a great idea, as they have complementary needs and the onions keep insect pests away from the carrots.

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About the author, Kate Williams
I always dreamed of becoming a journalist but life wanted it otherwise. As soon as I have some time to myself, I share here my discoveries and information that I find interesting.
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