Companion Planting Reference.
Companion Planting Reference
What plants help each other grow, and which to keep apart. Outdoor garden + small farm reference.
The "Three Sisters" — classic companion planting
The original North American companion planting tradition: corn, beans, and squash grown together.
- Corn provides the vertical structure for beans to climb
- Beans fix nitrogen that corn (a heavy feeder) needs
- Squash covers the ground with broad leaves, suppressing weeds and shading the soil to conserve moisture
This three-way relationship has been used in indigenous agriculture for thousands of years. It's a model of how companion planting can work mechanically (structure + nitrogen + ground cover) rather than as folklore.
Mechanisms of companion planting
| Mechanism | How it works | Examples |
|---|---|---|
| Pest deterrence | Aromatic plants confuse or repel pest insects | Basil → repels aphids, whiteflies, hornworms; planted near tomatoes |
| Beneficial insect attraction | Flowers attract predators / pollinators | Yarrow, dill, cilantro flowers attract ladybugs, lacewings, hoverflies |
| Nitrogen fixation | Legumes deposit N for next crop or neighbors | Beans, peas, clover near heavy feeders |
| Trap cropping | "Sacrificial" plants attract pests away from main crop | Nasturtiums for aphids; mustard for harlequin bugs |
| Allelopathy (negative) | Plants release chemicals that inhibit others | Black walnut, fennel, brassicas inhibit nearby crops |
| Allelopathy (positive) | Plants release beneficial root exudates | Mustard cover crops fumigate soil-borne pathogens |
| Spatial efficiency | Different root depths or canopy shapes coexist | Carrots (deep) + onions (shallow); lettuce under tomato |
| Microclimate | Tall plants shade or shelter sensitive ones | Corn shading lettuce in late spring |
A note on "lore vs evidence"
Companion planting traditions are mixed quality. Some recommendations have strong agronomic evidence (Three Sisters; brassica-allelopathy; legume nitrogen fixation; aromatic herb pest deterrence). Others are folklore with less validation.
The reference tables below combine well-validated relationships (drawn from peer-reviewed agronomy and university extension publications) with established traditional practices. For research-grade decisions, prioritize tested relationships; for hobby gardens, the worst case is often "no harm done."
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