Companion Planting: What Works, What Doesn't, and the Evidence

The Three Sisters

The most important companion planting system for survival gardens. Developed and practiced by indigenous peoples of North America for centuries. Each crop benefits the others.

Planting the Three Sisters:

1. After last frost, form flat-topped mounds approximately 18 inches wide, 4 feet apart.
2. Plant corn in a circle around the top — 4-6 seeds per mound, 6 inches apart.
3. When corn is 6-8 inches tall, plant 4-6 beans around each corn stalk.
4. When beans sprout, plant 3-4 squash around the base of the mound.

Why it works:

• Corn provides structure for pole beans
• Beans fix nitrogen, reducing soil depletion
• Squash leaves shade the ground, reducing moisture loss and weed germination
• Together: much less weeding, no fertilizer needed, and maximized production per square foot

Nutritional value of the combination: Corn lacks the amino acid lysine (found in beans). Beans lack the amino acid methionine (found in corn). Together, corn and beans provide all essential amino acids — this is why bean-and-corn combinations appear in nearly every traditional agricultural culture globally.

Companion Planting Reference Table

Beneficial Pairings (Evidence-Based or Well-Supported Traditionally)

| Plant | Good Companions | Reason | |-------|----------------|--------| | Tomatoes | Basil, carrots, parsley | Basil may repel aphids and thrips; limited research but widely practiced | | Tomatoes | Marigolds (French, Tagetes patula) | Marigolds deter whitefly and reduce nematode populations in soil | | Brassicas (cabbage, broccoli, kale) | Dill, celery, nasturtiums | Nasturtiums attract aphids away from brassicas (trap crop) | | Brassicas | Marigolds | Reduces cabbage moth activity | | Cucumbers | Radishes | Radishes may deter cucumber beetles; short-season so don't compete long | | Corn | Beans, squash | Three Sisters — see above | | Carrots | Onions, leeks | Carrot fly is confused by allium smell; some research support | | Beans (bush) | Carrots, celery, summer savory | Summer savory traditionally believed to deter bean beetles | | Roses | Garlic | Garlic may reduce black spot and aphid pressure on roses | | Squash | Nasturtiums | Nasturtiums trap aphids; squash aphids prefer nasturtiums |

Antagonistic Pairings (Evidence-Based Incompatibility)

| Plant | Bad Companion | Reason | |-------|--------------|--------| | Any vegetable | Fennel | Fennel produces allelochemicals that inhibit most vegetables; grow isolated or not at all | | Tomatoes, peppers | Black walnut trees (within drip line) | Juglone from roots and fallen leaves is toxic to nightshades | | Beans, peas | Onions, garlic | Alliums inhibit legume growth and nitrogen fixation | | Brassicas | Strawberries | Both heavy feeders; competition reduces both | | Potatoes | Tomatoes | Both nightshades — share blight (Phytophthora infestans). Keeping apart reduces spread. |

Neutral (Common Claims Without Strong Evidence)

| Claim | Assessment | |-------|-----------| | Marigolds deter all insects | Evidence supports French marigolds (T. patula) for whitefly and nematodes. African marigolds (T. erecta) less effective. | | Basil improves tomato flavor | No credible research supports flavor improvement in adjacent crops. | | Chamomile improves everything near it | Insufficient evidence for specific companion claims. Chamomile is a useful garden plant but claims are overstated. | | Lavender deters deer | Some supporting evidence but highly variable depending on deer population pressure. |

Practical Application

If pressed for time: Plant French marigolds (Tagetes patula, not African Tagetes erecta) throughout the garden. They have the most evidence-backed pest deterrence of any common companion plant.

If limited space: Use the Three Sisters to maximize output and reduce inputs on your most important calorie crops.

If dealing with nematodes: French marigolds planted thickly for a full season, then tilled in, significantly reduce soil nematode populations. This is well-documented.

What to avoid more than what to seek: Keeping fennel separate, understanding walnut allelopathy, and not planting beans with alliums will prevent real production problems.

Trap Cropping

A specific companion planting strategy worth knowing: deliberately planting an attractive crop to draw pests away from the primary crop.

Nasturtiums attract aphids. If planted near brassicas or beans, aphids colonize the nasturtiums preferentially. Pull and discard infested nasturtium plants to remove large aphid populations.

Blue Hubbard squash is extremely attractive to squash vine borers and cucumber beetles. Planted at the garden perimeter, it can draw these pests away from other squash varieties. The trade-off: you may lose the Hubbard plants.

Radishes bolting to flower attract flea beetles away from brassica seedlings. Plant a sacrificial row of radishes near brassicas at transplant time.