Companion Planting Calculator

Interactive botanical companion planting matrix and garden bed polyculture audit. Discover mutually beneficial plant partners, avoid allelopathic enemies, and control pests naturally.

Method reviewed by Plant Calculator Team·Updated August 2026

How this is worked out

Companion Match = curated lookup of beneficial partners and antagonistic pairings for the selected crop(s)

Relationships are drawn from established companion-planting practice: nitrogen fixation, aromatic volatile pest confusion, beneficial-insect attraction, canopy/root-zone layering, and documented allelopathic or shared-pest conflicts. The bed checker flags every antagonistic pair among the crops you select.

Assumptions

Companion synergies operate via four biological mechanisms: nitrogen fixation, pest masking aroma, predatory insect attraction, and vertical canopy layering.

Antagonistic pairings involve allelopathic biochemical growth inhibitors (e.g. juglone from walnuts, fennel root exudates, allium-legume nodulation inhibition).

Combining deep taproots (carrots, tomatoes) with shallow fibrous root systems (lettuce, onions) maximizes soil volume extraction without nutrient competition.

Polyculture planting enhances beneficial insect biodiversity and reduces pest damage by up to 60% compared to monocultures.

The science of companion planting: plant synergies, allelopathy & pest defense

Companion planting replaces vulnerable monoculture plantings with resilient, biologically active polycultures. By coordinating structural architecture (tall climbers shading cool greens), root zone layering (deep taproots opening pathways for shallow fibrous feeders), and chemical insect deterrence, companion planting maximizes organic garden yields without synthetic chemical pesticides.

Core Mechanisms of Plant Synergy

1. Atmospheric Nitrogen Fixation (Legumes): Peas, green beans, and clover form symbiotic root nodules with Rhizobium bacteria, converting atmospheric nitrogen (N₂) into bioavailable ammonium (NH₄⁺) that nourishes neighboring heavy-feeding brassicas and corn.
2. Volatile Terpene Insect Masking (Aromatic Herbs): Strongly scented herbs (basil, dill, rosemary, mint, sage) emit airborne terpenes that mask the visual and chemical silhouette of vegetables, preventing pest insects from locating host leaves.
3. Trap Cropping & Dynamic Nutrient Accumulation: Nasturtiums lure destructive black bean aphids and flea beetles away from tender vegetable crops, while deep-rooted comfrey draws subsoil potassium and calcium up into its leaf tissue for surface mulching.

Master Companion Planting Quick Reference Guide

Master Companion Planting Chart for Garden Vegetables
Crop	Beneficial Companions	Incompatible Antagonists	Primary Synergy Benefit
Tomato	Basil, Marigold, Carrot, Borage, Chives	Fennel, Potato, Cabbage, Walnut	Hornworm defense & flavor enhancement
Carrot	Leek, Rosemary, Sage, Lettuce, Radish	Dill, Parsnip, Fennel	Alliums repel carrot rust fly
Cabbage & Kale	Thyme, Mint, Chamomile, Celery, Onions	Tomato, Strawberry, Pole Beans	Aromatic herbs deter cabbage loopers
Cucumber	Corn, Peas, Radish, Sunflowers, Dill	Potato, Sage, Aromatic Mint	Corn stalks provide living climb trellis
Sweet Corn	Beans, Winter Squash, Melon, Cucumber	Tomato (shares corn earworm)	Traditional "Three Sisters" nitrogen synergy
Bush Beans	Corn, Cucumber, Summer Squash, Rosemary	Onions, Garlic, Leeks, Chives	Alliums inhibit legume root nodulation

Three Worked Companion Bed Examples

1. The Classic Mediterranean Bed (Tomato + Sweet Basil + French Marigold)

Tomatoes provide height and partial afternoon sun filtering. Sweet basil emits aromatic volatile linalool that repels thrips and hornworms. French marigolds secrete root alpha-terthienyl that eliminates harmful root-knot nematodes in the soil.

2. The Indigenous Three Sisters Guild (Corn + Pole Beans + Winter Squash)

Tall sweet corn stalks serve as structural climbing poles for vining beans. Pole beans fix atmospheric nitrogen for high-demand corn. Broad, prickly winter squash leaves carpet the soil surface as a living mulch, retaining moisture and deterring raccoons.

3. Brassica Pest Shield Guild (Cabbage + Thyme + Yellow Onions)

Cabbage heads are protected by thyme's intense thymol scent, which disorients cabbage white butterflies and reduces egg laying. Interplanted yellow onions emit sulfur volatiles that repel flea beetles and aphids.

Common Companion Planting Mistakes

Planting Alliums (Garlic/Onion) next to Legumes (Beans/Peas): Sulfur compounds excreted by allium roots actively kill the beneficial Rhizobium nitrogen-fixing bacteria on legume roots, stunting bean production.
Planting Florence Fennel inside vegetable beds: Fennel produces strong allelopathic exudates that stunt tomatoes, beans, brassicas, and peppers. Always grow fennel in isolated perimeter containers.
Grouping crops that share catastrophic pests: Tomatoes and sweet corn both host the destructive Helicoverpa zea (known as the tomato fruitworm or corn earworm). Keep them separated by at least 20 feet.

Plan crop rotations and bed layouts with our Crop Rotation Calculator, Square Foot Planner, and Planting Calendar.

Common questions

What is companion planting in gardening?

Companion planting is the agricultural practice of placing complementary plant species close together to enhance pest control, improve pollination, maximize spatial utilization, provide structural physical support, and stimulate biological nutrient cycling.

What is plant allelopathy?

Allelopathy is a biological phenomenon where a plant releases biochemical exudates (from roots or leaves) into the surrounding soil that inhibit the germination, root growth, or survival of neighboring plant species. For example, fennel and black walnut trees produce strong allelopathic chemicals that stunt most garden vegetables.

Why should tomatoes and potatoes not be planted together?

Tomatoes and potatoes belong to the same Solanaceae (nightshade) family and share the exact same fungal pathogens, specifically Early Blight (Alternaria solani) and Late Blight (Phytophthora infestans). Planting them in proximity allows blight spores to amplify and devastate both crops simultaneously.

How do aromatic herbs protect vegetables from insect pests?

Aromatic herbs like basil, rosemary, dill, and mint release volatile terpenes and essential oils that mask the scent of host vegetable crops, confusing flying insect pests (such as aphids, flea beetles, and tomato hornworms) while attracting beneficial predatory hoverflies and parasitic wasps.

Select Garden Crop or Polyculture Bed