Companion Planting Chart: 30 Plant Pairings That Actually Work

Pest Control Pairings (1-10)

These pairings use volatile compounds, physical barriers, or biological interactions to reduce pest damage on the target crop. The pest reduction percentages below come from controlled field trials.

1. Tomato + Basil

Mechanism: Basil emits methyl chavicol, linalool, and eugenol, which repel tomato hornworm (Manduca quinquemaculata) and whitefly (Bemisia tabaci). Spacing: Plant 3 to 4 basil plants (Ocimum basilicum 'Genovese') within 12 inches of each tomato plant. Measured effect: Iowa State University trials (2019) found 40 percent fewer aphids and 20 percent higher fruit yield in tomato-basil interplantings compared to monoculture tomatoes. Tip: Pinch basil flower buds to maintain volatile oil production. Flowering basil redirects energy from leaf oil production to seed production, reducing the repellent effect by 50 to 60 percent.

2. Cabbage + Marigold

Mechanism: French marigolds (Tagetes patula) release alpha-terthienyl from their roots, which is toxic to root-knot nematodes (Meloidogyne incognita) in the soil. The above-ground flowers also attract hoverflies (Syrphidae), whose larvae consume cabbage aphids (Brevicoryne brassicae). Spacing: Plant 1 marigold every 18 inches along the cabbage row, alternating with cabbage plants spaced 18 inches apart. Measured effect: University of California, Riverside trials (2018) showed a 65 percent reduction in root-knot nematode galling on cabbage roots when grown with French marigolds compared to cabbage alone. Use 'Nemagone' or 'Single Gold' marigold varieties, which produce the highest alpha-terthienyl concentrations.

3. Carrot + Onion

Mechanism: Onion foliage emits sulfur compounds (diallyl disulfide, diallyl trisulfide) that mask the scent of carrot roots from carrot rust fly (Psila rosae). Carrot foliage releases compounds that repel onion maggot (Delia antiqua). Each plant protects the other from its primary pest. Spacing: Interplant carrots and onions in alternating rows 4 inches apart, or in a checkerboard pattern with each plant 4 inches from its nearest neighbor. Measured effect: Rothamsted Research (UK) trials (2017) found 70 percent fewer carrot rust fly eggs and 55 percent fewer onion maggot larvae in interplanted plots compared to monoculture plots.

4. Cucumber + Radish

Mechanism: Radish plants repel cucumber beetle (Acalymma vittatum) through volatile isothiocyanate compounds released from their foliage. Spacing: Sow radish seeds (Raphanus sativus 'Cherry Belle') 6 inches from the base of each cucumber plant. Allow the radishes to flower (do not harvest them). Flowering radishes produce stronger volatile emissions than vegetative radishes. Measured effect: University of Georgia trials (2020) showed 60 percent fewer cucumber beetle adults on cucumber plants interplanted with flowering radishes compared to cucumber monoculture.

5. Pepper + Basil

Mechanism: Basil's volatile oils repel aphids (Myzus persicae) and spider mites (Tetranychus urticae), two of the three most damaging pepper pests. Spacing: Plant 2 basil plants per pepper plant, positioned 8 to 10 inches from the pepper stem. Measured effect: University of Florida trials (2021) measured 45 percent fewer aphids on pepper plants grown with basil compared to peppers alone. The effect was strongest when basil was planted before peppers were transplanted, allowing the basil to establish its volatile emission profile before peppers were vulnerable.

6. Squash + Nasturtium

Mechanism: Nasturtium (Tropaeolum majus) acts as a trap crop for squash bug (Anasa tristis). Squash bugs prefer nasturtium leaves to squash leaves and concentrate their feeding on the nasturtiums, reducing damage to the squash crop. Spacing: Plant nasturtiums 12 to 18 inches from the squash hill, between hills. Use 1 nasturtium per squash plant. Measured effect: Cornell University trials (2019) found that squash interplanted with nasturtiums sustained 50 percent less squash bug damage than squash grown alone. Check the nasturtiums every 3 days and hand-pick squash bugs from the foliage to prevent the pest population from building to levels that spill over onto the squash.

7. Eggplant + Thyme

Mechanism: Thyme (Thymus vulgaris) emits thymol and carvacrol, which repel Colorado potato beetle (Leptinotarsa decemlineata). Eggplant is a preferred host for this beetle, and the damage can defoliate a plant within 2 weeks. Spacing: Plant 3 to 4 thyme plants around the base of each eggplant, within 6 inches of the stem. Measured effect: University of Maryland trials (2020) showed 35 percent fewer Colorado potato beetle adults and 50 percent fewer egg masses on eggplant grown with thyme compared to eggplant alone.

8. Lettuce + Chives

Mechanism: Chive foliage (Allium schoenoprasum) releases sulfur compounds that repel aphids and Japanese beetles (Popillia japonica), both of which damage lettuce leaves. Spacing: Plant a border of chives around the lettuce bed, spaced 6 inches apart. Measured effect: Ohio State University trials (2018) found 50 percent fewer aphids on lettuce bordered by chives compared to lettuce without chive borders. The chive border must be established before lettuce is planted. Transplant chive divisions 4 weeks before sowing lettuce seed.

9. Broccoli + Sweet Alyssum

Mechanism: Sweet alyssum (Lobularia maritima) produces nectar that attracts hoverflies (Syrphidae) and parasitic wasps (Braconidae and Ichneumonidae). Hoverfly larvae consume cabbage aphids (Brevicoryne brassicae) at a rate of 30 to 50 aphids per larva per day. Parasitic wasps lay eggs in cabbage looper (Trichoplusia ni) and diamondback moth (Plutella xylostella) larvae, killing them before they reach damaging size. Spacing: Sow sweet alyssum as a living mulch between broccoli rows, 6 inches apart. Measured effect: Michigan State University trials (2021) measured 60 percent fewer cabbage aphids and 40 percent less diamondback moth damage on broccoli interplanted with sweet alyssum compared to broccoli alone.

10. Beans + Rosemary

Mechanism: Rosemary (Salvia rosmarinus) emits 1,8-cineole and camphor, which repel Mexican bean beetle (Epilachna varivestis). Spacing: Plant 1 rosemary plant every 4 feet along the bean row. Rosemary is a perennial; plant it once and it protects beans for years. Measured effect: University of Kentucky trials (2019) found 40 percent fewer Mexican bean beetle adults on bush beans (Phaseolus vulgaris 'Blue Lake 274') grown within 3 feet of rosemary compared to beans grown 20 feet from rosemary.

Nitrogen Fixation and Nutrient Pairings (11-18)

Legumes fix atmospheric nitrogen through a symbiotic relationship with Rhizobium bacteria in their root nodules. This fixed nitrogen becomes available to neighboring plants when the legume roots exude nitrogen compounds into the soil or when the legume is terminated and incorporated as green manure.

11. Tomato + Bush Bean

Mechanism: Bush beans fix 20 to 40 pounds of nitrogen per acre during a single growing season. Tomato roots growing within 12 inches of bean roots access a portion of this fixed nitrogen. Spacing: Plant bush beans (Phaseolus vulgaris 'Provider') 12 inches from the base of each tomato plant, 9 beans per square foot in a 3x3 grid. Measured effect: North Carolina State University trials (2020) showed 15 percent higher tomato yield when interplanted with bush beans compared to tomatoes with no legume companion. The effect was equivalent to applying 1 pound of 10-10-10 NPK per 100 square feet.

12. Corn + Pole Bean + Squash (Three Sisters)

Mechanism: Pole beans fix nitrogen for the corn, corn provides a trellis for the beans, and squash shades the soil to suppress weeds and retain moisture. This Iroquois polyculture has been documented in agronomic trials at Cornell University. Spacing: Plant corn in blocks (not rows) with 12 inches between plants. Sow pole beans (Phaseolus vulgaris 'Kentucky Wonder') 6 inches from each corn stalk after the corn reaches 8 inches tall. Plant squash (Cucurbita pepo 'Early Prolific Straightneck') 24 inches from the corn-planting area, 1 hill per 4 corn plants. Measured effect: Cornell trials (2018) showed that the Three Sisters intercrop produced 30 percent more total calories per acre than the same three crops grown in monoculture, with no fertilizer input beyond the nitrogen fixed by the beans.

13. Pepper + Pole Bean

Mechanism: Pole beans fix nitrogen that benefits the heavy-feeding pepper plants. Spacing: Plant pole beans on a trellis 18 inches from the pepper row. Measured effect: University of Illinois trials (2021) measured 12 percent higher pepper yield when grown adjacent to pole beans compared to peppers alone. The effect was strongest in soils with low initial nitrogen (below 30 parts per million).

14. Lettuce + Pea

Mechanism: Peas (Pisum sativum 'Oregon Sugar Pod II') fix 40 to 60 pounds of nitrogen per acre. Lettuce, a leafy crop with high nitrogen demand, benefits from the exuded nitrogen. Spacing: Plant peas on a trellis at the north edge of the lettuce bed. Lettuce grows in the shade of the pea trellis during the warm part of the day, which reduces bolting in temperatures above 75 degrees Fahrenheit. Measured effect: University of New Hampshire trials (2019) showed 20 percent heavier lettuce heads when grown within 18 inches of peas compared to lettuce grown alone.

15. Spinach + Fava Bean

Mechanism: Fava beans (Vicia faba 'Aquadulce') fix 80 to 120 pounds of nitrogen per acre, the highest fixation rate of any annual legume. Spinach is a heavy nitrogen feeder that produces 30 to 40 percent more leaf mass with adequate nitrogen. Spacing: Plant fava beans 18 inches apart in a row, with spinach sown between the bean plants at 4-inch spacing. Measured effect: Washington State University trials (2020) measured 35 percent higher spinach yield when interplanted with fava beans compared to spinach alone. The fava beans also provide a second crop: the young fava bean pods are edible and harvest in 75 to 90 days.

16. Cucumber + Radish (Nitrogen)

Mechanism: Radishes are not legumes, but their deep taproots (12 to 18 inches) break up compacted soil layers, allowing cucumber roots to penetrate deeper and access nutrients and moisture that shallow-rooted cucumbers cannot reach on their own. Spacing: Sow radishes between cucumber hills, 6 inches apart. Measured effect: University of Wisconsin trials (2019) found that cucumbers interplanted with radishes produced 10 percent more fruit than cucumbers alone, attributed to improved root penetration in compacted soils.

17. Tomato + Borage

Mechanism: Borage (Borago officinalis) accumulates trace minerals (calcium, potassium, magnesium) in its leaves. When borage leaves die back and decompose, these minerals become available to nearby tomato roots. Borage also attracts pollinators, increasing tomato fruit set by 15 to 20 percent. Spacing: Plant 1 borage plant per tomato plant, 12 inches from the stem. Measured effect: University of Georgia trials (2021) showed 18 percent higher tomato yield and 25 percent fewer tomato hornworm eggs on tomatoes interplanted with borage compared to tomatoes alone.

18. Strawberry + Bush Bean

Mechanism: Bush beans fix nitrogen that benefits the strawberry plants during fruit production. Strawberries are heavy nitrogen feeders during the fruiting period (May through June). Spacing: Interplant bush beans between strawberry rows, 12 inches from the strawberry crowns. Measured effect: University of Minnesota trials (2020) measured 15 percent higher strawberry yield when interplanted with bush beans compared to strawberries alone. The beans also produce a secondary harvest of 0.5 to 1 pound per plant.

Growth Enhancement Pairings (19-24)

These pairings improve growth through physical protection, microclimate modification, or growth-stimulating root exudates.

19. Tomato + Parsley

Mechanism: Parsley (Petroselinum crispum 'Italian Giant') attracts parasitic wasps (Anagrus spp.) that prey on tomato hornworm eggs. Parsley also has a deep taproot that brings nutrients from subsoil layers to the surface. Spacing: Plant 2 parsley plants per tomato, 10 inches from the stem. Measured effect: Purdue University trials (2019) found 30 percent fewer hornworm larvae on tomatoes interplanted with parsley compared to tomatoes alone.

20. Lettuce + Tall Flowers (Sunflower, Cosmos)

Mechanism: Tall flowers provide afternoon shade that reduces lettuce bolting by 3 to 5 degrees Fahrenheit. Lettuce bolts when temperatures exceed 75 to 80 degrees Fahrenheit for sustained periods. Spacing: Plant sunflowers (Helianthus annuus 'Mammoth Grey Stripe') on the west side of the lettuce bed, 24 inches apart. The shade falls on the lettuce during the hottest part of the afternoon. Measured effect: University of Vermont trials (2020) showed that lettuce grown in the shade of 6-foot sunflowers bolted 10 to 14 days later than lettuce grown in full sun, extending the harvest window by 30 to 40 percent.

21. Radish + Spinach

Mechanism: Radishes germinate in 3 to 5 days and mark the row, making it easier to identify the planting area while slower spinach seeds germinate over 7 to 14 days. The radish roots also break soil crust, improving spinach seedling emergence. Spacing: Sow radish and spinach seeds in the same row, alternating every 2 inches. Measured effect: Oregon State University trials (2019) found 25 percent better spinach stand establishment when inter-sown with radishes compared to spinach sown alone.

22. Cucumber + Sunflower

Mechanism: Sunflowers attract pollinators and serve as a living trellis for lightweight cucumber varieties. Spacing: Plant sunflowers 24 inches apart and train cucumber vines (Cucumis sativus 'Spacemaster') up the sunflower stems using soft twine. Measured effect: Pennsylvania State University trials (2021) showed 20 percent higher cucumber yield when grown on sunflower trellises compared to cucumbers grown on the ground, attributed to improved air circulation and reduced powdery mildew.

23. Kale + Sweet Alyssum

Mechanism: Sweet alyssum attracts syrphid flies that consume cabbage aphids, the primary pest of kale. The alyssum also serves as a living mulch that suppresses weeds between kale plants. Spacing: Sow alyssum between kale plants, 8 inches apart. Measured effect: University of California, Davis trials (2020) measured 55 percent fewer aphids on kale interplanted with sweet alyssum compared to kale alone. The living mulch also reduced weed biomass by 70 percent.

24. Potato + Horseradish

Mechanism: Horseradish (Armoracia rusticana) releases glucosinolate compounds from its roots that repel Colorado potato beetle. The deep root system also improves soil structure for potato tuber development. Spacing: Plant horseradish at the corners of the potato bed, 24 inches from the nearest potato plant. Horseradish is invasive; contain it in a 12-inch-deep root barrier or grow it in large pots sunk into the ground. Measured effect: Cornell University trials (2018) found 30 percent fewer Colorado potato beetle adults on potatoes grown with horseradish compared to potatoes alone.

Pairings to Avoid (25-30)

Some plant combinations actively harm one or both partners through chemical inhibition (allelopathy), competition for the same resources, or disease amplification. The six pairings below are documented in the scientific literature as incompatible.

25. Tomato + Fennel

Problem: Fennel (Foeniculum vulgare) produces fenchone and anethole, allelopathic compounds that inhibit the growth of tomatoes, beans, kohlrabi, and most other garden vegetables. Evidence: University of Rhode Island trials (2019) showed 25 percent lower tomato yield when grown within 4 feet of fennel compared to tomatoes grown 20 feet from fennel. Recommendation: Plant fennel in a dedicated bed at least 20 feet from the vegetable garden. Fennel is an excellent companion for dill (both attract parasitic wasps) but should be isolated from other crops.

26. Bean + Onion

Problem: Onion roots exude sulfur compounds that inhibit the nitrogen-fixing bacteria (Rhizobium) on bean roots. When Rhizobium activity is suppressed, beans fix less nitrogen and produce smaller yields. Evidence: University of Minnesota trials (2018) showed 20 percent lower bean yield when interplanted with onions compared to beans alone. Recommendation: Separate beans and onions by at least 4 feet, or grow them in different beds.

27. Tomato + Walnut

Problem: Black walnut (Juglans nigra) and butternut (Juglans cinerea) produce juglone, an allelopathic compound that is toxic to tomatoes, potatoes, eggplant, peppers, and other solanaceous crops. Juglone is present in all parts of the tree (roots, leaves, bark, nut hulls) and persists in the soil for 2 to 5 years after the tree is removed. Evidence: Tomato plants within the drip line of a black walnut tree show wilting, yellowing, and death within 2 to 4 weeks. Recommendation: Do not plant solanaceous crops within 50 feet of a black walnut tree. If the tree has been removed, test the soil for juglone before planting solanaceous crops. Raised beds with imported soil can bypass juglone contamination in the native soil.

28. Cucumber + Potato

Problem: Cucumbers and potatoes share susceptibility to Phytophthora blight (Phytophthora infestans), the pathogen that caused the Irish Potato Famine. Growing them in proximity increases the inoculum load in the garden and raises the risk of both crops being infected in the same season. Evidence: University of Maine trials (2020) showed that Phytophthora infection spread 40 percent faster in gardens where potatoes and cucumbers were interplanted compared to gardens where they were separated by 50 feet. Recommendation: Separate potatoes and cucumbers by at least 50 feet, or grow them in different years with a 3-year rotation between them.

29. Cabbage + Strawberry

Problem: Both crops are susceptible to verticillium wilt (Verticillium dahliae). Growing them together increases the soil population of this pathogen, reducing the productive lifespan of the strawberry bed. Evidence: University of California, Davis trials (2019) found that strawberry plants grown after cabbage had 30 percent higher verticillium wilt incidence than strawberries grown after beans or corn. Recommendation: Do not follow strawberries with cabbage or vice versa. Separate them by at least 3 years in rotation.

30. Mint + Everything

Problem: Mint (Mentha x piperita) spreads by aggressive rhizomes that can extend 3 to 4 feet from the parent plant in a single season. The rhizomes outcompete neighboring plants for water and nutrients, forming dense mats that smother slower-growing vegetables. Evidence: Mint reduced yield of adjacent pepper plants by 35 percent and basil by 40 percent in University of Connecticut trials (2020) when planted within 12 inches. Recommendation: Grow mint in containers only. A 12-inch pot confines the roots and prevents garden invasion. If you want mint in the garden, sink the pot into the soil with the rim 1 inch above ground level to prevent rhizome escape over the top.

Quick Reference: All 30 Pairings at a Glance