Companion planting chart
Which plants help each other, and which to keep apart, with the reason and how strong the evidence is. Grade A–B pairings come from research or extension services; grade D is traditional lore, worth trying but not proven.
| Plant | With | Effect | Why | Evidence |
|---|---|---|---|---|
| Black walnut | Tomato | avoid | Black walnut releases juglone from roots, leaves and hulls, which is documented to injure or kill this sensitive species. | B |
| Black walnut | Cherry tomato | avoid | Black walnut releases juglone from roots, leaves and hulls, which is documented to injure or kill this sensitive species. | B |
| Black walnut | Potato | avoid | Black walnut releases juglone from roots, leaves and hulls, which is documented to injure or kill this sensitive species. | B |
| Black walnut | Bell pepper | avoid | Black walnut releases juglone from roots, leaves and hulls, which is documented to injure or kill this sensitive species. | B |
| Black walnut | Blueberry | avoid | Black walnut releases juglone from roots, leaves and hulls, which is documented to injure or kill this sensitive species. | B |
| Black walnut | Rhododendron | avoid | Black walnut releases juglone from roots, leaves and hulls, which is documented to injure or kill this sensitive species. | B |
| Black walnut | Dwarf apple tree | avoid | Black walnut releases juglone from roots, leaves and hulls, which is documented to injure or kill this sensitive species. | B |
| Tomato | Potato | caution | Both are hosts of late blight (Phytophthora infestans), which can spread between them. | B |
| Cucumber | Zucchini (courgette) | caution | Cucurbits share powdery mildew, so disease can move between neighbouring plants. | C |
| French marigold | Tomato | good | Evidence is mixed: French marigold may suppress some root-knot nematodes when grown as a cover crop, not as an interplant. | D |
| Florence fennel | the Solanaceae family | caution | Fennel releases growth-inhibiting compounds from roots and foliage; lab and garden reports show poorer growth of tomato, pepper and other nearby crops, so keep it in its own corner. | C |
| Rhubarb | the Brassicaceae family | good | Traditional lore says rhubarb planted near brassicas helps against clubroot and pests; the evidence is weak and not confirmed by trials. | D |
| Garlic chives | the Rosaceae family | good | Gardeners plant garlic chives among roses to deter aphids and black spot; the evidence is weak and mostly anecdotal. | D |
| Wormwood | Fennel | avoid | Absinthin and related compounds released by wormwood have been shown to inhibit the growth of fennel and several other herbs in classic allelopathy trials. Keep them well apart. | C |
| Black walnut | Pear | avoid | Black walnut roots, leaves and hulls release juglone, and extension lists name this fruit as sensitive: it can wilt, stunt or die within the root zone and drip line. | B |
| Black walnut | Nashi pear (Asian pear) | avoid | Pear (Pyrus) is on extension juglone-sensitive lists; Asian pear is expected to react the same way, though it is not named separately. | C |
| Black walnut | Siberian crab apple | avoid | Apple (Malus) is on extension juglone-sensitive lists; crab apples are in the same genus and are expected to be affected, though not named separately. | C |
| Black walnut | Muscadine grape | avoid | Grape (Vitis) is on extension juglone-sensitive lists; the muscadine is in a related group and is expected to be affected, though evidence is limited. | C |
| English walnut | Dwarf apple tree | caution | English walnut also produces juglone, usually in lower amounts than black walnut, so sensitive plants such as apple may do poorly under or near its canopy. | C |
| Creeping juniper | Siberian crab apple | avoid | Cedar-apple rust (Gymnosporangium juniperi-virginianae) alternates between junipers and Malus; crab apples are often heavily infected and spores can travel hundreds of metres. | B |
| Creeping juniper | Quince | avoid | Cedar-quince rust (Gymnosporangium clavipes) alternates between junipers and quince, hawthorn and related pome fruit, causing fruit and shoot lesions. | B |
| Creeping juniper | Pear | caution | Pear trellis rust (Gymnosporangium sabinae) needs both pear and juniper, especially savin (Juniperus sabina), to complete its life cycle; keep them apart or choose resistant cultivars. | B |
| Pear | Dwarf apple tree | caution | Fire blight (Erwinia amylovora) infects both pear and apple, and bees carry it from blossom to blossom, so an outbreak in one can reach the other. | B |
| Pear | Quince | caution | Pear and quince are both highly susceptible to fire blight, and quince is commonly used as a pear rootstock, so the bacteria can pass between them. | B |
| Pear | Nashi pear (Asian pear) | caution | Both Pyrus species are susceptible to fire blight and share pear psylla and pear scab, so disease in one threatens the other. | B |
| Siberian crab apple | Dwarf apple tree | caution | Crab apples and apples share fire blight, apple scab and cedar-apple rust, so a heavily infected crab apple can be a source for orchard trees. | B |
| Apricot | Tomato | caution | Verticillium wilt fungus persists for years in soil where tomatoes grew and kills apricot trees, so avoid planting apricot on such ground. | B |
| Apricot | Potato | caution | Verticillium wilt fungus persists for years in soil where potatoes grew and kills apricot trees, so avoid planting apricot on such ground. | B |
| Apricot | Strawberry | caution | Verticillium wilt fungus persists in strawberry beds and can infect apricot trees planted into that soil, so avoid following strawberries with apricot. | B |
| Peach | European plum | caution | Brown rot (Monilinia fructicola and M. laxa) infects blossoms and fruit of all stone fruit, so mummified fruit on one tree can infect its neighbours. | B |
| Peach | Apricot | caution | Brown rot (Monilinia) infects blossoms and fruit of all stone fruit, so mummified fruit on one tree can infect its neighbours. | B |
| Sweet cherry | Sour cherry | caution | Sweet and sour cherries share brown rot, bacterial canker and cherry leaf spot, so they can reinfect each other. | B |
| Gooseberry | Blackcurrant | caution | Gooseberry and blackcurrant share American gooseberry mildew (Podosphaera mors-uvae) and currant aphids, so spread between neighbouring bushes is likely. | B |
| Gooseberry | Redcurrant | caution | Gooseberry and redcurrant share American gooseberry mildew and currant aphids, so spread between neighbouring bushes is likely. | B |
| Sweet orange | the Rutaceae family | caution | Asian citrus psyllid spreads huanglongbing (citrus greening) between citrus and relatives such as orange jasmine (Murraya). Keep new citrus away from infected or neglected hosts. | B |
| Avocado | the Lauraceae family | caution | Laurel wilt (Raffaelea lauricola), carried by redbay ambrosia beetles, kills avocado and wild laurels such as redbay and sassafras in the south-eastern US. | B |
| Guava | the Myrtaceae family | caution | Myrtle rust (Austropuccinia psidii) infects guava, feijoa, Surinam cherry and many other Myrtaceae; it spreads by airborne spores between them. | B |
| Garden phlox | Bee balm | caution | Both garden phlox and bee balm are very prone to powdery mildew; planting them together in a humid, crowded spot raises disease pressure, so give them good air circulation. | B |
| Delphinium | Larkspur | caution | Delphinium and larkspur share the same slugs, snails and mildew problems, and both are poisonous, so keep them away from play areas and grazing pets. | C |
| Mexican marigold (cempasúchil) | Tomato | good | Tagetes roots release thiophenes that suppress root-knot nematodes, but the effect is documented mainly for dense marigold stands grown before the crop, not for a few plants beside tomatoes. | C |
| Firethorn (pyracantha) | Wall cotoneaster | caution | Pyracantha and Cotoneaster are both hosts of fireblight (Erwinia amylovora), so an infected plant can pass the bacterium to the other via insects and rain. | B |
| Firethorn (pyracantha) | Red-tip photinia | caution | Pyracantha and Photinia are both susceptible to fireblight, so plantings of the two close together share the same disease pressure. | B |
| Japanese flowering quince | Wall cotoneaster | caution | Chaenomeles and Cotoneaster are both pome-fruit relatives that can host fireblight, so the disease can move between them in warm, wet spring weather. | B |
| Eastern red cedar | Common hawthorn | caution | Cedar-hawthorn rust needs both a juniper (eastern red cedar) and a hawthorn to complete its life cycle; spores can travel hundreds of metres, so keeping them about 50 m or more apart only lowers the risk. | B |
| Eastern red cedar | Dwarf apple tree | caution | Cedar-apple rust alternates between eastern red cedar and apples or crabapples; planting them close together raises the chance of leaf spot and fruit lesions on apples. | B |
| Common juniper | Rowan | caution | Gymnosporangium rust fungi spend part of their life on common juniper and part on rowan and related Sorbus; both being nearby sustains the cycle. | B |
| Common juniper | Common hawthorn | caution | Hawthorn and common juniper are alternate hosts of a Gymnosporangium rust (hawthorn-juniper rust), so keeping them apart reduces infection pressure. | B |
| Scots pine | Japanese black pine | caution | Both pines are susceptible to pine wilt disease, carried by the pinewood nematode and spread by Monochamus longhorn beetles in warm-summer areas. | B |
| Norway maple | Sugar maple | caution | Maples share Verticillium wilt, a soil-borne fungus that persists for years; replanting a susceptible maple where an infected one died risks reinfection. | B |
| Cider gum (eucalyptus) | Lettuce | avoid | Eucalyptus leaf litter and volatile oils inhibit seed germination and seedling growth of other plants in laboratory and field studies; effects in gardens are likely local and under the canopy. | C |
| Japanese spurge (pachysandra) | Boxwood | caution | Pachysandra and boxwood are both hosts of boxwood blight (Calonectria pseudonaviculata), so the pathogen can move between them; avoid planting pachysandra under or beside boxwood where the disease occurs. | B |
| Mountain laurel | the Ericaceae family | caution | Mountain laurel and rhododendrons are hosts of Phytophthora ramorum (sudden oak death), which spreads from infected foliage between neighbouring plants. | B |
| Smoke tree | the Solanaceae family | caution | Smoke tree is very susceptible to Verticillium wilt, which persists in soil after tomato, potato or aubergine crops; avoid planting it where those crops have wilted. | C |
| Alliums (onion family) | Legumes (pea family) | caution | Onions, garlic and leeks are said to stunt beans and peas (allium root exudates may reduce nodulation). Traditional companion-planting advice; controlled trials are few, so treat it as a soft preference. | D |
| Legumes (pea family) | Alliums (onion family) | caution | Beans and peas are traditionally kept away from onions, garlic and leeks. Traditional companion-planting advice; controlled trials are few, so treat it as a soft preference. | D |
| Fennel | Nightshades | caution | Fennel is reported to suppress tomatoes, peppers and many other vegetables nearby; grow it in its own corner. | C |
| Fennel | Legumes (pea family) | caution | Fennel is reported to suppress beans and peas growing next to it. | C |
| Fennel | Coriander (cilantro) | caution | Fennel and coriander are reported to inhibit each other's seed set; keep them apart if you save seed. | C |
| Dill | Carrot | caution | Mature dill can cross-pollinate with carrot family seed crops and shares carrot fly; young dill near carrots is fine. | C |
| Nightshades | Nightshades | caution | Nightshades share late blight, early blight, Verticillium wilt and Colorado potato beetle; mix with other families and rotate the bed. | B |
| Potato | Nightshades | avoid | Potatoes are the main source of late blight (Phytophthora infestans) that spreads to tomatoes and other nightshades within a few metres. | B |
| Cucurbits (squash family) | Cucurbits (squash family) | caution | Cucurbits share powdery mildew, cucumber beetles and mosaic viruses; good airflow and spacing reduce spread. | C |
| Acid-lovers | English lavender | caution | Acid-loving plants need pH 4.5–5.5, lavender prefers neutral to alkaline soil; one bed cannot suit both. | B |
| Eastern white pine | Blackcurrant | avoid | White pine blister rust alternates between five-needle pines and currants/gooseberries; some US states restrict planting Ribes near white pines. | B |
| Eastern white pine | Redcurrant | caution | White pine blister rust alternates between five-needle pines and Ribes; redcurrants are less susceptible than blackcurrants but can still host it. | B |
| Eastern white pine | Gooseberry | avoid | White pine blister rust alternates between five-needle pines and gooseberries; keep them well apart. | B |
| Sunflower | Nightshades | caution | Sunflower residues and roots release phenolic compounds that can slow nearby seedlings; transplants are less affected. | C |
| Legumes (pea family) | Sweet corn | good | "Three sisters": climbing beans use corn as a support and fix nitrogen; the benefit to the corn is modest but real in low-input soils. | C |
| Pumpkin | Sweet corn | good | "Three sisters": squash leaves shade the soil between corn, reducing weeds and evaporation. | C |
| Onion | Carrot | good | Interplanted onions can mask carrot scent from carrot fly; studies show a small effect that needs about 4 onion rows per carrot row. | C |
| Nasturtium | Brassicas (cabbage family) | good | Nasturtiums act as a trap crop for cabbage white butterflies and blackfly, drawing some pressure away from brassicas. | C |
| Basil | Tomato | good | Basil is said to improve tomato flavour and repel pests; Traditional companion-planting advice; controlled trials are few, so treat it as a soft preference. | D |
Reference values for planning, not horticultural, veterinary or medical advice. Varieties, soils and local climates differ: check with a local nursery or extension service.