Vegetable,Good Companions,Keep Apart From,Why It Works
Tomatoes,"Basil, carrots, marigolds, borage, parsley","Fennel, cabbage, corn, potatoes",Basil repels aphids & whiteflies; marigolds deter nematodes
Peppers,"Basil, carrots, tomatoes, marigolds","Fennel, brassicas",Basil improves flavour and repels aphids
Carrots,"Onions, leeks, lettuce, tomatoes, peas, rosemary","Dill, parsnips",Onions repel carrot fly; carrots repel onion fly
Cucumbers,"Beans, peas, radishes, sunflowers, nasturtiums","Potatoes, sage, aromatic herbs",Radishes repel cucumber beetles; nasturtiums act as trap crop
Beans,"Corn, squash, cucumbers, carrots, potatoes","Onions, garlic, shallots, fennel",Beans fix nitrogen into soil for corn & squash
Onions & Garlic,"Carrots, tomatoes, lettuce, beets, roses","Beans, peas, asparagus",Strong scent confuses and repels many pest insects
Lettuce,"Carrots, radishes, strawberries, tall plants (shade)","Parsley, celery",Shaded by tall neighbours in summer heat — extends season
Brassicas,"Nasturtiums, dill, thyme, onions, beets","Tomatoes, strawberries, peppers, pole beans",Nasturtiums trap aphids away from plants
Corn,"Beans, squash, pumpkins, cucumbers, melons","Tomatoes, celery",Three Sisters — classic polyculture with mutually reinforcing benefits
Potatoes,"Beans, peas, cabbage, marigolds, horseradish","Tomatoes, peppers, eggplant, cucumbers, squash",Keep away from tomato family — shares blight disease
Basil,"Tomatoes, peppers, asparagus, oregano","Sage, thyme","Repels aphids, whiteflies, spider mites — plant throughout garden"
Marigolds,"Almost everything — especially tomatoes, peppers",None significant,"Suppress soil nematodes; repel aphids, whiteflies, cabbage moths"
Nasturtiums,"Cucumbers, squash, brassicas, fruit trees",None significant,"Trap crop — aphids prefer nasturtiums, leaving vegetables alone"

Source: GrowersGuide.ca — https://growersguide.ca/blog/companion-planting-guide