Companion planting refers to strategically mingling plants with others that will support their growth and not hinder it. Reasons to companion plant include:

Organic Pest Management. Some plants will deter the pests that damage others.

Increase Pollination. Flowering plants will attract the insects necessary for the pollination of their friends. These flowering plants could also attract the beneficial insects that deter harmful insects.

Protection. Some plants will cast shade over friends who don’t fare well in full sun. Others appreciate protection from the wind and elements.

Improved Yield. It is said that some plants produce more abundant or more flavorful crops when planted near friends. The opposite can also be said, so avoid planting near enemies. Some companions go well together because their preferred care is similar, ie. they require the same amount of water, soil, sun, etc.

Asparagus
Friends: Aster, Basil, Cilantro, Marigolds, Nasturtiums, Oregano, Parsley, Peppers, Sage, Thyme
Enemies: None.

Basil
Friends: Tomatoes, Asparagus, Oregano, Peppers
Enemies: None.

Broad Beans
Friends: Not picky.
Enemies: Onions

Bush and Pole Beans
Friends: Broccoli, Brussels Sprouts, Cabbage, Cauliflower, Collards, Kale, Kohlrabi, Turnip, Carrots, Celery, Swiss Chard, Corn, Cucumber, Eggplant, Peas, Potatoes, Radishes, Strawberries.
Enemies: Chive, Garlic, Leeks, Onions, Marigolds, Beets (Bush Beans are ok near Beets)

Soya Beans
Friends: Corn
Enemies: None.

Beets
Friends: Bush Beans, Corn, Garlic, Broccoli, Brussels Sprouts, Cabbage, Cauliflower, Collards, Kale, Kohlrabi, Turnip, Leeks, Lettuce, Mint.
Enemies: Pole Beans.

Brassicas (Broccoli, Brussels Sprouts, Cabbage, Cauliflower, Collards, Kale, Kohlrabi, Turnip)
Friends: Chamomile, Rosemary, Sage, Dill, Mint, Nasturtium.
Enemies: Eggplant, Peppers, Potatoes, Tomatoes.

Carrots
Friends: Beans, Broccoli, Brussels Sprouts, Cabbage, Cauliflower, Collards, Kale, Kohlrabi, Turnip, Chives, Leeks, Onions, Peas, Peppers, Pole Beans, Radishes, Rosemary, Sage, Tomatoes.
Enemies: Dill, Parsnips, Potatoes.

Celery
Friends: Beans, Broccoli, Brussels Sprouts, Cabbage, Cauliflower, Collards, Kale, Kohlrabi, Turnip, Cucumber, Garlic, Leek, Lettuce, Onion, Tomatoes
Enemies: None.

Chives
Friends: Carrots, Tomatoes, Broccoli, Brussels Sprouts, Cabbage, Cauliflower, Collards, Kale, Kohlrabi, Turnip.
Enemies: Beans, Peas.

Cilantro
Friends: Potatoes.
Enemies: None

Collards
Friends: Tomatoes.
Enemies: None.

Corn
Friends: Amaranth, Beans, Beets, Cucumber, Dill, Melons, Parsley, Peas, Potato, Soya Beans, Squash, Sunflower
Enemies: Celery, Tomatoes

Cucumber
Friends: Asparagus, Beans, Broccoli, Brussels Sprouts, Cabbage, Cauliflower, Collards, Kale, Kohlrabi, Turnip, Celery, Corn, Dill, Lettuce, Onion, Peas, Radish, Tomatoes, Nasturtium.
Enemies: Potatoes, Sage.

Dill
Friends: Broccoli, Brussels Sprouts, Cabbage, Cauliflower, Collards, Kale, Kohlrabi, Turnip, Corn, Cucumber, Lettuce, Onion.
Enemies: Carrots, Tomatoes

Eggplant
Friends: Amaranth, Beans, Marigolds, Peas, Peppers, Spinach, Thyme.
Enemies: Fennel

Fennel
Friends: Doesn’t play well with others.
Enemies: Beans, Kohlrabi, Tomatoes, nearly all others.

Garlic
Friends: Roses, Beets, Broccoli, Brussels Sprouts, Cabbage, Cauliflower, Collards, Kale, Kohlrabi, Turnip, Celery, Lettuce, Potatoes, Strawberries, Tomatoes.
Enemies: Peas, Beans.

Kohlrabi
Friends: Beets, Broccoli, Brussels Sprouts, Cabbage, Cauliflower, Collards, Kale, Kohlrabi, Turnip, Cucumbers, Onions.
Enemies: Peppers, Pole Beans, Strawberries, Tomatoes.

Leeks
Friends: Beets, Celery, Carrots, Onions, Spinach.
Enemies: Beans, Peas.

Lettuce
Friends: Beets, Broccoli, Brussels Sprouts, Cabbage, Cauliflower, Collards, Kale, Kohlrabi, Turnip, Carrots, Celery, Chervil, Cucumbers, Dill, Garlic, Onion, Radish, Spinach, Squash, Strawberries.
Enemies: None.

Melon
Friends: Corn, Marigold, Nasturtium, Pumpkin, Radish, Squash, Sunflowers.
Enemies: Potatoes

Mint
Friends: Broccoli, Brussels Sprouts, Cabbage, Cauliflower, Collards, Kale, Kohlrabi, Turnip. Note: Mint is invasive, so consider planting it in containers and/or spreading the cut leaves around the brassicas.
Enemies: Parsley

Onions
Friends: Chamomile, Summer Savory, Beets, Broccoli, Brussels Sprouts, Cabbage, Cauliflower, Collards, Kale, Kohlrabi, Turnip, Carrots, Dill, Leeks, Lettuce, Strawberries, Tomatoes.
Enemies: Asparagus, Peas.

Oregano and Marjoram
Friends: Asparagus, Basil, Broccoli, Brussels Sprouts, Cabbage, Cauliflower, Collards, Kale, Kohlrabi, Turnip
Enemies: None.

Parsley
Friends: Asparagus, Carrots, Chives, Corn, Onions, Tomatoes. Sprinkle leaves around Asparagus and Roses.
Enemies: Mint

Peas
Friends: Beans, Carrots, Celery, Corn, Cucumber, Eggplant, Parsley, Peppers, Potatoes, Radish, Spinach, Strawberries, Turnips.
Enemies: Onions

Peppers
Friends: Asparagus, Basil, Carrots, Cucumbers, Eggplant, Endive, Oregano, Parsley, Rosemary, Squash, Swiss Char, Tomatoes.
Enemies: Beans, Broccoli, Brussels Sprouts, Cabbage, Cauliflower, Collards, Kale, Kohlrabi, Turnip, Fennel

Potatoes
Friends: Bush Beans, Celery, Corn, Garlic, Marigolds, Onions, Peas.
Enemies: Asparagus, Broccoli, Brussels Sprouts, Cabbage, Cauliflower, Collards, Kale, Kohlrabi, Turnip, Carrots, Cucumber, Melons, Parsnips, Rutabaga, Squash, Sunflower.

Pumpkins
Friends: Corn, Pole Beans, Melons, Squash, Sunflowers.
Enemies: Beets, Onions, Potatoes, Carrots, Turnips.

Radish
Friends: Nasturtiums, Beans, Beets, Celeriac, Chervil, Cucumber, Lettuce, Mint, Parsnip, Peas, Spinach, Squash, Tomatoes.
Enemies: Agastache (aka Anise Hyssop), Potatoes.

Rosemary
Friends: Beans, Broccoli, Brussels Sprouts, Cabbage, Cauliflower, Collards, Kale, Kohlrabi, Turnip, Carrots.
Enemies: None.

Sage
Friends: Broccoli, Brussels Sprouts, Cabbage, Cauliflower, Collards, Kale, Kohlrabi, Turnip.
Enemies: Cucumbers.

Spinach
Friends: Broccoli, Brussels Sprouts, Cabbage, Cauliflower, Collards, Kale, Kohlrabi, Turnip, Eggplants, Leeks, Lettuce, Peas, Radish, Strawberries.
Enemies: Potatoes

Squash
Friends: Corn, Lettuce, Melons, Peas, Radish, Borage, Marigolds, Nasturtiums, Sunflowers.
Enemies: Broccoli, Brussels Sprouts, Cabbage, Cauliflower, Collards, Kale, Kohlrabi, Turnip, Potatoes,

Strawberries
Friends: Beans, Borage, Garlic, Lettuce, Onions, Peas, Spinach, Thyme
Enemies: Broccoli, Brussels Sprouts, Cabbage, Cauliflower, Collards, Kale, Kohlrabi, Turnip, Fennel

Summer Savory
Friends: Beans, Onions.
Enemies: None.

Swiss Chard
Friends: Beans, Broccoli, Brussels Sprouts, Cabbage, Cauliflower, Collards, Kale, Kohlrabi, Turnip, Onions.
Enemies: None.

Thyme
Friends: Broccoli, Brussels Sprouts, Cabbage, Cauliflower, Collards, Kale, Kohlrabi, Turnip, Strawberries.
Enemies: None.

Tomatoes
Friends: Nasturtiums, Asparagus, Basil, Beans, Borage, Carrots, Celery, Chives, Collards, Cucumber, Garlic, Lettuce, Marigolds, Mint, Onion, Parsley, Peppers.
Enemies: Dill, Broccoli, Brussels Sprouts, Cabbage, Cauliflower, Collards, Kale, Kohlrabi, Turnip, Corn, Potatoes.

Turnips
Friends: Mint, Peas.
Enemies: Pumpkins.

Remember to rotate your crops each year so that your soil maintains nutritional balance and to avoid soil borne diseases. Growing a garden doesn’t only require physical endurance, but mental agility, too! Finding a different garden layout each year can be a challenge, but hopefully this list of friends and enemies will help you create a garden full of happy plants!