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Spelling Out Companion Planting
Other good combinations in the garden include broccoli with cucumbers, since broccoli reduces the attraction of striped cucumber beetles, leeks near carrots because they ward off carrot flies, and onions by potato plants since they chase away the potato-munching Colorado beetle. Never plant carrot and dill close by; being of the same family, they tend to influence one another to become more like the other, making the carrots woodier and stronger-flavored, and the dill milder and with weaker stems.

Some specific flowers and herbs reap big benefits for certain vegetables, too. Marigolds deter Mexican bean beetles, cabbage pests, and spur growth in roses. They reduce the number of nematodes in the soil and conversely attract hoverflies, which eat up destructive aphids. White marigolds in particular work well around cabbage family plants, their white flowers attracting the cabbage-loving white moth away from the heading plants.

Mint works to encourage plant growth, for some unknown reason, with all kinds of squash and brassicas. I haven't tried it yet, but wonder-herb tansy supposedly repels ants, aphids, cabbage worms, Colorado beetles, Japanese beetles and squash bugs.

Some plants are so highly antagonistic to one another, they stand in a separate category. Never plant:
  • carrots, cauliflower or potatoes with tomatoes
  • asparagus with onion and potato
  • beans with chives, fennel or garlic
  • potato with pumpkin and summer squash OR
  • peas with onion, garlic and shallots.

As for petunias in the onion patch, I know of no specific benefit to the lovely petunia for growing amidst the onions. But if I were to plant them close to the onion row, their pink, white and purple nodding heads among the stalwart green tops of the onions would make me smile. And a smiling gardener, I'm pretty sure, can work longer and harder every time, ensuring a better garden!