on the top of this page where you see the search box, type in companion planting and search this forum and you will find 2 pages of post on this topic.

Mostly it's folklore. People plant, have "success" and think it's due to the combo at hand.

Likewise, some plants aren't supposed to do well when planted near each other. Again, it's chance, or poor gardening, that explains the "effect."

i dunno, i planted a row of sunflowers last year next to my garden and the jap beetles swarmed them instead of my veggies. whether that was luck, chance, fluke or a real result i have no clue. i also had NO aphids last year for the first time ever.

the only real drawback was my wife thought i planted the sunflowers for seeds to eat, when i planted them as sacraficial just to see if they worked or not.

I plant flowers in the vegetable garden mainly to attract pollinators, and as barrier crops for seed saving purposes. I'm with Jean, I have yet to see any conclusive evidence that companion planting repels insect pests. Anecdotal evidence (I used it, and had no .....) means nothing, unless a control (unprotected) planting of the same vegetable, at the same site, had problems. The most common myths seem to be for marigolds & nasturtiums.

However, I am not saying that flowers are not useful for pest control. There are several ways they can be helpful.

To the extent that companion planting breaks up large mono-cultures, it can be helpful in reducing the spread of insects or diseases. This strategy works best if the adjacent plants are of a different genera, with different insect pests. For example, you would not want tomatoes, eggplant, and potatoes to be adjacent to one another, since they are closely related - as is the flower Nocotiana. Flowers & herbs are seldom in the same genera as cultivated vegetables, so they are generally effective as companions... but with a little planning, the same effect can be achieved with edible plants.

Flowers & herbs can be used as trap crops, as can other plants. I have observed some insects that are attracted more to the weeds in my garden, than to the vegetables. Mallows will attract white flies... but since okra is also a mallow, that strategy could potentially backfire. I grew an unusual solanaceae, "Litchi Tomato", which acts as a trap crop for Colorado potato beetle.

The problem with using trap crops as a pest control strategy is that you are attracting harmful insects... so if planted as a preventative measure, it could actually make things worse. Trap crops are best used outside the garden, to attract the target insect & kill them.

Flowers can also be useful for attracting & holding beneficial insect populations. Cleome, for example, attracts wasps... and as scary as that thought might be, wasps control several garden pests. Other flowers feed the adults of insects that are predatory as larvae, such as Syrphids (hover flies). And while this is not a flower, I'll mention that yardlong beans are attractive as feeding sites for ladybugs... so if you want to keep ladybugs from migrating, you might consider growing them.

When I planted marigolds in one earth box with tomatoes, those tomatoes got spider mites.

In the earthbox without marigolds, no spider mites

The exact opposite desired consequence. I won't do it again

I've had great success with using santolina bushes in my produce garden. The scent of santolina wards off many insect pests. Because santolina is a small, low growing bush I plant it around the perimeter of the garden.

I also use French marigolds planted throughout the produce garden. Lavender is supposed to be helpful. I only have one lavender in the produce garden thus far, but will be adding more.

I had trouble with thousands of red spider mites in one quadrant of a flower garden. I planted chrysanthemums and marigolds there. I haven't seen any spider mites in that garden for a couple of years.

There are some plants which will attract pest insects, as noted by davidandkasie, which saves other plants from infestation. And, there are plants which repel the insects. I tend to go for the plants which repel.

Many folks object to birds in their produce gardens and put up things to frighten birds away. However, birds can eat a lot of insects!

Lorna

Hmmm--for pest control, I buy praying mantis egg cases. But, I do like to grow greek oregano and basil under my tomatoes, as they act as a living mulch--and they are right there to pick when I am making sauce lol!

I also grow sunflowers to attract bees :D

The wasps and ladybugs are all over my dill this year, too--so I guess what I read about dill attracting beneficial insects has held true for me.

The Japanese Beetles are swarming my Marigolds.
They 'LOVE' em. They are the reason I quit trying
to grow beans. If I grow both the beans and the
Marigolds together, the beetles leave the Marigolds
alone and swarm the beans. It's a no-win situation.

I leave self sown poppies, nasturtiums and feverfew amongst the vegetables because I like the look. They attract pollinators but also black aphids and they are a great place for slugs and snails to lie up during the day. You just have to weigh up the pros and cons for yourself.

I love interplanting flowers and herbs in my veggie garden. I enjoy that the butterflies and hummingbirds join my in the garden while I'm working. I like that its pretty! I have no pollination problems whatsoever in my gardens. My garden includes bee balm, lavender, California poppy, garlic chives, french marigolds, sunflowers, zinnias, cosmos, and lamb's ears among the veggie plants. I have noticed that dill will stunt tomato plant growth; so I keep those plants apart. Do the flowers repel insects? Maybe...but they certainly attract me into the garden and I squish the bugs that I don't want.