Yes, insects. This is a whole other topic (of which I wrote an article that will be published in the July edition of the West Island Quarterly, so I'll share that with you once it's out).
Companion planting can be done with veggies, herbs, annuals & perennials. It's a great way to fill up empty holes in all your gardens, allows for diversity (which insects love) and utilizes your soil nutrients in a much more efficient manner.
I'll talk about veggies first. Believe it or not, some veggies will flourish when planted with others or they will just rebel & refuse to do anything for you no matter how much manure, compost or fertilizer you give them.
One of my favourite combinations begins with tomatoes. Now these have a whole health regime unto themselves, so why not give them a bit more help by planting things around them as aids. Borage will help repel the tomato worm, basil is said to aid in flavour while keeping mosquitoes & horn worms away (plus if you are eating a tomato, most times you'll eat it with basil). Because dill requires rich soil to be productive, it makes sense to place it close to tomatoes too & by planting lettuce around the base of the tomato plants, you are providing a mulch cover for the tomatoes & a bit of shade for the lettuce.
~ greenhouse bed - May 13 ~This is my year to conquer my tomato demons & to do that, I'm using companion planting to it's fullest. I've got my lettuce crop a little further away from the tomatoes than planned, but they are larger than I had anticipated. I guess I need to start eating them! Along the left I've got borage starting to come up & just finally planted the dill. On the far right will be calendula - a pot marigold which is supposed to repel nematodes (a type of roundworm that is parasitic & will eat your plants from the bottom up) & certain beetles. Of course, the way my calendula grow, they will start to flower sometime in September - just in time for fall sowing of lettuce, but of no use for the summer tomatoes.
Other combinations include: beans & summer savory (which is supposed to aid in the bean flavour while growing) but don't plant garlic or onions or peppers by your beans or they will not be happy with the competition.
Has anyone heard of the Three Sisters? I've come across this many times in my web research & am intrigued. It involves corn in the center, pole beans around the corn & finally squash around the outer edge. The beans will grow up the corn & the squash will provide a ground cover preventing weeds & the corn will use the nitrogen in the soil that the beans add. I would love to try it if I didn't have such a love-hate relationship with corn...
I'll list off a few other combinations that I hope to try this year:
* kale & sage
* lettuce & beets & peas
* onions with beets, carrots
You can use herbs too to aid in plant health. As mentioned with tomatoes, basil will repel mosquitoes & horn worms, borage keeps the tomato worm at bay, but if you are infested with aphids trying growing some sacrificial plants such as nasturtiums (which actually attract the aphids to themselves) or catnip (in a pot to keep it under control). You can remove the infested pots & treat them away from your precious tomatoes & basil & replace them again. Leeks grown with carrots will tame the carrot fly population & mint or rosemary deals with certain moths (cabbage moth), mosquitoes & beetles.
I've read that fennel - the leafy variety vs the bulb variety, as far as I know - is disliked by most veggies, so put it in the perennial bed towards the back - but where you can still reach it, as the leaves are so delicious in salads or as a cheat for dill.
Every garden & gardener will find different things that work well together. Those are usually closely kept secrets, imparted only with care & lotsa trust. Now that I'm starting to pay greater attention to how my garden grows, I can't wait to find out my own winning combination that allows me to let nature take back control...
Check out on-line resources as well for plant preferences, companion planting & natural insect repellents (or insect attractants because if you are growing food to bring in the predatory insects, they will take care of those other ones for you!)
The Farmer's Almanac - Pests & Problems
Farmer's Almanac - Plant Companions
Farmer's Almanac - Three Sisters Companions
West Coast Seeds - Companion Planting