What flowers can I grow with my veg to deter pests?
Thomas Moug, by email
Stefan says: There's little or no scientific evidence that the actual presence of flowers or anything else such as garlic, which is often suggested, has a deterrent effect on pests. The theory is that some chemicals are given off by the plants to produce a pest-controlling action, but if you think about it, the effect of the wind and open air must mean that anything, even if it existed, would instantly be diluted and blown away.
The only possible example I'm aware of is that the roots of some species of tagetes produce a chemical that has some deterrent effect on certain eelworms (nematodes) in the soil, but that's an exception.
But having said this, if you look at my own kitchen garden you'll see flowers scattered among the vegetables. They're mostly calendulas, which self-seed every year. So why are they there? For two reasons. First because they look really attractive and second because bright orange or yellow-coloured flowers are good at attracting insects and among these insects will be ladybirds and hoverflies that, directly or indirectly (as adults or larvae), may have some value in diminishing some pests, aphids especially. But the key word is 'some'; it doesn't guarantee my vegetables are aphid-free, although it does help.
You also ask about growing carrots with lettuce. You can grow anything with anything and I do in my garden.
You'll find some carrots close to lettuce, potatoes close to kale, rocket alongside beans and so on. This is because the area I devote to my kitchen garden is relatively small and not large enough for me to operate a truly strict three-course rotation, which scientifically is certainly the ideal. For pest and disease limitation, having three more or less equal areas devoted to root crops, to brassicas and to peas and beans and swapping them year by year is certainly desirable; with lesser crops like lettuce fitted in here and there.