There are many who advocate Companion planting is a beneficial practice but there is no scientific evidence to prove this.
It is often claimed that some plants repel certain insects, but due to the lack of scientific evidence, could the reverse also be true? that is; do they attract more of the same insect species?
Some research has found that insects identify plants by using chemical receptors on their feet.
It was found that insects have to land on the same type of foliage on four consecutive occasions (4x) before they are able to identify a particular plant.
If it fails to identify its target plant it goes elsewhere.
Although this research was carried out on brassicas there could be wider implications, e.g.
If pests are homing in on say carrots, and these are interplanted with say any other crop, the pests may well fly on, having failed to locate the necessary 4x identity check on their targeted plant.
If this can be be proved to be true then this would be good reason for interplanting (companion planting)
Similar research has found that many insects land on green surfaces but avoid brown surfaces such as soil.
They also found that using green artificial plants are just as effective as companion plants as were living green plants.
This suggests that insects do not discriminate between green surfaces on the basis of odour or colour.
So what does this mean?
Does it mean that carrots are more susceptible to carrot fly when grown in isolation? yet if grown among anything else with green foliage, they are are not so susceptible?
It would seem so, and the fact that even with interplanting / companion planting, carrots can still be attacked, simply because the fly landed 4x on the carrots as opposed to the other greenery by chance.............. not by choice!
At best it could be said that interplanting / companion planting can lessen the odds of an attack purely on the number of plants in the area, not the type of plants.
It is a widely held view that pungent or aromatic plants deter pests, but again research has failed to confirm this.
In fact the reverse has been found on some research that has been done.
For example; a test on the theory that French marigold deters whitefly found the whitefly happily colonised the marigolds.
Because of this, this attribute can be put to a good use in so far as: once the marigolds are infested, they can be sprayed to keep the population low enough to stop migration.
In other words: Just planting Marigolds isn't sufficent to control infestation transferring to other plants e.g. tomatoes.
On the smell issue the only beneficial findings are that smell / odour from plants can attract pollinators.
Similarly plants that produce high levels of nectar will also attract beneficial insects.
Meaning it can be advantageous to interplant (companion plant) aromatic nectar filled plants along side crops that require pollination e.g. crops such as fruit and beans.
Aromatic nectar filled plants can also attract insects such as hoverflies and ladybirds who’s larvae are natural predators for some varieties of aphid, making them a good organic pest control.
It was mentioned above that French marigolds deters whitefly, and it would seem they don't!
However they (the marigolds) can be put to another use and that is to interplant / companion plant them around the perimeter of a bed that is susceptible to slug / snail damage.
Slugs/snails love marigolds, so by using marigold plants as sacrificial plants, one will always know where to find slug populations..........under the marigolds!
Finding slugs/snails during daylight hours is often quite difficult, so by making a habitat such as this should help to make control a little easier.
Once these habitats are established one can use various methods of control e.g.
Lift the slugs / snails manually and dispose of them, spray the marigolds with a liquid slug killer so that when the slugs/snails chew on the plant they ingest the killing agent, or you could spread a few slug pellets under the marigolds.
It would seem that companion planting in certain instances is beneficial to the gardener but it must be said the whole subject is in need of more research.
The following list of companion planting is said to work for the reasons given;
Asparagus - prevents microscopic nematodes from attacking the roots of tomatoes.
Chervil - keeps aphids off lettuce.
Chives - onion scent wards off aphids from chrysanthemums, sunflowers and tomatoes.
Coriander - helps to repel aphids.
Dill - attracts aphid eating beneficial insects likes hoverflies and predatory wasps.
Garlic - deters aphids and is particularly good planted with roses.
Tansy - strongly scented plant deters ants.
Yarrow - this boosts vigour in other plants and accumulates phosphorous, calcium and silica, which can benefit homemade compost when plants are added to the heap.
It also attracts many beneficial creatures such as hoverflies and ladybirds.