I would like to find out what exactly means that two or more plants (more specifically vegetables) are good companions and could/should be planted together?
Does that mean plants just tolerate each other or there are other beneficial factors involved?
How someone determines which plants are good companionons?
There are some general principles to this, for example
Use tall plants to create shade to help stop smaller plants from bolting to seed (e.g. sweet corn + spinach)
Use plants with strong scent (e.g. herbs and some flowers, such as marigolds and sunflowers) to repel insect pests from other plants.
Mix fast growing and slow growing plants, so the fast growing ones keep down the weeds around the slow growers (e.g. lettuce or radish, with anything slow growing)
Use insect-friendly plants to encourage predators on pests, and pollinating insects.
Use sacrificial plants to keep pests off the main crop (e.g. aphids seem to prefer nasturtiums to many other plants)
There are a few particular combinations that are not "obvious" but seem to work - for example it is claimed that the smell of carrots deters leek moths and the smell of leeks deters carrot fly.
The basic method of discovery is pretty much the same as for everything garden-related - notice what works and what doesn't. Of course there is no guarantee that what worked (or seemed to work) for somebody else will actually work for you, but that's just part of the learning process.
Great question, False! Seriously. The main premise that companion planting came from was 'like needs make good partners'...something like that. Birds of a feather...? Different species that enjoy the same pH, the same amount of water and sunlight, the same formulation of fertilizer the same Zone of habitat and temperature.
Plants love to be cuddled up with other plants. There is a fine line where all plants are getting their needs and sunlight as well as having air circulation between too many plants and too little resources and air circulation.
This also includes planting certain Genus together such as brassicas; cabbage/broccoli/brussel sprouts/cauliflower. These plants should not be planted in the soil they were grown in for at least 1 year best is 2 years. Solanaceae is another headache for 'rotation' practices in the garden; the tomato/potato family. To extend my garden real estate I use pots to grow tomatoes and cabbage and this year I have to plant potatoes in pots...always always always using sterile potting medium for any plant in a pot. I am careful to dump out used medium into specific beds or into my compost pile and consider the year of rotation. This helps to eliminate big diseases such as tobacco mosaic virus and early and late blight spores.
Companion planting has gotten off track because of garden mythology which has become rampant these days. I've not seen any science that explains why marigolds 'protect' other species from certain insects. Lining the perimeter of a plant bed with marigolds is very pretty but I've yet to spend any time or real estate on what I feel is foo foo gardening. And the marigolds won't do well planted with lettuces for instance. Flowering plants need to have lower nitrogen in relation to phosphorous and potassium or they won't flower. Lettuce needs at least equal proportions of N P and K or bolting will be earlier and faster.
There are other companion planting 'tricks' such as planting peas or cucumbers with corn using the corn for supporting the vines. This needs to be done carefully so that the vines do not suck up all of the chemistry (nutrients) the corn will need. Corn is planted first, in some areas they plant it in the fall. The vines are planted after the corn is 2 feet high.
Hope this helps. Lots of people think planting certain plants with other certain plants creates a symbiosis for some reason or another. Perhaps there are instances where there is scientific reason to plant two different species together for mutual benefit other than common sense reasons. Then I shall weigh the benefits of the combination versus the expenditure of my garden real estate.
The real reason for companion gardening is common sense. Group like needs together to get the most out of your garden real estate. Neighborhoods.
There are quite a number of online videos regarding companion planting, as well as many books available on the subject, but general information regarding the basic principles is here https://en.m.wikipedia.org/wiki/Companion_planting.
People round the world have been using companion planting for a decades, and for various reasons; pollination purposes, space constraints and pest control are some of the main ones.
Some plants which may be grown for pest control purposes in the UK are English marigold (Calendula species) which are thought to deter aphids, whilst also attracting hoverflies, which predate on aphids. Nasturtiums are attractive to both aphids and some caterpillars which like brassicas, so they might be planted as a 'sacrificial' crop where cabbages grow to attract the caterpillars and aphids away from the brassica crop. Whilst companion planting can be helpful, its not a guarantee that crops will remain pest free, but it is an environmentally friendly way of reducing problems and is a common system used in organic gardening, where there is a desire to avoid man made pesticides and work with nature rather than battering it into submission.
Some crops may be grown together or close to one another because one benefits the other; more information regarding plants used for companion planting is contained in the Wikipedia link.
