Incompatible garden plants
Hi and thanks for contacting Ask an Expert.
I have gardened for many years and never used the companion planting guides that seem to abound on the Internet. In researching your question, I find very little scientific information to give you. I interpret your question to mean: will certain combinations be better together or will they be detrimental to each other if planted in close proximity. Most of the information I found is anecdotal, given from one gardener to another.
Mother Earth News has two articles about companion planting you might want to read:http://www.motherearthnews.com/organic-gardening/companion-planting-guide-zmaz81mjzraw?pageid=2#Page...
However there are other definitions of companion planting which I do use and they work well in my organic garden.
Trap cropping. Sometimes, a neighboring crop may be selected because it is more attractive to pests and serves to distract them from the main crop. This might be mustard planted to trap flea beetles and keep them away from your broccoli/cauliflower. Mustard comes up very fast and they will eat their hearts out with that crop and leave the broccoli alone.
Legumes— such as peas, beans, and clover—have the ability to fix atmospheric nitrogen for their own use and for the benefit of neighboring plants. This adds beneficial nitrogen in the soil that the other plants can easily take up.
Pest Suppression. Some plants exude chemicals from roots or aerial parts that suppress or repel pests and protect neighboring plants. The African marigold, for example, releases a nematode repellent—making it a good companion for a number of garden crops.
Beneficial habitats are another type of companion plant interaction. The benefit is derived when companion plants provide a desirable environment for beneficial insects and other arthropods—especially those predatory and parasitic species which help to keep pest populations in check. You can now purchase these predators online.
The two Mother Earth News links will give you some specific examples of what you can plant together. I think it is loosely based on some scientific data. However, to grow a successful garden (one that grows your food and gives you satisfaction) start with the soil. Add composted materials, organic compounds, organic fertilizers and when the plants are growing, mulch around them to protect them and the moisture in the soil. It has been my experience that most new gardeners do not add enough organic material (compost) nor do they fertilize correctly. Follow the directions on the organic fertilizer bags/boxes for the specific plant.
If you have any further questions, please feel free to contact us again. I hope I have helped you in your quest for companion planting information.
I wrote an addition to my response, do not know if that goes to you, so here it is again. The onion/bean question is mostly a no. The onion (allium) which includes chives and shallots and all alliums does not necessarily do well with beans/peas. However, the broccoli/cauliflower combo is fine. They are in the same family, need the same type of nutrients and grow well together. However, plant rotation is a must. The pests that plague broccoli will set up residence year after year if you leave the broccoli/cauliflower in the same bed.