Mixed results in my experience:
1) I don't find that Marigolds are particularly effective at repelling pests...but they are very good at attracting pollinators.
2) I always plant basil around tomatoes not to improve their taste but to repel aphids and it seems to be effective.
3) I always use the "three sister" approach in corn with beans and melons instead of squash...and yes, I consider it effective.
4) Perhaps the most effective companion planting I use is sweet peas with potatoes. Very effective and creamed peas and new potatoes is a big favorite dish here.
5) I use sunflowers as a trap for stink bugs on tomatoes...and find it very effective if the sunflowers are planted relatively close to the tomatoes.
6) I make extensive use of cover crops to both control weeds and to replenish soils. These are powerful tools that I use to precede a crop and to follow a harvested crop. Various legumes alfalfa, clovers, vetch, peas, are often used in combo with root plants like turnips and daikon radish. Summer covers include Sunn Hemp and field peas. Using these cover crops in rotations augmented with composted cow manure have enabled my garden veggies to grow completely without any synthetic fertilizers and/or pesticides, fungicides, or herbicides...and the soil to consistently test out "No N, P, K " required.
