This is a great thread! Though I've been slow to get started, I've been working on a number of different companion planting ideas. From my research a while back, a few of the big things to make room for all over the place were alliums (pest control), legumes (nitrogen fixing), yarrow (enhanced essential oils), marigold (if I can keep it happy enough to reseed!; pest control), artemisia (pest control), and ruta (pest control). I'm sure there's more, but these are the ones that stood out to me as both very effective and favorable.
So I've planted Yarrow throughout my orchard, as a preferred groundcover around the trees. If kept mowed (no flowers), it makes a good alternative lawn for sunny spaces! Very soft ferny leaves.
I've got various kinds of Artemisia getting started around my propagating area... and is it a coincidence that this year that area has not been pestered by pests to any significant degree? I had been thinking it had to do with a high diversity of plants, but maybe it actually is the artemisia at work.
I have plans for Ruta graveolens (Rue) to be planted along the street with all my flowering cherries, because one of the things it repels is beetles. Every summer those trees have been getting virtually defoliated by Japanese Beetles... and maybe that is why there seems to be a new dead one every year...
So I'm hoping that the Rue will work to keep them off those trees. In one year, I had some planted at the base of the end tree... which happened to also be the prettiest... and I wonder if that is -why- it was the prettiest?
Legumes and Alliums we all know about, although it should be noted that they don't mix well. Something about the onions inhibits the legumes, so plan wisely. Also, since I don't want to be planting bulb onions all over my yard, I've been looking for my favorite ornamental types to act as companions. The best of both worlds!
Finally, beneficial nematodes that I applied one spring made the difference between peach borer the year previous, and none since that single application around the tree. (I used Pot Poppers from Amazon, because it had the right kind of nematode I was looking for in an easy to use form). Apparently they're good against a lot of soil pests, so I use them anywhere such might be an issue. Beneficial microbes can similarly be used against disease agents, and also soil pests. It sure makes gardening more fun not to have to run into these problems!
