Ask Mr. Smarty Plants

We're not exactly sure what you mean but we'll take a stab at it. Because at the Lady Bird Wildflower Center, we are all about plants that are native not only to North America but the area in which they are being grown, we feel that plants native to the same area should grow well together because they are adapted to the same climate, rainfall and soil. Since you live in Idaho, that means that if you had some Minuartia obtusiloba (twinflower sandwort), which is native to Idaho, and needs full sun, and can grow in dry, rocky soil, it would probably do fine. But, if you tried to plant some Claytonia caroliniana (Carolina springbeauty), which is native to Vermont, needs part shade and a wet or moist soil, the seond plant would probably not do well, or just die, because it was not adapted to the same conditions as the plant native to Idaho. 

That's one way to decide which plants will grow best together. Another way is how much sun or shade the plants need. Even if you're dealing with two plants native to the same place, like Idaho, if one needs sun and one needs shade, and they are planted together in one shady place, what do you think will happen? For instance, suppose you wanted to plant some Eriophyllum lanatum (common woolly sunflower), which needs lots of sun, and right next to it Aquilegia caerulea (Colorado blue columbine), which needs shade, what do you think would happen if you planted them both in the sun? In the shade?

We are also wondering if perhaps you were thinking of symbiosis when you asked your question. Symbiosis is a Greek word meaning "living together." You can read the complete definition from this website by MSN Encarta Symbiosis. There are three types of symbiosis:

Mutualism is when there are benefits to both members of the relationship. In plants, this can be seen in the coexistence of  certain species of algae and fungi that together compose lichens. You might want to look at the website Lichen Biology and the Environment, which is an extract from the book Lichens of North America by Irwin M. Brodo and Sylvia and Stephen Shernoff.

Antagonistic symbiosis, or parasitism, when one organism receives no benefits and is often harmed while supplying nutrients or shelter to the other. A good example of this is Phoradendron tomentosum (Christmas mistletoe), which is not native to Idaho (and you should be glad) but is to Texas. Mistletoe is not only supported by its host tree, but also draws some nourishment from that tree. It definitely does the tree no good. 

Commensalism is most commonly an association between two non-parasitic species, harmless to both and in which one organism benefits. The best example of this in the plant kingdom is  explained in this Wikipedia article Epiphytes. An epiphyte native only to Texas is Tillandsia baileyi (reflexed airplant), usually called "ball moss." It likes oak trees, and needs some shade, so it tends to be on inner branches of its host trees, and many think that it is killing the tree, because those branches are usually aging naturally. 

If we still didn't answer your question, maybe you could ask again and explain to us just what you are looking for.

Phoradendron tomentosum

Tillandsia baileyi