Companion planting - myth or reality?

I've heard of companion planting and read about people using it on a few blogs (here and here and here), but due to a general lack of planning in my garden anyway, I hadn't got around to implementing it myself yet.  It has also been pointed out to me that I one of the reasons my peas didn't succeed this season is because I planted them next to spring onions, so I have been thinking about paying more attention to companion planting.

The peas don't like growing with the onions, but why??

Part of the reason I haven't taken a huge interest in companion planting so far is an uneasiness about the science behind it.  I don't like to just blindly follow instructions about which plants like to grow together, I like to know why something works, so I can modify the system to work for me in my garden.  For example, with the peas, I assumed that the reason for not planting peas and onions was that the nitrogen from the peas will make the onions too leafy with not enough growth underground, however if I'm growing spring onions then it would be ok.  I don't see how the onions can impact the peas, but maybe there's some plant/soil chemistry interaction that I don't know about, see how I need to know the WHY so I can make decisions!

Time for a bit of google research and I found a few interesting articles.....
Is Companion Planting Scientific?
The Myth of Companion Planting

The Art of Companion Planting

Companion Planting

I can see that the reasons that companion planting may work, are varied and include:

1. alleopathic interactions (both good and bad)
2. confusing pests 
3. preventing the spread of diseases
4. enhancing environmental conditions for growth (mostly about shading young seedlings with wide-leaved plants)

The last three points can be achieved just by mixing up the garden in the way that I do now.  If there's a gap for a seedling I'll pop it in there and it will be shaded by other plants, which will be pulled out eventually, leaving more room for the seedling to develop.  The mixed up vegetables should also confuse pests and prevent the spread of diseases.  Some of the recommendations that come from the northern hemisphere for specific companions to deter pests up there may not help us down here in Australia, with different pests attacking our plants, so I'm happy to just use a general mix up, rather than stick to a plan.

Zucchinis and corn are supposed to like growing together, but can you see the corn?

I'm still a bit confused about how to also apply a rotation program, as this kind of depends on having similar plants in the same garden bed and moving them each year, how then do I mix them up?  (By the way, I love this post about garden planning strategies ).  

Anyway, the first point above is the interesting one, and requires more information to put it to use.

Allelopathy is a biological phenomenon by which an organism produces one or more biochemicals that influence the growth, survival, and reproduction of other organisms. These biochemicals are known as allelochemicals and can have beneficial (positive allelopathy) or detrimental (negative allelopathy) effects on the target organisms- Wikipedia

You really need to know the alleopathic interactions of each plant to know which can be planted together, and maybe that's why I had a problem with the peas next to the onions, but it seems that most of the research in this area has been targeted, as usual, at commercial applications for allelopathy - using the chemicals to develop herbicides.  Its so frustrating that this research effort is wasted when they could be working out how to grow vegetables in organic gardens/larger scale farms more efficiently!  Although I guess its pretty difficult to design an objective study of plant alllelopathy, so I should just use the associations that have been developed over the years based on experience and see what does work for me.

My next problem is finding a system that I want to follow.  As this post found out after comparing several different companion planting guides, they don't all agree!  They recommended that you at least consider the plants that are consistently reported to not grow well together and try to avoid those combinations, which includes peas and onions (damn!).  They have compiled a summary of several commonly used guides to assist.  There is also a summary table (which even includes some reasons for bad neighbours, I like it!!!) on this post from Sustainable Gardening Australia, which I think will be useful.  I have copied it into a word doc, deleted the veges that I don't usually grow and printed it out for easy reference in future (I almost need to laminate it and put it on the garden gate so I remember to use it!).

If you are interested in reading about other potential gardening myths, Linda Chalker-Scott has reviewed a number of them, with some interesting and well-researched articles on her website.

Do you use companion planting in your garden?  Does it work for you?