This won't be knew to most people, but I thought perhaps a good topic for discussion and collection of knowledge?
I was doing some googling to find out if all those soursobs in my front yard will give my chooks indigestion, and I came accross the natural sequencing farming forum
I was particularly interested in the bit on caltrop (three cornered jacks, bindi) as that was literally the ONLY thing growing in my garden when I moved in. Six months later, the next crop has come up, but it's been mostly smothered out by mallow (as recently identified by this forum!) and clover, all self seeded. The only places the caltrop is still growing in where the soil is particularly poor, and where I haven't mulched. I've been pulling it up (have you ever stepped on a three cornered jack? Then you wouldn't find the little delicate purple flowers it's putting out now to be pretty!) but reflecting on it's niche in my garden at the same time.
Researching soursobs, I've found that 'feeding them to death' is a common control method, because they are suited to poor nutrient soils. The same is true for caltrop. That would explain my abundatn crops! In past gardens (I'm in SA, by the way) I've found that sowthistle, dandelion and milkthistle, etc grow in low nutrient soil, and their deep roots do wonders for it. My pet rabbits love them, so I've never considered them a weed anyway, and let them grow large enough to make a real impact on the soil, until you could see worm castings piling up at the base of the plants. In my current garden they are only growing on the very edges of garden beds, in the dampest parts of my garden.
Grass is invading the beds that were previously covered by ivy and other allopathic leaves, and I'm leaving them to do what they do until I have other things to plant there. After the grass has been there a few weeks, the mallow follows.
What 'weeds' have you noticed in your garden, and where? What niche do they fill? I know there are plenty of smart, observing people with far more knowledge than I!