There’s long been that question, tossed around almost philosophically: what defines a weed?
The prevailing answer offers a certain existential satisfaction: Any plant qualifies, if it's growing out of place. Right?
Well, not really. The fact is, plants don't grow out of place. Nature doesn't allow it. When we look at most farm soils from an ecological perspective, the plants we usually call weeds aren't the out-of-place ones. Rather, the plants we're cultivating are.
In the ecology-based approach to farming, we have a perfectly distinct and useful definition for what a weed is. It all rests on natural succession, a concept that anyone managing soil biology needs to know. When we understand succession, it seems perfectly reasonable that aggressive weeds would overtake any conventional farm or garden.
Succession is the gradual shift of a natural area over time, from one kind of plant-and-microbe community to the next. It’s a concept grounded in ecology, involving the interactions between plants and microbes, with the whole process rooted in a soil’s most recent disturbance event.
In farming, disturbance is visited upon soils most effectively through tilling. After thousands of years of human agriculture, we are finally learning something that now seems obvious: tilling ruins our soils, and the first symptom is always an aggressive weed problem. Different plants may show up after plowing, depending on geography and climate; here in Northern California we can get overrun by various thistles and running grasses.
So why does this happen? Again, succession. It goes like this:
The soil environment following disturbance is a bleak world. Microbial communities are destroyed, leaving only scattered survivors, mostly bacteria. Certain plants thrive in this chaotic environment, quickly setting seed and outcompeting what previously grew. These are the plants we call weeds. Relatively few of them are of much use to us.
As bacterial communities begin developing again, shorter grasses begin growing, feeding on the type of nitrogen made available by those bacteria. After many years, taller grasses may rise to dominance; then shrubs and vines, then trees. As the pattern progresses, fungal communities reestablish and proliferate underground until, eons later, dense groves of towering trees are found growing in soil that harbors many times more fungal biomass than bacterial.
Every plant is at home somewhere along this spectrum, and we use this knowledge to promote growth of whatever plant type we’re cultivating. When we re-introduce fungi that once again proliferate, those fungi change the soil’s chemistry so that it favors higher-successional plants – including most of the productive plants we grow commercially.
With a higher-successional soil profile, we can further introduce ground-cover plants that thrive at the same successional stage. A good example is clover, which performs best in soil with a fungal-to-bacterial biomass ratio of anywhere between 0.6 and 2.0, depending on type. This makes it a suitable companion for a range of crops, from low-growing vegetables to shrubs to trees.