It just doesn't seem fair. When you clean a surface, it ought to stay clean, unless you put something on it that dirties it. Right?! So why is it that, after you've worked hard to weed a flower bed or vegetable garden and you have a beautifully pristine expanse of bare soil in between each plant, the weeds show back up? Seemingly like magic! Sometimes they seem to spring up overnight.
Well, actually, no, there's a very good reason for this to happen in nature. Weeds are nature's way of "preserving its capital", to use economic terms. So what do I mean by "capital"?
In an ecosystem, soil is one of the most important resources available and one of the hardest resources to manufacture. Originally soil formed from bedrock. It takes thousands of years for soil to form: soil formation involves the slow action of water and weather (especially the freeze/thaw cycle), acids from lichens, and eventually, organic debris and the action of plant roots. Soil is NOT a substance that it's wise to waste, because its replacement takes thousands of years.
Enter most "weeds". What we consider weeds are usually fast growing plants that produce thousands and thousands of seeds which transport readily, in a variety of ways, and which require sunlight and open soil to germinate and grow. Once established in a good spot, these plants will reproduce rapidly and repeat the cycle of producing thousands and thousands of easily transportable seeds. What's the point of this process?
Well, these same plants' seeds and seedlings cannot compete in a crowded, shaded location. In a setting like that - a more established setting - they get outcompeted by other plants that don't have to start from scratch every year. Plants like perennials, with well established roots that can send up tall stalks in a short period of time and dominate the sunlight, will outcompete them. Plants like shrubs and trees whose roots are not only established, but that can leaf out and block the sunlight seemingly overnight each spring, will outcompete them.
In fact, if you stop to think about it, if humans do nothing to a piece of land, the vegetation changes through the years in a fairly predictable pattern. First, the open ground gets covered with "weeds", often a mix of annual grasses and annual, broad-leafed, flowering plants. Pretty soon, a few perennials start to show up and establish themselves and, before long, there are some seedlings from woody plants beginning to grow. As the perennials and woody seedlings grow up, the "weeds" that need full sunlight and open soil, so that they can start from scratch every year, start disappearing. Eventually, the vegetation continues to change until only plants that can germinate and grow in shady, crowded conditions dominate. This entire process is called ecological succession; the final types of vegetation that will grow in an area are known as the "climax vegetation."
So what happens if the climax vegetation gets removed? Succession begins again - with the "weeds" whose seeds are easily transportable showing up. Their job, if you will, is to hold the soil in place until more permanent vegetation can take over and maintain the capital, a.k.a. the soil, that has built up over thousands of years.
The more the soil is disturbed, the more "weeds" will appear.
What we are doing when we garden or farm is to continually remove the perennial vegetation and take the soil back to its most vulnerable state - bare and open. Nature wants to stabilize that soil as soon as possible, so that the soil doesn't wash or blow away. In come the weeds.
The gardener - or farmer - cusses the weeds and clears the soil again, so that ONLY the plant he/she wants to grow will be there. But rarely is the desired plant able to fill the cleared space rapidly enough and completely enough to truly stabilize the soil. So in come the weeds again. And the cycle continues.
The moral of my tale? Understand what nature is trying to accomplish with "weeds" and plan accordingly. Learn to work WITH nature, rather than fighting against it. Mulching between plants, for example, helps tremendously because it covers (and helps stabilize) the soil. Mowing high (or not mowing at all) is likely to leave much less soil exposed and thus make an area less amenable to "weed" growth.
Why do I keep putting the word, "weed," in quotes? Because a weed is simply a plant in the wrong place. To nature, bare soil is a form of an emergency and what we humans call "weeds" are nature's emergency response team. Therefore, to nature, annuals are important...and aren't weeds at all.