Weeds are commonly referred to as "plants out of place." Any plant can be a weed when it is growing where it shouldn't — even the most revered native plant.
To understand weeds we must understand their role. They heal out-of-balance ecologies and push succession forward. They are "nature's Band-Aids." Early soils are shallow, low in organic matter and typically have a high pH. The only organisms that grow well are bacteria.
As soils weather, annuals arrive. They put all their energy into quick growth and seed production. The grasses generate large amounts of organic matter and the broad-leaved plants send down taproots to help deepen the soils and bring up mineral nutrients. Annuals grow and die, season after season, working to reduce erosion, build up organic matter, establish an organic mulch layer and deepen the root zone.
Leguminous plants that have a symbiotic relationship with rhizobia bacteria also grow in these early soils. They are able to fix atmospheric nitrogen into a plant-available form and quickly add to the soil's fertility.
As the amount of organic matter increases in early soils, saprophytic fungi, the decomposers, start to grow. Humic acids are developed. The pH of the soil starts to decrease. Over time, perennials are able to take root and later, as the process progresses, woody plants. Eventually other soil organisms proliferate, including the mycorrhizae fungi that assist perennials and woody plants in symbiotic relationships. As these changes in the ecology occur, the early annuals and legumes are seen less and less. They are no longer needed.
But they do not disappear completely. They have evolved to have seeds that are viable for decades and even centuries. They're dormant in the soil, waiting until they are needed. When there is a catastrophic event such as a fire or a flood, they are there — the first responders to quickly do their job. They stop erosion and prepare the soil for later plants. When their job is done they again go into dormancy.
But what if the catastrophic ecological disaster is manmade? What if the land is cleared for a road, a subdivision or a shopping center? What if salty synthetic fertilizers and toxic pesticides are applied to the soil?
What happens is that these same early responders quickly germinate to heal the situation. But now they are in the landscape and gardens, unwanted, and have become weeds.
It is sad that the same plants that have evolved for millions of years to fix ecologies are considered weeds in the garden setting. It is even sadder to see the typical reaction, killing them with herbicides, further degrading the ecology they were there to heal.
Because of their tenacity and the long viability of their seeds, this cycle can go on for years while the collateral damage of the chemical controls continue to wreck havoc on the environment.
Note that in a natural setting, when an ecology reaches a certain successional trophic level, the weeds are no longer needed and they go into dormancy, awaiting the next disturbance. The gardener's goal is to act to help them quickly push succession forward by mimicking their results so that they are no longer needed. This is accomplished by creating a condition where the soil's organic matter, the mulch layer, the soil's microorganisms and the pH of the soil are far enough along that the first responders are not needed and their seeds remain dormant.
Establishing a healthy layer on top of the soil is the first step. This can be accomplished by adding a layer of organic mulch over a layer of compost. These layers can then be maintained by allowing leaves and plant debris to compost in place or by adding subsequent layers of organic mulch as it decomposes and is incorporated into the soil. Topdressing fine compost on turf areas and ground covers can help accomplish the same task.
Sheet mulching is an important tool for the gardener to be able to quickly smother perennial weeds and interrupt the annual weed seed cycle. Sheet mulching adds a layer of recycled cardboard to these layers and accomplishes what would take "nature's Band-Aids" years to do.
Niche management or weed displacement is another tool. Keeping a habitat niche occupied by healthy plants that compete for sun, water and nutrients makes weeds unnecessary. Maintaining this new homeostasis by promoting the creation of humus, soil biodiversity and niche occupancy is the key to holistic weed control and reducing the use of herbicides.