Why do weeds decrease productivity? Well, they are the greedy kids that grab the most M&M's at snack time. They are better at using available resources quickly. Resources for plants include sunlight, water and soil nutrients. Weeds out-compete crop plants for some or all of those resources. Crop plants are usually somewhat wimpy competitors. Plant breeders have selected for tender, juicy fruits or pretty flours or large seeds, which often come at the expense of defensive or offensive plant growth strategies.
Our hours of weeding enabled us to become intimately familiar with three weeds: crab grass (3), yellow wood sorrel (4) and spiny amaranth (5). Oddly enough, each of these weeds has a different strategy for success as a weed.
Most of us know crab grass, even if we don't realize it. Crab grass is a vicious, ubiquitous, nonnative weed found throughout most of North America. It is a weed of agriculture, lawns, sidewalks, and vacant lots. Crab grass's strategy is to dominate territory throughout the summer. It puts as much energy into roots as it does leaves as it develops its death grip on the soil. It is an annual that leaves holes in lawns when it dies back in the Fall after it has deprived all neighboring plants from all necessary resources. Removal of crabgrass requires tough hands and strong muscles and also a strong tolerance of soil disturbance by neighboring plants as the crabgrass clings to the soil even as it leaves the earth.
Wood sorrel is ironically the sweetest of the three weeds to pull. While it would taste sour due to its oxalic acid content, it comes out of the ground as smooth as honey - a great relief to a weeder who is fed up with crab grass. Wood sorrel grows quickly, and it puts little energy into maintaining its turf. It just needs a little patch of soil to get in, grow some seeds and get out. It is done with its life cycle early in the summer.
Spiny amaranth blends strategies of the other two plants and adds its own twist to the mix. It grows quickly but also holds on tightly to the soil. For a little flourish, it grows sharp spines all along it's stems discouraging predators and pullers.
From an evolutionary perspective, each of these plants has found a successful strategy. Their growth patterns yielded plants that could use available resources to produce offspring with similar characteristics to the parent plants. Each survives in a similar habitat with a different answer the the ultimate biological question, "How do I survive and pass on my DNA?". After yesterday, at least a few of those weeds lost the evolutionary battle to their wimpier onion competitors. Onions, though, have their own secret weapon. They have managed to be useful and appealing enough to humans that humans are willing to fight off their competitors for them!
David W. Monks and Jonathan R. Schultheis, 1998. Critical Weed-Free Period for Large Crabgrass (Digitaria sanguinalis) in Transplanted Watermelon (Citrullus lanatus)Weed Science. Vol. 46, No. 5 (Sep. - Oct., 1998), pp. 530-532
(2) A hectare is about 2.5 acres, which would take about 5 hours to mow with a push mower. No breaks.