Weeds compete with crops for water, light, and nutrients thereby reducing crop yield and quality. Competitive effects of weeds are density dependent, with increasing weed density causing increasing crop yield loss. The magnitude of loss varies based on the crop and weed species present and the timing of competitive interactions. Carrot, beet and Alliums, for example, are small-seeded, slow to emerge, and are poorly competitive early in the growing season. Larger-seeded crops (e.g. pea, bean, and corn), tubers (potato), or transplanted crops start with a significant initial size advantage over germinating weeds. A first principle of weed management is to establish this size advantage, and then maintain it with early weeding or mulching to reduce the density of competition weeds. To this end, successful weed management requires good foundational agronomic and horticultural practices regarding seedbed preparation, fertility and moisture management, timing and density of planting, choice of variety, as well as necessary early management of insect pests and plant pathogens. Remember, in plant competition, “the big get bigger!”
Weed Biology and Ecology
Weed identification is now easier than ever, with apps like “Picture This” getting better every year. While apps still perform poorly in identifying grasses and weed seedlings, recognizing your mature weedy flora is a great place to start. Weeds of the Northeast by Uva, Neal and DiTomaso is another useful reference for weed identification.
Weedy plant species often share certain traits or characteristics that contribute to their success as the early colonists of disturbed sites, which is after all, the ecological job or “niche” of a weed. Fundamental weedy traits include rapid growth, high amount of seed production, and most notably, seed/bud dormancy to ensure offspring germinate and attempt to grow over several future years-an impressive bet-hedging strategy. Weedy species are also known for their “plasticity,” in other words, genotypic flexibility whereby an individual may exhibit different morphology in response to its environment (e.g., bushy growth in full sun but erect growth in a dense crop).
Annual plants complete their life cycle in one year: seeds germinate, seedlings grow to maturity, flower, and reproduce all within a single growing season. Winter annuals germinate in late summer or fall, overwinter, and set seed the following spring. Summer annuals germinate in spring or early summer and set seed before fall, often in response to shorter days. Timing of germination and thus emergence is affected by species specific dormancy characteristics and environmental conditions including light quality (e.g., affecting phytochrome), temperature, moisture, gas exchange, nitrate, among many other factors. These in turn are dramatically affected by soil disturbance (tillage or cultivation). Thus, it should be no surprise that annual weeds dominate the weedy flora of vegetable farms that rely on soil disturbance for residue management, seedbed preparation, amendment incorporation and weed control.
Weed seedbanks vary widely across farms. The seedbank refers to weed seeds on the soil surface or buried in the soil. Generally, seedbanks are larger on organic farms compared to farms using herbicides. Larger seedbanks result in a higher density of weed seedlings, or greater “weed pressure” as described by some farmers. The “bank” metaphor is useful in thinking about management, specifically the aim to reduce “credits” to the bank, i.e., avoid or reduce weed seed rain, while encouraging “debits” or losses. A common misperception is that weed seeds last “forever” in the soil, so “why bother to manage the seedbank?” While it is true that a seed may occasionally last many years, perhaps decades, by far, most seeds germinate or die in the first year. In fact, many annual weed seeds have a half-life of less than one year. In other words, 50% of the seeds produced this season will be gone in less than a year. This is true for several important weeds in our region, including, redroot pigweed, common lambsquarters, hairy galinsoga, yellow foxtail, and crabgrasses.
Vegetable farmers are fortunate in having many options related to weed seedbank management. Consider a field that will be used for a sequence of short season crops like radish or leafy greens. Shallow tillage for seedbed preparation in this case serves to encourage weed germination, the most important of seedbank debiting mechanisms. This way, crop harvest can be completed and residues incorporated before weeds mature, thus preempting seed rain.
Emergence periodicity is a useful aspect of weed biology that can be used to optimize fallowing or stale seedbed events that aim to reduce the germinable weed seedbank. Shallow tillage breaks seasonal dormancy for species ready to germinate. Thus, if targeting summer annuals, shallow tillage in June or July will encourage germination, while winter annual species will remain in the seedbank.
Perennial weeds can live for more than one year and while most produce seed, vegetative propagation by stolons, rhizomes, or roots are generally more important. Tillage operations often drag perennial weed fragments from sod headlands or farm roads into vegetable fields. Shallow tillage around field margins throughout the growing season can establish a fallow zone to avoid this dispersal from tillage.
Monitoring weeds is an important but often neglected part of a weed management plan. Weed maps of field areas are extremely helpful in planning weed control strategies. A weed map can illustrate problem areas so that growers can target specific problems in specific areas and help plan for future crop rotations. Over time, weed maps can show shifts in weed pressure and indicate the possible need for a strategy change. Maps can also highlight the importance of managing dispersal, e.g., avoiding tillage that drags quackgrass rhizomes from grass alleys into fields, or working around a weed patch to avoid tillage dispersal.
Dispersal is critical to the success of weeds, but rarely a priority for management. It is widely thought that weed seeds are blowing in from neighbors, hitching rides from birds or mammals, or washing in with irrigation or surface waters. These are important dispersal mechanisms, but they are rare events, generally moving only a very small fraction of a batch of seeds. In fact, well over half of most seeds are dispersed right around the base of the mother plant, and most remaining seeds not much further. In a natural setting, these seeds would be very crowded, with intense competition among the weeds when a cohort germinates the following year. In a farmed system, however, tillage events serve to disperse the seeds, spreading them out to the benefit of the weed. These local dispersal factors drive the patch dynamics that will be evident in your weed maps, with dense areas where reproduction was high, lower density radiating from the patch where tillage moved seeds, and then many other locations where weeds may be absent.