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Tillage prompts germination

Most weed species have very small seeds. Hence the newly emerged seedlings are tiny and incapable of competing with established vegetation. Consequently, these species have been naturally selected to respond to environmental cues that indicate the absence of competing vegetation. In natural situations, vegetation is usually only absent if the soil has been recently disturbed. Thus, weeds often respond to cues associated with soil disturbance (e.g., tillage). These cues include light, and especially red light (in contrast to the green light that passes through a plant canopy), fluctuation in temperature between day and night, high soil temperatures, the absence of volatile substances released by anaerobic metabolism, and the presence of nitrate in soil water. High light levels at the soil surface occur when competing vegetation is absent. Similarly, high soil temperatures and large day to night fluctuations in soil temperature occur when the soil is exposed to direct sunlight by day and radiative cooling to the sky at night. Turning the soil vents ethanol, aldehydes and other substances that accumulate within soil particles due to insufficient oxygen for complete digestion of carbohydrates by seeds, roots and microbes. Finally, the increase in warmth and oxygen associated with tillage stimulates microbes to consume organic matter and thereby release nitrogen containing compounds that other microbes turn into free nitrate ions.

Many species respond to several of the germination cues discussed above. Thus, for example, a few lambsquarters seeds will germinate in the dark at constant temperature and no nitrate. More will germinate if any one of these three cues are present, and most will germinate if all three cues are present.

As a result of the changes in soil properties following tillage, many weed seeds are able to detect (i) that they are near the soil surface, and (ii) that competing vegetation and dead organic materials have been removed. Consequently, a flush of germination usually follows tillage provided the soil is moist enough for seed germination.