Characterizing the response of weeds to canopy shade is important for improved understanding of crop–weed competition and weed population dynamics. In 2000 and 2001, field studies were conducted in central New York state to examine the influence of three neighbor types (none, broccoli, or broccoli plus winter rye) and two locations (between or within rows of broccoli) on the morphology, phenology, and seed germination characteristics of Powell amaranth. Reductions in light availability and in the ratio of red-to-far red light were associated with increases in (1) partitioning of dry weight to stem tissue, (2) stem elongation, and (3) specific leaf area. Canopy shade also resulted in fewer main leaves at flowering and a reduced rate of leaf appearance but had no effect on the number of days to flowering. The relationship between Powell amaranth fecundity and aboveground dry weight was allometric, with both parameters declining significantly under competition. The weight of seeds produced did not vary significantly according to the competitive environment experienced by the maternal parent. However, the germination percentage of viable seeds was 40 to 50% lower for seeds maturing on plants grown under competition than without competition. Reductions in the number of main leaves at flowering and increased seed dormancy may be adaptive responses to canopy shade. Both mechanistic crop–weed competition models and population dynamic models would benefit from incorporation of data on the phenotypic plasticity of morphology, phenology, and seed germination characteristics of weeds.

In shallow flooded parts of rich fens Mentha aquatica might thrive in deeper water than Epilobium hirsutum but previous experiments have provided no clear indication that the flooding tolerance of these species differs. In this study we investigated, by measuring growth, biomass allocation and vegetative reproduction, whether the impact of water level on vegetative reproduction might produce different lower boundaries on water level gradients. There was a striking contrast between biomass production at high water levels and the field distribution of both species. After 18 wk, the mean biomass of E. hirsutum grown in waterlogged and flooded conditions was 82% and 54%, respectively, of the mean biomass production of drained plants. Biomass of waterlogged and flooded M. aquatica was reduced to 57% and 37% in drained conditions. Waterlogged and flooded E. hirsutum had swollen stem bases and invested a high proportion of biomass in adventitious roots. Stems of M. aquatica did not swell, formed few adventitious roots and maintained an equal proportion of below-ground roots at all water levels. The effect of water level on vegetative reproduction corresponded well with the lower hydrological boundaries of both species. When waterlogged and flooded, most rhizomes of E. hirsutum emerged from above-ground parts of the stem base and were oriented in an upward direction. Plants in flooded soil allocated less biomass to rhizomes and also reduced the number and size of rhizomes. Rhizome formation of M. aquatica on the other hand was not directly affected by water level and only depended on plant size. These differences in vegetative reproduction are discussed in relation to the different abilities of both species to oxygenate their below-ground roots. It was concluded that the mode of adaptation to soil flooding might also affect vegetative reproduction and, therefore, a species' ability for long-term persistence in soil-flooded habitats.