Velvetleaf (Abutilon theophrasti Medik.) is a troublesome broadleaf weed that competes with crops for resources such as soil moisture. Water stress can affect the ability of weed species to grow and produce seeds. The objective of this study was to determine the effect of degree of water stress on the growth and fecundity of A. threophrasti using soil moisture sensors under greenhouse conditions. Abutilon threophrasti seeds collected from a corn (Zea mays L.)–soybean [Glycine max (L.) Merr.] field were grown in silty clay loam soil, and plants were maintained at 100%, 75%, 50%, and 25% soil field capacity (FC) corresponding to no, light, moderate, and high water-stress conditions, respectively. Water was added daily to pots based on soil moisture levels detected by a Meter Group 5TM sensor to maintain the desired water-stress level required by treatment. Plants maintained at 100% FC had the maximum number of leaves (28 leaves plant−1), followed by 21 and 15 leaves plant−1 at 75% and 50% FC, respectively. Abutilon threophrasti at 100% and 75% FC achieved maximum plant height (108 to 123 cm) compared with 83 cm at 50% FC. Abutilon threophrasti maintained at 75% FC had the greatest growth index (79,907 cm3) followed by 72,197 cm3 at 100% FC and 64,256 cm3 at 50% FC. Seed production was similar at 100%, 75%, and 50% FC (288 to 453 seeds plant−1) compared with 2 seeds plant−1 at 25% FC. This is because the majority of plants maintained at 25% FC did not survive more than 77 d after transplanting. Seed germination was 96% to 100% at 100%, 75%, and 50% FC compared with 20% germination at 25% FC. Abutilon threophrasti can survive ≥50% FC continuous water-stress conditions, although with reduced leaf number, plant height, and growth index compared with 75% and 100% FC.

Carolina redroot [Lachnanthes caroliniana (Lam.) Dandy] is a frequent weed of New Jersey cranberry (Vaccinium macrocarpon Aiton) bogs that competes with the crop for nutritional resources. Studies were conducted in 2018 to determine the effects of planting depth, soil moisture, lighting conditions, rhizome water content, and duration of rhizome submersion under water on L. caroliniana shoot emergence, vegetative growth, and rhizome development. Only planting depth greater than 12 cm significantly reduced shoot emergence (54%), biomass shoot and root production (27% and 65%, respectively), and rhizome formation (65%) compared with a 2-cm depth. Complete inhibition of new rhizome production was observed when the rhizome water content dropped to 30%. Soil moisture ≤30% decreased shoot biomass by ≥53% compared to 60% soil moisture, but marginally affected root biomass and had no impact on rhizome formation. Rhizome submersion for at least 120 d had minor effect on shoot emergence but reduced plant biomass by ≥28% and completely inhibited the formation of rhizomes. Finally, shading did not influence emergence but had a more dramatic effect on root and shoot biomass, which were reduced by 53% and 75%, respectively, and prevented the development of new rhizomes. This study demonstrates the plasticity of L. caroliniana to drought stress or long-lasting flooding conditions, therefore preventing consideration of cranberry bed temporary flooding or limitation of irrigation volume and frequency as viable management options. Sanding would not provide a layer of material sufficiently thick for reducing L. caroliniana shoot emergence. Reducing the quantity of light reaching the soil with black tarps or promoting rapid crop canopy closure are options that can complement the use of mesotrione for controlling L. caroliniana. Future research should address the practicality of these options, especially in bogs with low L. caroliniana pressure when early-summer weed regrowth occurs following dissipation of PRE herbicide activity.

Palmer amaranth (Amaranthus palmeri S. Watson) is the most problematic weed in agronomic crop production fields in the United States. The objective of this study was to determine the effect of degree of water stress on the growth and fecundity of A. palmeri using soil moisture sensors under greenhouse conditions. Two A. palmeri biotypes collected from Nebraska were grown in loam soil maintained at 100%, 75%, 50%, 25%, and 12.5% soil field capacity (FC) corresponding to no, light, moderate, high, and severe water stress levels, respectively. Water was regularly added to pots based on soil moisture levels detected by Watermark or Decagon 5TM sensors to maintain the desired water stress level. Amaranthus palmeri plants maintained at ≤25% FC did not survive more than 35 d after transplanting. Amaranthus palmeri at 100%, 75%, and 50% FC produced similar numbers of leaves (588 to 670 plant−1) based on model estimates; however, plants at 100% FC achieved a maximum height of 178 cm compared with 124 and 88 cm at 75% and 50% FC, respectively. The growth index (1.1×105 to 1.4×105 cm3 plant−1) and total leaf area (571 to 693 cm2 plant−1) were also similar at 100%, 75%, and 50% FC. Amaranthus palmeri produced similar root biomass (2.3 to 3 g plant−1) at 100%, 75%, and 50% FC compared with 0.6 to 0.7 g plant−1 at 25% and 12.5% FC, respectively. Seed production was greatest (42,000 seeds plant−1) at 100% FC compared with 75% and 50% FC (14,000 to 19,000 seeds plant−1); however, the cumulative seed germination was similar (38% to 46%) when mother plants were exposed to ≥50% FC. The results of this study show that A. palmeri can survive ≥50% FC continuous water stress conditions and can produce a significant number of seeds with no effect of on seed germination.

Dormant weed seeds are difficult to kill and, with the exception of soil sterilization methods, current weed control techniques are not effective against dormant weed seeds. This study was conducted to investigate the effects of temperature, light, soil moisture, burial depth, ethephon, and potassium nitrate (KNO3), and their interactions on the germination of false chamomile seeds under controlled conditions. False chamomile was photoblastic and thermophilic and germinated rapidly within the first 4 days of incubation under all conditions. Regression analysis indicated that ethephon and KNO3 had quadratic effects on false chamomile germination. Ethephon enhanced germination more than KNO3. There was a linear reduction in germination with increased storage time, and it is projected that zero germination should occur after 9 yr of storage at room temperature. Soil moisture had a positive linear effect and a positive interaction with ethephon on false chamomile germination. Very little germination occurred when seeds were buried 5 cm deep even with high soil moisture.