Large crabgrass [Digitaria sanguinalis (L.) Scop.] (Poaceae), Virginia buttonweed (Diodia virginiana L.) (Rubiaceae), and cock’s-comb kyllinga (Kyllinga squamulata Thonn. ex Vahl) (Cyperaceae) are common problematic weeds in turfgrass and crop production systems in the Southeast. Large crabgrass and cocks’s-comb kyllinga are prolific seed-producing summer annuals (Lowe et al., 1999; McCarty et al., 2001). Virginia buttonweed, a perennial dicot, can produce abundant fruit on branched stems reaching 100 cm in length (Baird and Dickens, 1991). These weed species inhabit landscapes encompassing many different soil types and textures.
Weed seed germination and emergence depend on many factors, including species, light, temperature, pH, osmotic stress, and oxygen (Baird and Dickens, 1991; Benvenuti, 2003; Benvenuti et al., 2001; Chase et al., 1999; Chauhan and Johnson, 2008; Lowe et al., 1999; Oliveria and Norsworthy, 2006; Woolley and Stoller, 1978). After germination, burial depth, crop residue, soil air permeability, soil aggregation, and seed carbohydrate reserves resulting from seed size can influence weed emergence (Benvenuti, 2003; Chauhan and Johnson, 2008).
Soil texture and seed burial depth reportedly influence weed emergence. Porosity is associated with soil texture (Radford and Greenwood, 1970) and can affect germination and emergence resulting from diffusion of oxygen and volatile toxic metabolites contained in soil air space (Holm, 1972; Norton, 1986). Sandy soils have greater pore space resulting from increased particle size (Brady and Weil, 2002); therefore, gas diffusion may be increased. This allows for increased emergence in a predominantly sandy soil compared with a soil with higher clay content [e.g., jimsonweed (Datura stramonium L.)] (Benvenuti, 2003). Increased planting depth can also affect germination and emergence (Benvenuti and Macchia, 1995). Large crabgrass emerged when seeds were present within 8 cm of the soil surface. Approximately 60% of large crabgrass populations emerged when buried at 2 cm, whereas less than 10% emerged from a depth of 6 cm (Benvenuti et al., 2001). Similar to large crabgrass, Virginia buttonweed emergence 14 d after planting occurred from planting depths of 8 cm, but maximum emergence (40%) was found at a depth of 2 cm (Baird and Dickens, 1991). Green kyllinga (Kyllinga brevifolia Rottb.) is also greatly inhibited by deeper planting depths, ceasing emergence at soil depths of 2 cm or greater (Molin et al., 1997).
Emergence of weed seeds at the soil surface also varies across species. Large crabgrass, buckhorn plantain (Plantago lanceolata L.), giant foxtail (Setaria faberi Herrm.), and green kyllinga all had lower emergence at the surface when compared with emergence from buried seed (Benvenuti et al., 2001; Fausey and Renner, 1997; Molin et al., 1997). In contrast, redroot pigweed (Amarathus retroflexus L.), wild mustard [Brassica kaber (DC.) L.C. Wheeler], and black nightshade (Solanium nigrum L.) surface emergence was greater than when these weed species were buried (Benvenuti et al., 2001). Such differences can be attributed to physiological characteristics including seed carbohydrate reserves (Benvenuti, 2003; Chauhan and Johnson, 2008), seedcoat dormancy (Buhler et al., 1997), water-imbibing requirements (Wilson and Witkowski, 1998), and inhibitory substances (Baskin and Quarterman, 1969).
As a result of physiological differences among weed species, previous research evaluated emergence of various species separately. Therefore, interactions between weed species could not be determined. Standardizing the emergence response of multiple weed species to a selected planting depth will allow for comparison across weed species and botanical families to better determine if interactions exist among weed species, planting depth, and soil texture. Therefore, research was conducted to determine the effects of soil texture and burial depth on emergence of large crabgrass, cock’s-comb kyllinga, and Virginia buttonweed.
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