Seedbed preparation is the next step following site selection. The goal prior to seeding is to have an environment that optimizes seed germination and seedling establishment. Ideal seedbeds are very firm below planting depth, have friable surface soil, and are free from competition with resident vegetation and weed seeds (Vallentine 1989). These ends can be achieved with both conventionally tilled and no-till systems.

Having clean fields with minimal weed competition can be a major factor for successful switchgrass establishment and the amount of residue also can be a factor in timing of planting. Zarnstorff (1990) reported greater stand success with later seeding dates when sowing into rye (Secale cereale L.) stubble and that shorter stubble heights supported increased seedling numbers. Excess herbage residue on fields can hinder seed placement, thus preventing proper soil-seed contact (Wolf and Fiske 2009). As seen with other crops, residues can also provide safe haven for slugs and other pests that prey on the emerging seedlings (Hammond 1996; Luna and Staben 2002; Vernava et al. 2004).

Tillage can be an effective method of seedbed preparation and residue removal, although tilling can be more expensive and has potential to expose susceptible sites to erosion. Because firm seedbeds are essential for switchgrass establishment, it is imperative that tilled fields be firmly packed at or just prior to seeding. Tillage also has the potential both to kill weed seedlings and to free weed seeds for germination; thus, weed conditions and management must be carefully considered with tillage.

No-till methods can be quite effective for switchgrass establishment. No-till systems conserve soil moisture, minimize soil erosion, require less fuel, and allow earlier entry of equipment into fields following precipitation events (Parrish and Fike 2005; Douglas et al. 2009). While no-till establishment has several advantages, residue management can be a prime concern given the issues of seed placement and pest habitat mentioned above.

Strategies of residue removal include harvest, or chemical burn followed by sufficient time to degrade the residue. Growing glyphosate-resistant crops such as soybeans on the site prior to switchgrass establishment can be an effective strategy for seedbed preparation. Glyphosate applications can decrease weed burdens, and, following harvest, the resultant stubble makes a suitable seedbed for planting switchgrass. However, caution should be used if prior production practices have utilized persistent herbicides that can prevent seed germination and growth (Douglas et al. 2009).

Burning field residues can also be an effective residue removal method for both tilled and no-till systems. In addition to removing crop residues, burns can kill small weeds and pests and reduce the size of the soil weed seed bank, thus decreasing competition for new seedlings (Wolf and Fiske

2009) . Burning may be especially useful for converting old pastures or abandoned field sites with large weed burdens, but, success is predicated on the temperature and speed of the burn.

At planting, an ideal seedbed—whether prepared with or without tillage—will enable placement of the seed at the proper depth (discussed in the following section) and in firm contact with the soil. This requires appropriate levels of soil compaction, which ensures rapid movement of water from the soil to the seed/seedling by improving capillary water flow. Increased moisture availability increases the likelihood of rapid, uniform germination, early seedling growth, and successful stand establishment (Bartholomew 2005). Too much compaction, however, can restrict the ability of seedlings and their roots to penetrate through the soil. Hudspeth and Taylor (1961) reported that switchgrass was able to germinate and emerge from 8 cm depth in loose soil, but only 10% of seeds emerged when compaction was 6.9 kPa and no seedlings emerged with pressure of 69 kPa. Too much compaction also affects oxygen diffusion, soil temperature, and light penetration, all of which influence germination and emergence (Hudspeth and Taylor 1961).

In contrast to over compacted soils, poor soil contact resulting from cloddy or loose soil or from excess residue can slow seed germination creating conditions for uneven emergence and subsequent seedling desiccation. Loose soils can also contribute to too great a seed depth when rains cause seed depths to be greater than ideal (Fig. 2). Such effects limit emergence and can lead to problems such as weed competition during the early establishment phase (Hall and Vough 2007).

Planting Depths

Proper planting depth is critical to successful switchgrass establishment, and many stand failures have occurred because seed placement was too deep. Appropriate depth maximizes emergence and seedling growth, and as a general recommendation, proper seed placement is difficult to regulate unless the seedbed is firm enough to prevent placing the seeds too deep (Masters et al. 2004), either directly or by soil washing into the planting furrow (Fig. 2). Typically seed should be covered with enough soil to maintain moist conditions for germination, but not so deep that the shoot cannot reach the surface (Zhang and Maun 1990; Roundy et al. 1993; Cosgrove and Collins 2003).

Although 1.5 cm is a common lower limit for planting depth, emergence from greater depth is possible (Zhang and Maun 1990). The ideal depth depends on soil texture and other soil physical properties (Aiken and Springer 1995; Evers and Parsons 2003), and deeper plantings are recommended for arid environments or on sandy soils where moisture limitations can slow imbibition, germination, and emergence (Newman and Moser 1988; Evers and Butler 2000; Evers and Parsons 2003).

Just as planting too deep is problematic, shallow plantings also can be the cause of stand failure. Shallow seed placement under drying conditions can cause seedlings to desiccate and die before they become established (Cosgrove and Collins 2003). This is especially so with bare soils which lose water more rapidly than those protected by litter (Winkel et al. 1991). Adventitious root formation may be compromised with shallow plantings —and the adventitious roots are the only roots that matter for the plant’s long-term survival (Parrish and Fike 2005).

Seed size can affect the appropriate seeding depth for many species, but the data on switchgrass seed size and germination, while suggestive of greater success with larger seeds, are not definitive. Larger seeds support more rapid germination and emergence (Aiken and Springer 1995) and more rapid adventitious root development (Smart and Moser 1999). However, these early advantages appear to be lost over time (Zhang and Maun 1990). Whether seed size affects competitive responses with weeds has not been definitively tested.