Switchgrass - Bioenergy Crop Plants - page 571

and Masters 2001). Establishment-year stands of 10 plants/m 2 are adequate but will require one
or more years to achieve full production yields. Stands of less than 10 plants/m 2 may need to be
overseeded or re-seeded. An on-farm study indicated that establishment-year stands of 40% or
greater, determined by a frequency grid, can be considered adequate for successful establishment
(Schmer et al. 2006).
The minimal germination temperature for switchgrass is 10°C (Hsu et al. 1985a). Temperature-
gradient-table studies with several switchgrass cultivars and seedlots demonstrated that near-
maximal germination was obtained from 19 to 36°C and optimal germination was between 27 and
30°C (Dierberger 1991). Optimum germination temperatures for switchgrass may be lower than
those for seedling development (Panciera and Jung, 1984). Seedling growth of switchgrass at 20°C
is much slower than at 25 or 30°C (Hsu et al. 1985b). Although seedlings develop slowly, planting in
early spring may be advantageous even though the soil is cold if the seed lot being used has dormant
seed. The cold soil may aid in breaking dormancy. Best stands in Iowa were obtained when planted
at early to mid-spring (Vassey et al. 1985). In the northeastern United States, a planting window
of 3 weeks before and 3 weeks after the recommended maize planting date has been suggested
(Panciera and Jung 1984). This general guideline for time of planting would be suitable in most
areas where switchgrass is adapted. In some areas “dormant plantings” are made very late in the fall,
late enough that the seed will not germinate. The seed remains dormant during winter and the cool
moist spring conditions result in a natural cold stratification. Switchgrass should not be planted in
late summer because it may not have time to develop sufficient cold tolerance before onset of winter.
Planting seed too deeply often leads to seeding failures with switchgrass and other small seeded
warm-season grasses. Switchgrass requires a firm seedbed, allowing the drill to plant the seed
approximately 1-2 cm deep. No-till seeding into crop residues or chemically killed sods is often
very effective (Samson and Moser 1982). Corrective applications of P or K should be made before
seeding but N applications are generally not made until the grass is established because it will
stimulate excessive weed growth during the seeding year.
Physiological seed dormancy of some cultivars and seedlots of switchgrass can result in seeding
failure. Although alive, dormant seed will not germinate under normally suitable conditions. Simple
dormancy will be broken if the seed is aged long enough or if it is given cold treatments or cold
stratified to break dormancy (Zheng-Xing et al. 2001). The normal germination test carried out
according to Association of Official Seed Analysts procedures (AOSA 1988) includes a period of
cold stratification where seed are allowed to imbibe water and are chilled at 4°C for 2-4 weeks to
break dormancy. The germination percentage on the seed tag represents the percentage of viable
seed but does not represent the actual amount of seed that will germinate upon planting because
of dormancy. Producers should conduct a germination test without chilling if they suspect dormant
seed and want to determine the percentage of seed that will germinate when planted. With time,
much of the dormancy will be broken if seed is stored for 1 year at room temperatures. Seed stored
for 3 or more years at room temperature may result in poor stands due to decreased vigor (Vogel
2002). Dormancy of switchgrass seed can be broken by stratification by wet chilling, but drying
can cause some of the seed to revert to a dormant condition (Zhang-Xing et al. 2001). Extended
stratification (>42 days) significantly reduced the percentage of switchgrass seed that reverted to a
dormant condition after drying (Zhang-Xing et al. 2001). It must be emphasized that switchgrass
seed should have high germination (>75%) and should not be older than 3 years to ensure successful
establishment. Old seed can have good laboratory germination but may have poor seedling vigor
and fail to produce acceptable stands under field conditions.
Variation exists among and within cultivars for seed size. Smart and Moser (1999) graded
switchgrass seed into lots differing in seed weight and evaluated the seed lots in field plantings.
Seedlings from the heavy seed had greater germination, earlier shoot and adventitious root growth
than seedlings from light seed but growth and development were similar 8-10 weeks after emergence.
Weed competition is one of the major reasons for stand failure of switchgrass. Seedlings do not
develop rapidly until conditions are warm, which is also the same time that annual weeds develop.