Seeding Rate

Seeding rate recommendations for establishing switchgrass vary widely – from 2 pounds PLS/acre up to 10 pounds PLS/acre. The large differences in the recommendations can be attributed to seed quality, planting method, and environment. A goal for switchgrass establishment is to plant at least 20 PLS/ square foot. To accomplish this goal, common seeding rate recommendations throughout the U.S., including the southern Great Plains, are 5 pounds PLS/acre for switchgrass. However, few recommendations have made any attempt to account for seed dormancy.

Seed dormancy is a significant issue to consider before planting switchgrass. Like most perennial grass seed, switchgrass seed exhibits varying degrees of dormancy that decreases as the seed ages. For example, seed less than one year old may have seed dormancy greater than 60 percent; whereas, the same lot of seed between one and two years old may have an insignificant amount of dormant seed.

Four options for deciding on the amount of switchgrass seed to plant are described, and all are related to managing seed dormancy at planting. None of the following establishment options precludes a reliable seed germination test or proper seedbed preparation. The recommended seeding rate using options 1, 2 and 3 is 5 pounds PLS/acre.

Establish switchgrass plot near Stillwater, Oklahoma.

Option 1 is to plant seed that has been in storage for at least a year. The rationale is since seed dormancy declines over time, the older, dormant seed will usually germinate the following year. However, the seed that were germinable earlier may not germinate now. This option would only be recommended for seed with a high ratio of dormant seed to germinable seed. Additionally, it would be necessary to have adequate cool, dry storage space available.

Option 2 is a combination of wet-cold stratification of the seed prior to planting. This method requires adequate cold storage. The rationale is the combination of cold temperatures and elevated relative humidity (40° F at 50 percent relative humidity) reduces seed dormancy. However, wet-cold stratification is a somewhat risky process. Seed is placed in porous sacks (not water tight) and submerged up to 24 hours in water. Excess water is allowed to drain, and the seed is placed in cool storage (40° F at 50 percent relative humidity) for two weeks until planting. The primary drawback to using this option is that once it has been initiated, it is necessary to carry the process completely through to planting. At this point, a final word of caution is necessary. Seed that is allowed to dry out following wet-cold stratification may have a higher amount of dormant seed present than before.

Option 3 is a dormant season planting. The rationale for using this option would be similar to the cold-wet stratification except the natural conditions would reduce the dormancy. Planting time of this option would normally occur during winter months, usually in January or early February. Under most conditions during this time, seed will not germinate, but they will be ready to germinate as soon as soil temperatures are appropriate. Essentially they will be moist and stored in a cold soil.

Alamo switchgrass seedling emerging
in somewhat dry conditions.

Option 4 is the establishment based solely on germinable seed. Depending on the seed test, the seeding rate may be different than the recommended 5 pounds PLS/acre. Planting more seed than needed may make this practice cost prohibitive. This option may be best to consider with seed that has a high germination (greater than 60 percent) and low dormancy (less than 20 percent).

Seed lots with the same percent PLS may be very different. Even though the dormant seed is alive, it will not germinate quickly. Only the germinable seed will germinate in the near future. It is possible to have seed with high germination and low dormancy or low germination and high dormancy (and a similar PLS) that will behave differently under field conditions due to the number of germinable seeds per pound. Based on this, it may be possible to reduce the seeding rate for switchgrass with a high ratio of germinable to dormant seed.

The calculated seeding rate for switchgrass on a 5 lbs PLS/acre basis, has a large effect on the number of viable seeds planted per square foot (16 vs. 29 in the first table). Using the germinable seed per square foot as a basis has a sizeable effect on seeding rate (7.1 vs 3.9 in the second table). Even though the overall effect of seedling density appears small, it may be possible to reduce the seeding rate by at least 1 lb/acre with switchgrass seed that has a higher germination and lower dormancy. To have 20 seeds/sq. ft. that will germinate quickly, very different amounts of bulk seed is required (7.1 lbs/acre at 35 percent germination vs. 3.9 lbs/ acre at 65 percent germination).

Comparison of switchgrass seed lots on a pure live
seed basis - 5 lbs PLS results in different potential
stands depending on the percent quick germination

Comparison of switchgrass seed lots on a germinable
seed basis - results in a different amount of bulk seed
needed to have 20 plants/sq. ft.