SEEDING RATES
Sowing the proper number of prairie seeds is critical to planting success. Planting too few seeds will result in a weedy plant community that will eventually be overtaken by non-native plant species; planting too many seeds will be costly and not result in a significant increase in prairie plant establishment.
Seeding rates can be separated into three groups; overall seeding rate for the seed mix, seeding rates for plant guilds, and seeding rates for individual species. Other factors influence seeding rates too, including the following: the kind of seeding equipment used for the project, the time of year the site is planted, and whether or not the site is prone to erosion. Determining the best combination of these factors can be a challenge.
The Iowa Prairie Seed Calculator will automatically adjust seeding rates for all these factors when your planting information is entered on the front page. To help you understand how these factors can impact a successful planting please follow the links on the side bar menu.
Overall seeding rate – Prior to seeding calculators, seed mixes were developed by using rough measurements like pounds per acre. Today, seed calculators allow us to be very precise and calculate the exact number of seeds per square foot. Here in Iowa, every prairie planting should have at least 40 Pure Live Seeds (PLS) sowed per square foot (Iowa NRCS 2011). Research has shown that native plant emergence for most prairie plantings is between 5% and 10% (Williams et al. 2007). If we assume that emergence is 5%, sowing 40 seeds per square foot sowed would produce 2 seedlings per square foot (40 x 0.05). A prairie planting needs at least one prairie seedling per square foot to develop into a successful native stand (Morgan et al. 1995). We are often asked what happens to the other ninety percent. Many seeds are eaten by granivores (mice, birds, insects) but other factors play a role as well, including seed loss because of too-deep planting, soil pathogens that kill the seed, seed blown or washed from the site by wind or water erosion and seed dormancy (Henderson and Kern 1999).
Plant Guild seeding rate – A high quality seed mix should contain species from each plant guild. A guild is a group of prairie species that share similar growth characteristics that include: C4 warm-season grasses, C3 cool-season grasses, sedges, legume forbs and non-legume forbs. Including all the guilds in a prairie seed mix will ensure the resulting stand will be diverse, weed resistant, and provide long-term plant community sustainability. There is an old saying that states, “what you sow is what will grow.” We believe this. A seed mix containing more grass than forb seed will result in a grass-dominated stand. Likewise, a mix containing more forb than grass seed will result in a forb-dominated stand. To maximize plant diversity of both grasses and forbs in the stand, 50% of the total seeds should be made up of grasses and sedges and 50% of the seed mix should include legume and non-legume forbs (Williams 2007). So if you plant the recommended 40 seeds per square foot; 20 of those seeds should be grasses and sedges, and 20 seeds should be forbs. For more information on seeding rates by guilds, see Table I.
Individual Species seeding rate – Seeding rates of individual species can be influenced by many factors, including the cost of the seed, how well a particular species can be established from seed, and both field and published research associated with prairie restoration and reconstruction. The methods used by this calculator were informed by all of these factors. Compassplant (Silphium laciniatum) and prairie blazingstar (Liatris pycnostachya) offer a constructive example of the process. We begin by calculating the seed cost of a species if it were sowed at one seed per square foot. To do this we need to determine how many compassplant seeds are in an ounce. Compassplant has 660 seeds in an ounce (Prairie Moon Nursery 2015). An acre has 43560 square feet. Planting one seed per square foot of compassplant for an acre will require 66 ounces of seed (43560/660). Average 2015 retail cost for compassplant is $6.52 per ounce. The total cost to sow one seed per square foot of compassplant would be $430.00 (66 ounces needed x $6.52 average 2015 retail cost). Clearly, $430.00 for one species in the seed mix exceeds any reasonable seed budget. Second, we determine how well the species establishes from seed. Newly planted prairies and greenhouse propagation for compassplant have shown that compassplant readily establishes from seed. If we assume that only 5% (see overall seeding rate section) of compassplant seeds will establish in the stand, compassplant sowed at a rate of 2 seeds per 100 square feet would be 871 live seeds sowed per acre (43560/100 x 2 seeds). This seeding rate would potentially produce 44 plants per acre (871 seeds x 0.05). Compassplant seed sowed at this rate would cost $8.60/acre (871 seeds/660 seeds per ounce X $6.52 average retail seed cost per ounce). We believe that potentially having 44 compassplants in an acre prairie planting is a reasonable starting point for a prairie restoration. For these reasons, compassplant is included at a lower seeding rate than other species in the seed mix.
Prairie blazingstar offers another example of how a combination of factors will affect seeding rate recommendations. Prairie blazingstar has 11,000 seeds in an ounce (Prairie Moon Nursery 2015). Planting one seed per square foot of prairie blazingstar for an acre will require 4 ounces of seed (43560/11000). Average 2015 retail cost for prairie blazingstar is $14.01 per ounce. The total cost to sow one seed per square foot of prairie blazingstar would be $56.00 (4 ounces needed x $14.01 average 2015 retail seed cost per ounce). In a diverse seed mix that includes 40 or 50 species, spending $56.00 on prairie blazingstar per acre would mean having to greatly reduce the seed cost of many other species in the seed mix. To make this species affordable so it can be included in a diverse seed mix, the cost of prairie blazingstar seed will have to be in the range of $10.00 per acre. Ten dollars worth of prairie blazingstar seed equates to 0.71 ounces or 7852 seeds per acre ($10.00 seed needed per acre/$14.01 average seed cost per ounce x 11,000 seeds per ounce). Unfortunately, prairie blazingstar typically establishes poorly (less than 1.0% emergence). The best one could expect from sowing 7852 prairie blazingstar seeds per acre would be 79 plants (7852 seeds x 0.01 emergence). Many resource managers would love to have more prairie blazingstar plants in an acre of planted prairie but the financial reality is that if more of these plants are desired, then one would have to accept fewer plants of other species in the seed mix. This process of comparing seed cost to potential emergence and its affordability in a diverse seed mix has been done for every species that we recommended in the seed calculator. The advantage with the seed calculator is that you can see what we recommend and then make your own choices based on your seed budget and species preference.
Sowing time and seeding rates – The time of year a prairie is sowed will have a significant impact on the plant composition in the stand. Sowing a diverse seed mix in the dormant season (fall to late winter or when soil temperatures are 38 degrees or lower) favors forb establishment. In contrast, a non-dormant planting when soil temperature are 50 degrees and above (early to mid-spring) favor cool-season grass and sedge establishment. Late-spring plantings favor establishment of warm-season grasses and some legumes. In northern Iowa, dormant planting is generally defined as occurring from Nov 01 – March 31 and non-dormant plantings occur between April 01 and June 15. Dormant planting favors forbs by vernalizing the seed (periods of freezing/thawing) which permit germination. Dormant planting a diverse seed mix will result in a stand dominated by forb plants; grasses (in particular warm-season grasses) don’t do as well. We don’t know exactly why some grasses establish poorly in a dormant seeding but to compensate for seed loss, we double the seeding rate of big bluestem (Andropogon gerardii), little bluestem (Schizachyrium scoparium), side-oats grama (Bouteloua curtipendula) , and indiangrass (Sorghastrum nutans) for dormant plantings. The seed calculator automatically adjusts the seeding rate of these species if dormant seeding is indicated. As you might expect, forbs that require vernalization will have lower germination when planted in the non-dormant time of year. So this same diverse seed mix planted in mid to late spring will have fewer forbs and more grasses in the stand. Note: The seed calculator does not adjust the forb seeding rates for spring plantings except in cases when seed is broadcast sowed (see planting methods section).
Planting Methods and seeding rates – There are three ways to sow prairie seed: broadcast seeding, hydroseeding (also considered a form of broadcast seeding), and drill seeding. Regardless of the seeding method used, it is essential that seed be planted at the proper depth (1/8” – 1/4”) and promote good seed-to-soil contact. Broadcast seeding is the least exact form of seeding in terms of promoting proper seed depth and seed to soil contact. The seed calculator will automatically increase the seeding rate for all species by 30% in your seed mix if seed is broadcast sowed.