Germination of Vernonia lettermannii and Monarda bradburiana
Perennial plantings installed with sufficiently diverse number of species in high-density spacing can contribute to sustainable, low-cost solutions for public and private landscapes. However, this alternative is currently available on a restricted basis due to the lack of germination documentation on many of the hardy herbaceous perennial forbs for our region. The long-term goal of this project is to complete germination documentation of herbaceous perennial forbs suitable to be grown in USDA Cold Hardiness Zone 5 to facilitate in-situ sowing of perennials.
Rationale & Significance
In-situ sowing of perennial forbs offers many advantages over the traditional establishment of transplanted nursery-grown cells for public and private gardens. Sowing onto a bed results in a plant community that is better adapted to its microclimate after establishment. Natural selection ensures seedlings better suited to local climatic conditions are the ones to thrive and grow to full maturity, effectively eliminating replacement costs due to transplanting shock and poor establishment in subsequent years.
This method offers both reduced up-front cost at the time of installation and reduced maintenance costs over the life of the planting. Forb seed is significantly less costly than nursery-grown plugs, and installation costs for sowing are lower than planting costs as a result of reduced labor and equipment. This makes it an essential technique to increase the feasibility of densely-planted landscaping projects, such as naturalized plantings and prairie restorations.
Despite its many benefits, in-situ sowing of perennial forbs as an alternative to nursery-grown transplants is currently being hampered by lack of information. Many herbaceous forb species that grow well in Zone 5 lack germination data, making it impossible to use them in custom seed mixes.
Two forbs for which germination data is scarce have been selected for this germination study: Vernonia lettermannii and Monarda bradburiana.
Vernonia lettermannii is native to Arkansas and Oklahoma, yet it is hardy to Zone 4. It grows in the cracks of rocks in the drainage area of the Ouchita River (Vernonia lettermannii 'Iron Butterfly', 2015). It is tolerant of poor soils, drought, and occasional flooding. Although not many studies have been done on Vernonia lettermannii, the germination requirements for Vernonia fasciculata have been well documented. Since they are closely related, we anticipate V. lettermannii and V. fasciculata will have similar germination requirements.
Monarda bradburiana is native to many central states, including Iowa. It grows in open, dry, rocky upland woods, savannahs, and pastures (Bradbury's Bee Balm, 2015). This plant is very fragrant, attractive to bees, and its essential oils can be used as an organic pesticide (Rohlfsen, 2015). Although not many studies have been done on Monarda bradburiana, the germination requirements for Monarda fistulosa have been well documented. Since they are closely related, we anticipate M. bradburiana and M. fistulosa will have similar germination requirements
Objective 1: Determine germination treatment for Vernonia lettermannii.
Objective 2: Determine germination treatment for Monarda bradburiana.
By determining germination treatments of two previously unstudied perennials, we are contributing to the completion of germination documentation of herbaceous perennial forbs suitable to be grown in USDA Cold Hardiness Zone 5.
Vernonia lettermannii and Monarda bradburiana are both subject to physiological dormancy and will require stratification after imbibement to release from dormancy.
A project sample of approximately 1,000 seeds have been collected. The seeds will be mechanically cleaned, then tested for viability by Tetrazolium Test using two replications of 50 seeds each. Scarification will not be attempted prior to imbibement, since the seed is small is unlikely to suffer from physical dormancy.
The Vernonia lettermannii seed will be disinfected with 10% chlorine solution to minimize fungal and bacterial infection during germination. Monarda bradburiana will not be disinfected, as this may cause damage to the seed. Following imbibement, the seed will be divided into the following treatments:
· Group A: No stratification
· Group B: 30 days moist-cold stratification
· Group C: 60 days moist-cold stratification
Moist-cold stratification temperature will be set to 4C. Seeds not undergoing stratification treatment will be germinated at 25C.
The germination percentage of Monarda bradburiana was similar across all treatments, including the unprimed seed. The moist-cold treatment showed improved germination by 10% at 30 days, and inhibited germination by 3% at 60 days. Assuming the seed was 100% viable, this may suggest that M. bradburiana may only suffer from a slight physiological dormancy.
The treatment did seem to have an effect on the date of first recorded germination, although overall vigor was not greatly affected. Vigor at the 30-day treatment was the highest of all, coinciding with the highest germination percentage.
The vigor calculations for M. bradburiana were done in 2 day increments instead of 7 day increments. All treatments had reached their respective highest germination percentage at 7 days, which precipitated higher resolution of the time scale.
The germination percentage of V. lettermannii was similar across all treatments, including the unprimed seed. Considering the seed was tested to be only 2% viable, these results could be interpreted as close to 100% germination of viable seed across treatments. Regretfully, the sample size is not large enough to draw any conclusions from these results.
The treatment did however seem to have an effect on vigor, which was directly proportional to the duration of the stratification. Vigor doubled at 30 days, with similar results at 60 days. Vigor and germination percentage were inversely proportional, with the highest vigor coinciding with the lowest germination count.
Although this species was not submitted as part of the original project proposal, it was added after V. lettermannii came back with low viability at testing, for back-up purposes. The germination percentage of L. microcephala was highest with unprimed seed, and decreased in primed seed. Germination was low across the board. Since viability was not tested prior to treatment, it is difficult to discern whether the results were due to dormancy restrictions, poor seed quality (dead seed, diseased seed, et cetera), or damage during the seed cleaning process.
The treatment did however seem to have a slight effect on vigor, which decreased by 2 days at the 30-day treatment and increased by 3 days at the 60-day treatment. Highest vigor was at the 60-day treatment, whereas highest germination coincided with no stratification treatment.
All three species exhibited extremely low germination percentage. Seeds were harvested late in the season, between October and November of 2014. This minimizes the risk of harvesting under-developed seed.
Another aspect that could have affected seed viability is handling during storage prior to imbibement. Monarda seed was stored at room temperature. Seed heads has been somewhat wet at harvest and they were dried prior to storage in a sealed margarine container.
Vernonia seed were rescued out of a compost pile. Some had dried on the plant due to early frosts and likely did not have a chance to fully mature. This likely explains the low viability found through the Tetrazolium testing.
Liatris seed are readily dehiscing seeds and some had dispersed prior to harvest. This could indicate that harvested seed may not have fully matured or ripened at harvest. If morphological issues had been anticipated, perhaps a moist-warm treatment would have been useful prior to the cold treatment.
For future studies, TZ testing of all species should be done in order to avoid the use of extremely low viability seed in germination studies.
Bradbury's Bee Balm. (2015, January 16). Retrieved from Illinois Wilflowers: http://www.illinoiswildflowers.info/savanna/plants/bradbury_balm.htm
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Monarda Fistulosa Seeds - Mintleaf Bee Balm. (2015, January 16). Retrieved from Outsidepride.com: http://www.outsidepride.com/seed/flower-seed/bee-balm/bee-balm.html
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Vernonia lettermannii 'Iron Butterfly'. (2015, January 16). Retrieved from Northcreek Nurseries: http://www.northcreeknurseries.com/plantName/Vernonia-lettermannii-Iron-Butterfly