More info for the terms: cohort, cover, density, fitness, frequency, litter, natural, vernalization
Common mullein reproduces entirely by seed and has no means of vegetative regeneration [48].
Pollination and breeding system: Self and cross pollination of common mullein flowers are both possible. If by the end of the day an open flower has not been visited by a pollinator, it is self pollinated ("delayed selfing") [10,52]. While common mullein flowers are visited by a variety of insects, only short- and long-tongued bees are effective pollinators (Pennell 1935, cited in [52]), [22]. In field and greenhouse studies, researchers found that flowers fertilized by natural, delayed selfing set less seed than flowers that were outcrossed. Flowers pollinated by delayed selfing produced 75% of maximum fruit set. Delayed selfing may be important to small common mullein populations that may fail to attract pollinators [39].
Plant height likely affects pollinator visits and method of pollination. Taller plant heights significantly (P<0.02) increased outcrossing rates for 3 populations of common mullein in northeastern Georgia and southwestern North Carolina. Plants over 4.9 feet (1.5 m) tall experienced 21% more outcrossing than shorter plants [22]. Findings were similar for 6 common mullein populations near Kingston, Ontario. Significantly (P<0.0001) more pollen was deposited on tall plants with a median height of 5.6 feet (1.7 m) and an average of 13.5 flowers than on short plants with a median height of 2.6 feet (0.8 m) and an average of 5.5 flowers. Flowers at the top of an inflorescence also received significantly (P=0.0003) more pollen than flowers at the bottom [88]. Researchers in both studies concluded that taller plants attracted more pollinators than short ones [22,88].
Seed production: Common mullein produces abundant seed, and branching and fasciation of the flower stalk can lead to even greater seed production. In a 3-year-old abandoned field in Michigan, common mullein produced between 0 and 749 seeds/capsule for an average of 208 seeds/capsule. Total seeds per plant averaged 175,000 [52]. In 1- to 4-year-old fields in southwestern Michigan, common mullein averaged 100,000 seeds/plant [53]. An "average, well developed" common mullein plant in North Dakota, "growing with little competition" and sampled at a time when seed production was likely at a maximum, produced 223,200 seeds [127,128].
Studies have shown that common mullein rosettes must reach a minimum size before flowering. In a 4-year-old field in Kalamazoo County, Michigan, all rosettes greater than 6.1 inches (15.5 cm) in diameter flowered. In the greenhouse, however, rosettes beyond that size did not flower, suggesting a vernalization period may be necessary for flowering in temperate areas [49,54].
Branched inflorescences produced significantly (P<0.0001) more seeds than unbranched inflorescences in common mullein populations near Kingston, Ontario. The likelihood of branching increased significantly (P=0.0001) with plant height and decreased significantly (P=0.049) with population size. Branching was also associated with weevil damage. There was a significantly (P=0.0195) greater proportion of fruits damaged in branched plants [90].
While branching was affected by several factors, the reason for fasciation of common mullein spikes in Hawaii has not been determined. Ansari [7] found no difference between the prevalence of bacteria in normal and fasciated flowers, and physical damage to the flowering spike actually decreased fasciation rates. Evidence of single gene inheritance was also lacking, since there was no statistical difference in the prevalence of fasciation in normal and fasciated progeny [7]. Fasciated plants produced up to 3 times the seed of normal plants [33].
Seed dispersal: Common mullein seeds have no morphological adaptations for long-distance dispersal. Most seeds fall very near the parent plant [52,53]. Maximum dispersal distances of up to 36 feet (11 m) are possible, but the median dispersal distance is 3 feet (1 m) [52,54]. In natural settings, long-distance seed dispersal is rare. However, the long-lived common mullein seed bank makes transport of soil from areas where common mullein currently or historically occurred a potential long-distance dispersal event [16].
Seed banking: The common mullein seed bank is persistent. Seeds have germinated after 100 years or more in the soil [71,99]. The method used to determine seed bank composition and size, however, may affect common mullein seed bank findings. Seed bank estimates are much greater with the seedling emergence method than with the seed extraction method [19]. Seed bank estimates may also be affected by sample size and sample location. Because abundant common mullein seed is produced and dispersal is limited, soil samples collected near a site once occupied by a prolific parent plant could skew seed bank findings [52,53,73].
In Denmark, common mullein seed germinated from archaeological soil samples dated to 1300 AD [99]. In the late 1800s in Michigan, Dr. W J Beal buried seeds and soil in open jars about 3 feet (1 m) below the soil surface. Later jars were exhumed and germination of the soil samples was monitored in the greenhouse. Common mullein germinated from soil buried for 5, 15, 20, and 35 years [34]. Common mullein also germinated from soil buried 100 years [71]. In a similar study initiated by Dr Duvel in 1902, seeds were buried with soil in pots at increasing depths: 8 inches (20 cm), 22 inches (56 cm), and 42 inches (107 cm). Some common mullein seed germinated from all depths and from all periods tested between 1 and 21 years of burial. Germination percentages, however, were erratic and did not vary consistently with depth or length of burial [46]. After 39 years of burial, common mullein germination rates were 48% and 35% from 22 (56 cm)- and 42 (107 cm)-inch depths, respectively [133]. Seeds have also germinated at low percentages (3%) after 60 months in the water of Washington's Chandler Power Canal. Germination was much higher (82%) after 60 months of dry storage [28].
Methods of detection compared: Common mullein seed bank density estimates using the seed extraction method were much lower than those from the emergence method on soil samples collected in southern Ontario. Very small common mullein seeds were likely washed away or otherwise missed in the extraction method. Overall, the 2 methods provided very different pictures of the site's seed bank composition and density [19].
Frequency and density of common mullein seed in soil collected from a 2-year-old woodland clearcut in southern Ontario using extraction and emergence methods [19] Method Frequency (%) Density Seed extraction 6 87 seeds/m² Seedling emergence 90 1,299 emergents/m²
Vegetation types compared: In most seed bank studies, common mullein was either absent or present at very low densities in the aboveground vegetation but still predominant in the seed bank. The common mullein seed bank can vary by vegetation type; however, patterns of variation are not consistent. It is likely that the soil area sampled and past land use are more important than current vegetation type. This idea is also discussed in Impacts and Control.
Common mullein seeds emerged from soil samples collected from 5 different vegetation types in the Mt Trumbull and Mt Logan Wilderness Areas of northern Arizona. Emergence was greatest from sites dominated by New Mexico locust (Robinia neomexicana) and lowest from sites dominated by old-growth ponderosa pine (Pinus ponderosa). Soil samples were collected in mid-September [123].
Common mullein seedling emergence from various vegetation types in northern Arizona [123] Canopy type Old-growth ponderosa pine Dense, pole-sized ponderosa pine Gambel oak
(Quercus gambelii) New Mexico locust Big sagebrush
(Artemisia tridentata) Number of emergents/m² 23 917 158 4,267 396
On limestone soils in Pennsylvania, common mullein seedlings emerged from soil samples taken from prairie, ecotone, and deciduous forest types. Common mullein was not present in the aboveground vegetation but emerged from 7 of 20 prairie, 6 of 20 ecotone, and 2 of 20 forest soil samples [81]. In the southern Appalachians, common mullein did not emerge from soil samples taken from a floodplain dominated by sedges (Carex spp.) or from soils taken from an adjacent site dominated by sapling red maple (Acer rubrum). A single common mullein seedling germinated from soil collected in a closed-canopy red maple forest [114].
Shrub-steppe: On ungrazed to heavily grazed areas dominated by antelope bitterbrush (Purshia tridentata) in the Okanagen Valley of British Columbia, common mullein occurred with much greater density in aboveground vegetation than in the seed bank. Aboveground density was 65 plants/m², while seed bank density was 0.2 seeds/m² [26]. It is important to note that researchers used the seed extraction method to characterize the seed bank. This method has been shown to underestimate common mullein seed abundance [19].
Coniferous forest: From 47-year-old loblolly pine (Pinus taeda) plantations in North Carolina, 840 common mullein seedlings/m² emerged from soil collections. The researcher noted that common mullein likely had not been present on the sites since canopy development [109]. The density of common mullein seedlings emerging from open-canopy ponderosa pine forests in northern Arizona was staggering. At depths up to 2 inches (5 cm), 4583 seedlings/m² emerged, and from 2- to 4-inch (5-10 cm) depths, 2,083 seedlings/m² emerged. Common mullein occurred in aboveground vegetation with an average frequency of 35% [3].
Deciduous forest: Common mullein seedlings emerged from soil collected in 6 of 8 deciduous forests in Tennessee's Anderson and Campbell counties. Stands were over 47 years old, and common mullein was not present in aboveground vegetation. Seedling density was greatest (93 seedlings/m²) in soil collected from yellow-poplar (Liriodendron tulipifera)-dominated sites, and the greatest abundance of common mullein seedlings came from 2- to 4-inch (5-10 cm) depths [37]. In 70- to 90-year-old mixed deciduous stands in the Yale-Myers Forest of northeastern Connecticut, common mullein seedlings emerged from mineral soil samples taken from midslope (33/m²) and ridgetop (17/m²) positions but not from valley sites. Soil samples to were taken to a depth of 2 inches (5 cm) [9].
Germination: Light and warm temperatures produce the greatest common mullein germination rates; however, some germination is possible in the dark and at burial depths of 1.1 inches (3 cm). Common mullein seeds are either nondormant or conditionally dormant. Seed collected from temperate climates is typically not dormant when temperatures are cool, but as temperatures increase, seeds show conditional dormancy or a narrowed range of suitable conditions for germination (Baskin and Baskin, cited in [12]).
Light, temperature and seed size: Common mullein seed germinates best with exposure to full light and warm temperatures, but several studies have shown that seeds exposed to cool or hot temperatures, drastically fluctuating temperatures, dark conditions, and very brief light exposure may also germinate. Soil disturbances can expose common mullein seeds to the light and increase germination. For more information, see Impacts and Control.
Seed size can also affect germination. Smaller common mullein seeds collected from old fields and roadsides of Michigan and Ohio had significantly (P<0.05) lower germination than medium and large seeds. Small seeds had the lowest and large seeds had the highest germination rates in both light and dark conditions [50].
Germination of small-, medium-, and large-sized common mullein seeds after 3 weeks in greenhouse [50] Seed size Average seed weight (mg) Percent germination in light Percent germination in dark Small 0.041 71.2% 19.2% Medium 0.056 90.4% 26.4% Large 0.070 92.8% 35.2%
Germination of common mullein seed is generally low in dark conditions, but increased temperatures may improve dark germination. In the laboratory, newly harvested common mullein seeds collected from the University of Michigan's Botanical Gardens germinated at over 90% in the light and about 2% in the dark. Older seeds were also light sensitive. When seeds were in soil or sand, germination in dark conditions was better, 24% to 34% [44]. No common mullein seeds collected from 2-year-old fields in southwestern Michigan germinated in the dark. However, germination increased to 38% after 5 seconds of light exposure; after 30 seconds of light exposure, germination increased to 63%, which was not statistically different from germination in full light [51]. Temperature affected successful germination in a dark germinator. Germination was very low in sustained cold temperatures but increased some when fluctuating temperatures reached highs of 68 °F (20 °C) or more [101]. When controlled studies were conducted on common mullein seed collected from low- and high-elevation roadside sites in western Nevada and northern California, germination percentages reached a high of 98% in the dark at alternating warm temperatures of 77 and 95 °F (25/35 °C). In the light, common mullein seed germinated at constant 104 °F (40 °C) and at alternating 0 and 104 °F (0/40 °C) temperatures [120].
Using field and greenhouse studies, researchers concluded that common mullein seed germination is possible throughout most of the year in light conditions. Seed was collected in early September from Wilson County, Tennessee, buried under 2.8 inches (7 cm) of soil in Lexington, Kentucky, for 1 to 25 months, and exhumed at monthly intervals. Germination rates of fresh-harvested seed were 0% at alternating temperatures of 56 °F and 43 °F (15/6 °C), 8% at 68/50 °F (20/10 °C), 97% at 86/56 °F (30/15 °C), and 95% at 95/68 °F (35/20 °C). Germination rates varied with season. Seeds removed in the winter had lower temperature requirements for germination. At high temperatures, 10% of seeds germinated in dark conditions, while none germinated in the dark at low temperatures [13].
Burial/canopy cover: Common mullein seed germinates best on the soil surface in areas with low canopy cover. Germination success generally decreases with increased depth of burial and increased canopy cover.
Emergence of common mullein in established Kentucky bluegrass (Poa pratensis) was significantly lower than emergence in litter or bare soil (P<0.0001). In litter or bare soil, emergence of common mullein was rapid and synchronous, and nearly 50% of maximum emergence occurred within 15 days of being sown [50]. Seed collected from 2-year-old fields in southwestern Michigan germinated at much lower percentages under a simulated canopy than under full light conditions [51]. After 2,500 seeds were sown in 1-year-old, 5-year-old, and 15-year-old fields at the W K Kellogg Biological Station, Michigan, common mullein emergence was greatest in 1-year-old fields with the greatest amount of bare ground. Survival of seedlings was evaluated in 1-year-old and 15-year-old fields; results are presented in Seedling establishment/growth below [53,54].
Seedling emergence with increasing field age and decreasing bare ground availability [53,54] Field age Percentage of bare ground Number of seedlings emerged 1 year 66 20.85a 5 years 10.8 6.3b 15 years <1 2.75b Emergence values followed by different letters are significantly different (P<0.05)
Factors other than darkness associated with burial may prevent germination. Using field and greenhouse studies, researchers found that fewer common mullein seeds germinated in the dark at spring temperatures after 2 years of burial than after 1 year of burial [13]. Common mullein seeds collected from northern California roadsides and tested in a greenhouse study germinated better under a litter layer than under a soil layer, and increasing depth of burial corresponded to decreased germination percentages [120].
Percentage of common mullein germination with increasing depth of burial [120] Burial depth (cm) Seed source Sierra County, CA Lassen County, CA Donner Summit, CA Elevation: 1,510 m
Mean annual precipitation: 625 mm Elevation: 1,380 m
Mean annual precipitation: 300 mm Elevation: 2,190 m
Mean annual precipitation: 1,000 mm Surface 53ab 68a 58b 0.1 25c 21bc 10cd 0.5 17cd 24b 8cd 1.0 7cd 10bc 5cd 2.0 6cd 8bc 1d 3.0 7cd 0c 3d 4.0 0d 0c 0d On top of litter 71a 72a 84a Under 1 cm litter 46b 27b 24c Means within a column followed by different letters are significantly (P<0.05) different.
Seedling establishment/growth: Predictions regarding common mullein's survival and flowering success can be made by measuring its rosette size. Successful establishment and rosette size are affected by site conditions and the availability of open sites.
Probability of common mullein survival and flowering generally increase as rosette size increases. In 4-year-old fields in Kalamazoo, Michigan, rosettes less than 3.5 inches (9 cm) in diameter failed to flower in the subsequent year, but all those greater than 16 inches (41 cm) flowered. Of the 1,006 plants studied, very few survived more than 2 years, and none survived more than 3 years [49]. Probability of dying or not flowering was greatest for small-sized rosettes in 24 common mullein populations from southern Canada, North Carolina, Texas, and Georgia. Very large rosettes over 28 inches (70 cm) in diameter also had a lower probability of survival than those of intermediate diameter. As latitude of the population increased, so did the likelihood that plants with small rosettes would remain vegetative (P<0.001) [110]. Findings were similar on Mauna Kea in Hawaii. Common mullein's probability of dying without flowering decreased and probability of flowering increased with increasing rosette size, which was typically greatest at the highest elevation sites. Rosettes over 10 inches (25.5 cm) in diameter had a 0.08 probability of dying, a 0.15 probability of remaining vegetative, and a 0.77 probability of flowering [7].
Emergence timing may or may not affect common mullein germination, rosette size, flowering, or survival. At sites ranging from 5,540 to 8,860 feet (1,690-2,700 m) elevation on Mauna Kea, common mullein survival and reproductive success were not affected by timing of cohort emergence [7], but timing of cohort emergence was critical to common mullein's survival and reproductive success in southwestern Michigan [48]. The fate of more than 7,000 common mullein seedlings was monitored for 3 years on the island of Hawaii. There were 4 emergence cohorts, but timing of emergence did not affect germination, rosette size, flowering, or survival. Some variation appeared to be related to elevation. Seedling density, rosette diameter, and leaf number were greater at high-elevation sites than at low-elevation sites, which had greater precipitation and more associated vegetation. However, probability of flowering was greatest at the lowest elevation sites. The proportion of plants that delayed flowering beyond 2 years of age was greatest at high-elevation sites [7].
Common mullein seedlings emerged in mid-May, mid-June, and mid-August, generally after 3 to 4 days of rain, in a 3-year-old field at Michigan's W K Kellogg Biological Station. None of the seedlings that emerged in August, the largest cohort, survived the winter. When neighboring vegetation was removed, survival increased [48].
Fate of common mullein plants with timing of emergence in a 3-year-old field in Michigan [48] Cohort Number of seedlings Probability of surviving winter Number of flowering plants Mean height of flowering plants (cm) Mean number of seeds/plant Overall probability of reproducing May 116 0.50 4 92.1 2,050 0.035 June 2,640 0.75 6 38.1 380 0.003 August 29,060 0 0 0 0 0.000
Open site availability: Like seed germination, common mullein seedling establishment is best on open sites. Time since disturbance and its relationship to open-site availability affects seedling size, survival, and reproductive success. Seedling growth was dramatically lower when seeds were sown in containers with established Kentucky bluegrass than when planted in litter or bare soil [50].
Final average dry mass (mg) of common mullein seedlings* in containers with bare soil, litter, or established Kentucky bluegrass [50] Bare soil Litter Kentucky bluegrass Kentucky bluegrass and litter 874-1,013 1,003-1,147 0.07-0.22 0.09-0.16 *First number is average for seedlings from small-sized seeds; last number is average for seedlings from large-sized seeds.
In southwestern Michigan, common mullein seedlings established and survived only in 1-year-old fields when seeds were sown in 1- and 15-year-old fields. Seedlings that survived to the end of the growing season (~20 weeks) on 1-year-old fields were restricted to bare areas. When openings were created in 15-year-old fields, seedling emergence increased and some seedlings established [53,54].
Decreased germination, lower survival, and delayed reproduction were typical in common mullein populations in North Carolina's Piedmont as time since disturbance increased. Common mullein seedlings that established 2 to 3 years after a disturbance had a greater chance of remaining vegetative in their 2nd year than those established in the 1st postdisturbance year. When areas were artificially disturbed, seedlings had a high probability of flowering in their 2nd year. The researcher concluded that the "successional age of the habitat determined the relative fitness of the biennial and triennial plants" [112].
Seedling density and seedling survival with increasing time since disturbance. Ranges include information from 2 to 6 common mullein populations [112] Time since disturbance (years) 1 2 3 Seedlings/m² 98.9-127.6 0-14.1 0-3.8 Fraction of seed pool germinating (%) 11.2-26.6 0-2.9 0-0.02 Seedlings surviving to end of 1st growing season (%) 14.9-17.8 0-2.5 0-7.1
Vegetative regeneration: Common mullein has no means of vegetative regeneration [48].