Breeding system: Purple loosestrife is a tristylous species (3 different style lengths), usually in a 1:1:1 ratio, indicating sexual reproduction is probably its most important means of regeneration . It is primarily an outcrosser, as self-pollination in purple loosestrife is rare, and has been shown to reduce seed production .
Pollination: Purple loosestrife is insect pollinated. Most reports indicate honeybees are the main pollinators [43,73]. Others include bumblebees [72,73], leaf-cutter bees and carpenter bees , as well as a variety of butterflies [72,73]. Hummingbirds have been observed taking nectar from purple loosestrife in British Columbia , although pollination by hummingbirds is undocumented.
Seed production: Purple loosestrife produces an immense number of seeds. Estimates of seed production rates range from just over 100,000 seeds per plant for young plants with single stems , to over 2.5 million seeds per plant for established plants with an average of 30 stems per plant . Although perennial, purple loosestrife is capable of producing viable seed during its 1st growing season . Seed output is largely a function of plant age, size, and vigor . Shoots growing in relatively dense stands tend to produce fewer and smaller inflorescences than those growing in more open areas .
Seed dispersal: Because seeds are small and light they are thought to be dispersed, at least in part, by wind [53,111]. However, Thompson and others  report observations that seedling densities decline sharply within a 33 foot (10 m) perimeter of the parent plant and seedlings are often distributed downslope from the parent plant rather than downwind, suggesting a limited role for wind dispersal. Dispersal via moving water is also likely [53,118,119]. Seeds and cotyledon stage seedlings are reportedly buoyant , although there are reports that purple loosestrife seeds don't float . Floating seeds may disperse to suitable sites for establishment. Seeds that sink may germinate while submerged, then rise to the surface and drift to suitable sites for establishment . Seeds may be transported in fur of mammals, plumage of waterfowl, mud attached to footgear, vehicle treads or cooling systems of outboard motors [53,128,129]. Thompson and others  also suggest birds may deposit ingested seeds in areas where wind or gravity-mediated dispersal seems unlikely.
Seed banking: Given its high seed output and ability to produce seed in its 1st growing season, purple loosestrife can establish substantial soil seed banks. Seeds may remain viable for at least 2 to 3 years [102,111], although the long-term viability of seeds stored in the soil seed bank remains under investigation . Seeds may remain viable even when subjected to saturating conditions. Viability of seeds that were stored underwater was tested at 4-month intervals under ideal germination conditions. Germination declined from an initial rate of 99% to 93% after 1 year and 80% after 2 years .
Purple loosestrife has the potential to dominate the soil seed bank where it becomes well established. Soil samples taken from within purple loosestrife stands in emergent wetlands in southeastern Minnesota contained an average of 37,963 purple loosestrife seeds per ft2 (410,000 /m2) in the top 2 in (5 cm) of soil. Seeds were distributed within this entire profile and seed density increased with proximity to the soil surface. Under greenhouse conditions chosen to promote germination, and using soil samples from the above source spread 0.4 in (1 cm) deep, recruitment failed to exhaust the seed bank [138,140]. From the same experiment, purple loosestrife seedlings were found in 91% of untreated (no herbicide) 6.6 x 6.6 feet (2 x 2 m) quadrats, the most frequently encountered species in the soil seed bank .
Germination: Germination is greatest in unshaded, wet soils, with temperatures >68 degrees Fahrenheit (20Âº C) . Shamsi and Whitehead  demonstrated germination is constrained at low temperatures between about 50 to 59 degrees Fahrenheit (10Â°-15Â° C), and no germination occurred below 57 degrees Fahrenheit (14Â° C). Experimental evidence indicates seed dormancy may be enforced by burial, with germination response decreasing linearly (p = 0.001, r2 = 0.89) from 90% at the soil surface to 0% at 0.8 in (2 cm), even under conditions known to promote germination in wetland plants . Any disturbance that redistributes seeds to within the upper 0.8 inch (2 cm) of soil is likely to promote germination. Although light exposure is a prerequisite for germination, length of exposure does not appear important . Purple loosestrife seeds are capable of germinating underwater .
Seedling establishment/growth: Favorable recruitment conditions are largely a function of disturbance that creates areas where little to no vegetation is present . Estimates of maximum initial seedling density vary greatly, from 926 to 1,852 foot-2 (10,000-20,000 m-2) on bare open mudflats  to 2.8 to 4.6 foot-2 (30-50 m-2) in vegetated semiflooded wetlands. In areas where large numbers of seeds are present in the seed bank, small changes in area favorable for establishment can yield large fluctuations in recruitment .
Growth is limited by cold temperature and is considerably slowed at around 46 to 50 degrees Fahrenheit (8-10Âº C) . Light availability can also limit growth and development. Under diminishing light intensities, vegetative growth is slowed, the numbers of flowers, fruits, and seeds per fruit are fewer, and the average dry weight of fruits declines, but there is no change in average dry weight of individual seeds . Growth is also affected by day length. Shamsi and Whitehead  found leaf area and plant dry weight were significantly (P<0.05) reduced when plants were subjected to a 9-hour photoperiod compared with a 16-hour photoperiod. Plants in the 9-hour treatment grew in a comparatively flattened, semi-prostrate condition.
Asexual regeneration: The rootstock is the main organ of perennation, and unaided wide vegetative spread is unlikely. New shoots arise from buds at the top of the rootstock . Root crowns expand annually to accommodate increasing numbers of shoots, but may reach maximum growth at around 20 inches (0.5 m) in diameter .
Purple loosestrife can consistently resprout in response to aboveground damage, often fairly rapidly. A greenhouse experiment showed 91% of clipped seedlings resprouted within 42 days . Live stems that are dislodged and buried can give rise to new shoots via adventitious buds [23,129].
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