Life History

As an herbaceous perennial, purple loosestrife grows from a woody crown every year. The timing of shoot emergence in the spring is affected by temperature, but normally shoots emerge from crowns in late April or early May. Rapid shoot growth begins after day-light length reaches 13 hours. Flower bud formation begins in mid-June to early July and inflorescence growth continues until the first killing frost (indeterminate growth). Early flowering is associated with atypically warm and dry weather. Flowers are pollinated by honey bees (Apinae), leaf cutter bees (Megachilinae), carpenter bees (Xilopinae), bumble bees (Bombinae), European cabbage white butterflies (Pieris rapae), common sulphur (Colias philodice), and wood nymph butterflies (Cercyonis pegala). After fall senescence, ridged dead stems can remain standing for one to two years.

Carbohydrates accumulate in the crown and roots during the late summer and fall. Overwintering shoot buds form on the crown in late August and early September and use minimal amounts of carbohydrates for respiration and maintenance during the winter. Starch is the primary non-structural carbohydrate in the roots and crowns, and averages 19 percent and 15 percent by weight in the roots and crowns, respectively, in populations in Minnesota. Starch content of crowns decreases following shoot emergence in the spring and continues to decline until flower bud formation. Starch levels increase during flowering and peak when plants die back in late September and early October. Water stress (drought) reduces starch accumulation in the crowns. Sucrose is the predominant soluble sugar in roots and crowns and average 1.3 percent of total dry weight in populations in Minnesota. The sucrose level in crowns is lowest during bud and early flowering, and increases to the highest level in late August and September. Pre-winter elevated sucrose concentrations have been associated with increased cold tolerance in alfalfa and leafy spurge.

Fresh seed viability is nearly 100 percent, and after two to three years of submergence, 80 percent of the seeds can maintain viability. In late spring or early summer, seeds germinate on bare, moist soil, but not under flooded or shaded conditions. The seed bank of purple loosestrife can be as high as 400,000 seeds/m2 and usually outnumbers seeds of native species. Seeds on bare, moist soil germinate in three to four days, generally faster than native species, and seedling growth rate is also faster than most native wetland species, giving purple loosestrife an advantage in capturing light and shading competitors. After germination, flowering can begin in eight to ten weeks, and new plants can set seed in one growing season. However, growth and competitive ability are diminished under low fertility conditions.

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Last Modified: 08/12/2008