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When spring metamorphoses into summer, the mimosas along our roadside burst into bloom. Most of our native species have already bloomed or are waiting for the cooler days of fall, but not Mimosa. From mid-June to late August, the pink powder-puff clusters of flowers present a refreshing burst of color in an otherwise sea of green. Another indication of its alien nature is its habitat. Mimosa thrives disease free along disturbed edges of roads and in vacant lots.
Mimosa is native to Asia ranging from China to Iran. It was brought to this country as early as 1745 as a horticultural planting. The more common name for this plant in the 30s was silk tree. Whatever its common name, it is not in the genus a Mimosa at all but is known scientifically as Albizia julibrissin.
The Mimosoidae is a large tropical group comprising 56 genera and up to 3,000 species. The only other member of this subfamily in the upstate is sensitive briar, Schrankia microphylla , a vine-like herb that folds its leaves when touched.
Members of the Mimosoidae have sweet scented power-puff flowers clusters. The showiness of the flowers is due to the long colored anther stalks (filaments) which very from yellow to deep rose in color. The 5 fused petals form a small tube at the very base of the flower. Clumps of flowers on an expanded stalk tip are technically known as heads. Bees are common pollinators of this subfamily. Although one African relative is pollinated by giraffes. Go figure!
The dainty fern-like leaves are actually quite large. The botanical description would go something like, leaves alternate and twice pinnately compound. The translation is as follows. All parts of a mimosa leaf are green. The leaf stalks (petioles) are swollen at the base and are attached to the stem so that the leaves are on alternating sides of the branch. The photosynthetic blade is subdivided into sections (pinna or leaflets) and these are divided again into sub-leaflets where most of the photosynthesis occurs. Such a twice divided leaf is very rare in South Carolina trees. The vein of the sub-leaflets is strange as well, for it is distinctly off center. But stranger still is the folding of the leaves at sunset or during a rainstorm and their unfolding again when the sun shines.
Why such a design? Large leaves provide a large surface area for photosynthesis but are susceptible to wind damage and provide more surface for evaporation. Mimosa leaves give with the wind and are less likely to be torn like a banana leaf. Such leaves also provide diffuse dappled light to all leaves and self shade. Mimosa grows in the open where the wind and light are most intense. Such a leaf shape is ideal for these conditions. The leaf folding may reduce water loss at night and reduces damage from hail and strong winds during storms.
The shape of the whole tree is quite different from the long lollipop shape of most temperate trees. Silk trees have crowns that are much broader than tall, a shape more common to savannas of Africa than to the Appalachian foothills. Mushroom shaped profiles are ideal for sun drenched savannas but are poorly suited to ice and snow storms.
The fruits are flat pods (legumes) which usually become curiously twisted when dry. The fruits splits open only after falling from the tree. These light curled pods blow along the ground, dispersing the seeds.
As we have disturbed more and more ground, mimosa has slowly become more a part of our dissected landscape. New seedlings begin flowering and fruiting within 3-4 years. Certainly not as great of pest as Kudzu, its rapid growth and disease free status is sure to make it a increasingly common site along our roadsides.
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