Trapa natans
water caltrop, water nut, water-chestnut
slender, young growth and flowering parts velutinous.
of floating rosettes bearing successively longer petioles toward outer edges of rosette, to 20 cm;
blade 20–40 × 25–60 mm, width greater than length, surfaces velutinous abaxially, glabrous adaxially.
20–25 mm diam., excluding spines;
horns 2–4, to ca. 10 mm.
tube 2 mm;
sepals 4–7 mm, keeled;
petals obovate, 8–15 mm.
= 48 (Poland, Japan), 96 (Japan).
Trapa natans
Trapa natans was first noted in Massachusetts in 1859 from an unknown origin and was recorded from the Charles River, Cambridge, in 1879. The hard, spiny fruits can cause severe puncture wounds and are slow to decay in lake and river bottoms. The species propagates by seed and by detached floating rosettes to form extensive floating mats that reduce oxygen, restrict light, crowd out native plants, and make navigation difficult. Populations grow rapidly and are difficult to eradicate except by sustained efforts over multiple seasons. Federal regulations now prohibit interstate sale and transport of T. natans. The species is considered rare and threatened in Europe.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
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