By Amy Weiss

Sep 26 2019

Ships transporting cargo and passengers use ballast to stabilize the ship at sea. The material of choice used to be solid ballast; which was just rubble, gravel, stones collected indiscriminately at the port. Along with this debris, came plant seeds, insects, and soil organisms.¹ Once reaching the destination, the ballast was dumped on the shore before loading the ship with cargo for the return voyage.

Ballast heaps and the areas around seaports were places botanists found many spontaneous non-native plants growing from these transported seeds—they often made herbarium specimens of the plants they found. While some plant species remained fun curiosities, some of these accidental plant introductions were able to establish themselves, and a small portion of those species went on to become serious invasive weeds. 

Herbarium records indicate that Italian thistle (Carduus pycnocephalus, seen above) was introduced into several United States ports in the 19th century through ship ballast. Conditions were generally unsuitable in the U.S. for this species to become permanently established, and it has only become a weed of concern in a few states, including California.²

Purple loosestrife (Lythrum salicaria) was accidentally introduced into the United States through ship ballast, but also intentionally introduced as an ornamental plant. The herbarium at the New York Botanical Garden has one of the earliest known collections of purple loosestrife in the US (seen below). The species has become a very aggressive invader of disturbed places and wetlands where it displaces native plant species (and the native wildlife that depend on them) and alters waterways.³

Modern ships use water as ballast, which means plant seeds rarely catch a ride this way anymore. Instead the ballast water brings along a new contingent of aquatic organisms that travel the globe.