My understanding us that many plants were introduced to North America via soil ballast dumped from foreign ships. American botanists understood this and actively searched ballast heap dumps for new species occurence. Pardon the sloppy citations but here are some 19th century examples.
Plants Introduced with Ballast and on Made Land. Addison Brown. Bulletin of the Torrey Botanical Club , Vol. 6, No. 45 (Sep., 1878), pp. 255-258 Ballast Plants In and Near New York City. A. B. Bulletin of the Torrey Botanical Club , Vol. 8, No. 12 (Dec., 1881), pp. 141-142 THE INTRODUCTION OF FOREIGN WEEDS IN BALLAST AS ILLUSTRATED BY BALLAST- PLANTS AT LINNTON, OREGON. J. C. Nelson Torreya , Vol. 17, No. 9 (September, 1917), pp. 151-160 Ballast Plants in Boston and Vicinity Chas.. E. Perkins Botanical Gazette , Vol. 8, No. 3 (Mar., 1883), pp. 188-190
