Ben Zimmer wrote:

Depending which government site you believe, tamarisk/saltcedar was
first imported to the US in either 1837 or the 1850s:

http://www.ars.usda.gov/is/pr/1999/990708.htm
http://www.nps.gov/whsa/tamarisk.htm

And here's yet another government website with different information:

http://www.nwcb.wa.gov/weed_info/saltcedar.html
It is believed that nurserymen on the east coast made the
first introduction of saltcedar to North America in 1823.

corroborated by:
http://www.werc.usgs.gov/hq/pdfs/bozeman1.pdf

More on the 1837 date, and a possible etymology for "tamarisk":

http://google.com/search?q=cache:www...ome/saltcedar/

By way of Europe and New Jersey, saltcedars came to the
Mojave Desert from central Asia, where, according to
Christie Robinson, Tamarix is "one of the oldest and most
dominant plant genera."

From Asia the saltcedar spread to the Middle East and Spain.
According to the Audubon Society, the botanical name Tamarix
recalls the Spanish river Tamaris.

A member of the Desert Crossroads Garden Club in Lucerne
Valley, Robinson is employed as education and public-
relations coordinator for the Mojave Desert Resource
Conservation District.

"Saltcedar was first introduced to North America as an
ornamental," Robinson said. "The earliest records we have,
dated 1837, are from a nursery in New Jersey."