It seems that little is known about the introduction of Dahlia in the USA, so I added some info to Wikipedia:
The exact date the dahlia was introduced in the United States is uncertain. The first Dahlia in the USA may be the D. coccinea speciosissima grown by Mr William Leathe, of Cambridgeport, near Boston, around 1929. According to Edward Sayers [66] "it attracted much admiration, and at that time was considered a very elegant flower, it was however soon eclipsed by that splendid scarlet, the Countess of Liverpool". Sayers als stated that "No person has done more for the introduction and advancement of the culture of the Dahlia than George C. Thorburn, of New York, who yearly flowers many thousand plants at his place at Hallet's Cove, near Harlaem. The show there in the flowering season is a rich treat for the lovers of floriculture : for almost every variety can be seen growing in two large blocks or masses which lead from the road to the dwelling-house, and form a complete field of the Dahlia as a foreground to the house. Mr T. Hogg, Mr William Read, and many other well known florists, have also contributed much in the vicinity of New York, to the introduction of the Dahlia. Indeed so general has become the taste that almost every garden has its show of the Dahlia in the season." In Boston too there were many collections, a collection from the Messrs Hovey of Cambridgeport was also mentioned.
In 1835 Thomas Bridgeman, published a list of 160 double dahlias in his Florist's Guide [67]. 60 of the choicest were supplied by Mr. G. C. Thornburn of Astoria, N.Y. who got most of them from contacts in the UK. Not a few of them had taken prices "at the English and American exhibitions".
The exact date the dahlia was introduced in the United States is uncertain. The first Dahlia in the USA may be the D. coccinea speciosissima grown by Mr William Leathe, of Cambridgeport, near Boston, around 1929. According to Edward Sayers [66] "it attracted much admiration, and at that time was considered a very elegant flower, it was however soon eclipsed by that splendid scarlet, the Countess of Liverpool". Sayers als stated that "No person has done more for the introduction and advancement of the culture of the Dahlia than George C. Thorburn, of New York, who yearly flowers many thousand plants at his place at Hallet's Cove, near Harlaem. The show there in the flowering season is a rich treat for the lovers of floriculture : for almost every variety can be seen growing in two large blocks or masses which lead from the road to the dwelling-house, and form a complete field of the Dahlia as a foreground to the house. Mr T. Hogg, Mr William Read, and many other well known florists, have also contributed much in the vicinity of New York, to the introduction of the Dahlia. Indeed so general has become the taste that almost every garden has its show of the Dahlia in the season." In Boston too there were many collections, a collection from the Messrs Hovey of Cambridgeport was also mentioned.
In 1835 Thomas Bridgeman, published a list of 160 double dahlias in his Florist's Guide [67]. 60 of the choicest were supplied by Mr. G. C. Thornburn of Astoria, N.Y. who got most of them from contacts in the UK. Not a few of them had taken prices "at the English and American exhibitions".