Mono bouquets in New York: choosing one flower by season and meaning
Mono bouquets in New York carry a date the American calendar leaves out: 8 March, which is no US holiday but is kept by Brighton Beach, south Brooklyn's "Little Odessa", home to Soviet emigrants since the mid-1970s. Mimosa, the traditional flower for that day, blooms from January to March, while greenhouse tulips stay in season until May. Choosing a single flower, then, starts with the month and with who is going to receive it. The city keeps its own list of dates for the rest of the year, set out in the city's flower calendar.
Mimosa in March, dahlias by autumn: a single-flower year in New York
New York's single-flower choices follow the calendar outside. Spring opens with bulb flowers, tulip, daffodil, hyacinth, and ranunculus, and lilac joins them by May. Further north and east, peonies open in late May and early June; they do not belong to this category, but they mark where spring hands over to summer. Hydrangea and sunflower carry the warm months, and dahlia and chrysanthemum take over by late summer and into autumn.
Chrysanthemum, gerbera, alstroemeria, lily, lisianthus and carnation are grown under glass and sit in the catalogue whatever the month. Almost four-fifths of the cut flowers sold in the US are imported, more than half of them from Colombia, which is why a greenhouse variety is rarely out of season here.

A desk in Midtown: the varieties that last two or three weeks
Midtown is the largest business district in the world, and Lower Manhattan's Financial District is New York's second centre of offices. For a desk in either one, the flower that counts is the one that keeps the longest: chrysanthemum, carnation, alstroemeria and a cut orchid all hold for two to three weeks in a vase. Chrysanthemum leaves turn yellow before the flower does, and stripping them off keeps the stem looking tidy.
A lily asks for one extra step at a desk: its pollen marks fabric, so the anthers come off once the flower has opened, before it goes anywhere near a sleeve.
Office delivery comes down to two details, the company name and the floor. Where a building runs a pass system, mono bouquet delivery in New York is handled at reception, and the sender is told who accepted it there.

Carnations by colour, chrysanthemums for good news
A bouquet of one flower carries its meaning undiluted, with nothing else in the vase to soften it. In the US, chrysanthemums usually read as good news, a flower for an achievement or a recovery. In France, Italy and Poland, large chrysanthemums are kept for funerals, worth remembering if the person receiving them grew up there.
Carnations carry a history of their own. In 1908 Anna Jarvis sent five hundred white carnations to the first Mother's Day service, and by the 1920s a red carnation was worn for a mother still living, a white one for one being remembered. That is a reasonable guide to colour, without assuming the custom still holds. Victorian guides to the meaning of flowers never agreed with each other either, so no single answer is the final word.
A good starting point is the month: tulips or mimosa in early spring, chrysanthemums or carnations for a desk that needs to last. Most orders are delivered on the day they are placed, and the exact timing for any bouquet shows on its own page.