Ask Mr. Smarty Plants

As far as I could determine (and I did a lot of searching), it is not illegal to pick wildflowers from the roadside in New York.  There are, however, some wildflowers that are illegal to pick and some places it is illegal to pick or destroy any plant in New York.

The wildflowers that are not legal to pick are those that are listed in the Protected Native Plants Program.  New York created the Protected Native Plants Program in 1989 to protect plants that are "endangered, threatened, rare or exploitably vulnerable".  You can see a list of these plants in 6 NYCRR 193.3.  As quoted on the NY Department of Environmental Conservation page:

"It is a violation of the Environmental Conservation Law §9-1503 to collect or destroy listed plants without the permission of the landowner. The regulation gives landowners additional rights to prosecute people who collect plants without permission."

There is also a list of Federally Protected Plants that are found in New York State.

The places you will be prosecuted for collecting or destroying any plants are in New York State Parks or Historic Preservation areas.  In state parks it is one of the "Prohibited Activities" listed in New York State Office of Parks Recreation and Historic Preservation Rules and Regulations:

Part 375.l(e): "Injury to property. No person shall make an excavation on or injure, destroy, deface, remove, fill in, tamper with or cut any real or personal property, tree or other plant life."

Even though picking wildflowers that aren't on the State or Federal list of protected plants from the roadside won't result in prosecution, activities associated with collecting wildflowers might result in problems for you.   Here are a few things to consider:

1. Be sure you are not on private property collecting flowers.   You could be cited for criminal trespass.
2. Be sure to park safely off the roadway on the same side as the flowers.  Don't cross the highway on foot.
3. Don't park where there are signs prohibiting parking.  Follow all traffic laws.
4. Don't dig up large clumps.  Take only a modest amount of wildflowers, leaving plenty of flowers to reseed the area.

Remember that, in general, wildflowers don't last very well as cut flowers.  Flowers captured on camera will last a lot longer!