Ask Mr. Smarty Plants

The USDA Plants Database has maps showing the location of plants reported to occur in North America.  For most species they also show records by counties.  Their database covers plants native to North America and plants that have been introduced to North America with a code telling whether each plant is native or introduced.  This is the source that I use most often to find out whether a plant is native to North America and where it occurs.

For your flower of interest, I'm supposing you mean "moon" flower.  The problem with common names is that they may be associated with several different plants, often plants that are not closely related.  So, since there are several possibilities for moon flowers or moonflowers, I'm not sure whether you mean:

• Ipomoea alba (Tropical white morning-glory), also known as moonflower.  Here is the distribution map from the USDA Plants Database. 
• Datura inoxia (Pricklybur or moonflower), also native to Central and South America but also introduced into Europe, Asia, Africa and Australia.  You can see the distribution map on the USDA Plants Database.
• Datura wrightii (Jimsonweed,thornapple or angel trumpet) is native to a large part of the United States. Here is the distribution map from the USDA Plants Database.
• Mentzelia pumila (Dwarf mentzelia)  Here is more information from Wikipedia and here is the distribution map from USDA Plants Database.
• Peniocereus greggii var. greggii (Nightblooming cereus) sometimes called moonflower.  You can see its distribution on the map on USDA Plants Database page.
• Hylocereus spp. such as Hylocereus undatus (Nightblooming cactus) that was introduced to and grows in Florida.
• Or, perhaps, you are thinking about another entirely different plant.

DiscoverLife.org is another place where you will find location maps.  Enter the species name (e.g., Ipomoea alba) in the Search slot to find their page for a particular plant.

SouthwestBiodiversity.org (SEINet) has maps.  Enter the species name (e.g., Ipomoea alba) in the Taxon Search slot to find their page for a particular plant.

To find any of these maps you will need the scientific name of the plant since, as you may have noticed, the common name given for Ipomoea alba varies with the different databases.  The scientific name will (usually) remain the same.  If it doesn't, the synonyms are usually cross-referenced on the databases.  To find the scientific names you can Google the common name and look at the different sites to find its scientific name.

On our Native Plant Database when you reach the description page for each species, if you scroll to near the bottom of the page to the section called ADDITIONAL RESOURCES and click on the link to USDA Plants, you will be taken to their page for the species with the distribution map.  Try this for Ipomoea alba (Tropical white morning-glory).