Information on Botanic Gardens in the United States of America, 
organised by national institutions and then state organisations and gardens.
Within each section, the botanic gardens are organised alphabetically by location and then size
It's going to take some time to develop this page and do it justice
​so it will be developed on a regular basis and updated periodically.
It starts with some of the major botanic gardens in the USA. 
Additionally this is the List of botanic gardens and arboretums in the United States which can be found in Wikipedia

National Botanic Gardens
​

U.S. Botanic Garden Conservatory - Located in Washington DC. Established in 1820, it is is one of the oldest botanic gardens in North America. Dedicated to demonstrating the aesthetic, cultural, economic, therapeutic and ecological importance of plants to the well-being of humankind

Noteworthy collections include economic plants, medicinal plants, orchids, carnivorous plants, cacti and succulents, Mid-Atlantic native plants and ferns.

State Botanic Gardens
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CALIFORNIA

Huntington Botanical Gardens - The Huntington Library is home to a famous botanical art collection. The gardens are themed and cover 150 acres (60.7 hectares).

There are 16 Gardens in total. A 10 acre Desert Garden has more than 5,000 species and is one of the largest outdoor collections of cacti and succulents in the world. Other gardens include the Australian Garden, the Jungle Gardens and the Japanese Garden

There is also a Huntington Botanical Gardens Herbarium. It has over 10,000 specimens in the archive of vascular plants from around the world. It has a particular emphasis on plants from Mexico, Central America and South America

Los Angeles County Arboretum and Botanic Garden
This site covers 127 acres and is located at 301 North Baldwin Avenue, Arcadia, CA, 91007 - in the historic Rancho Santa Anita area of Arcadia.  It aims to be an educational resource for environmental stewardship, the history of our region, the study of plants, and a place of learning and respite for the people of Southern California.

It is managed and operated jointly by the Los Angeles Arboretum Foundation and the Los Angeles County Department of Parks and Recreation.  You can access an archive of publications

Wikipedia: List of botanical gardens and arboretums in California 
Picture
Huntington Library Desert Botanical Garden in afternoon after and during rain - Various cactus and succulent plants. By Pamla J. Eisenberg from Anaheim, USA [CC BY-SA 2.0] via Wikimedia Commons
Other Gardens

Ruth Bancroft Garden
Address: 1552 Bancroft Road, Walnut Creek, CA 94598
The 3 acre site site is a  former walnut tree orchard. It now contains a 10,000 m² public dry garden containing more than 2,000 cactus, succulents, trees, and shrubs native to Africa, Australia, California, Chile, and Mexico. Its mission is to preserve this exceptional collection of water-conserving plants for the education and enjoyment of the public.
COLORADO

Denver Botanic Garden - One of the top botanical gardens in the USA covering 23 acres (9.3 hectares). Developed in the 1950s and opened in 1959.  This was one of the first gardens  in USA to focus on native plants and to champion environmentally responsible practices such as water conservation and biological pest control. Three unique sites reflecting diferent aspects of the local ecosystem: 
Picture
General view of Denver Botanic Gardens, 1007 York Street, Denver, Colorado, USA
ILLINOIS

Chicago Botanic Garden: 1000 Lake Cook Rd, Glencoe, IL 60022
  • The garden has 26 gardens and four natural areas on 385 acres on and around nine islands, with six miles of lake shoreline.
  • Ground was broken for the Chicago Botanic Garden in 1965 and it finally opened in 1972.
  • 2,587,596 plants representing 9,694 plant varieties of 232 plant families
  • It now has 50,000 members—one of the largest memberships of any U.S. botanic garden.
  • one of only 17 public gardens accredited by the American Association of Museums.
  • Its Lenhardt Library contains 110,000 volumes — including one of the nation's best collections of rare botanical books.
​MISSOURI
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Missouri Botanical Garden - Important for various reasons. It is a 19th century botanical garden which originally opened in 1859. It is
  • the oldest botanical garden in continuous operation in the USA.
  • a National Historic Landmark and
  • has one of the world's leading programs in botanical research. It covers 51.5 acres (20.8 hectares) in St.Louis, Missouri.
The Linnean House was built in 1882 and is the USA's oldest continuously operating greenhouse. The Climatron is a geodesic dome covering more than 0.5 acre with more than 1,200 tropical species.
Picture
Missouri Botanical Garden, 2345 Tower Grove Avenue, Saint Louis, Missouri (St. Louis City County, MO) Missouri Botanical Garden - site plan, drawn 1974-1977.
NEW YORK

List of botanical gardens and arboretums in New York: Wikipedia has a separate list of all the significant botanical gardens and arboretums in the U.S. state of New York.
New York Botanical Garden - This botanic garden has a mission to  study, exhibit, and teach about plants.

It was founded in 1891 and boasts some impressive statistics
  • a 250 acre site
  • more than one million living plants in its extensive collections of tropical, temperate, and desert flora
  • ​the largest remnant of the original forest which covered the area of New York - it has never been cut.
  • a major educational institution with one of the world's largest plant research and conservation programs
  • nearly a million visitors a year.
  • host for some of the major exhibitions associated with botany and botanical art
​
Picture
Reflecting Pool, New York Botanical Garden, The Bronx, New York City.
Brooklyn Botanic Garden ​
​This 52 acre garden was founded in 1910.  More than 10,000 different plants can be found in the garden. It contains a Native Flora Garden. When this was established in 1911 it was the first of its kind in the USA. It also includes the first Japanese Garden to be constructed in the USA. Its current research is focuses on research focused on understanding and conserving regional plants and plant communities. The current development programme includes new gardens the biggest of which is the new Water Garden.
Picture
Brooklyn Botanic Garden - Fall In the Native Flora Garden.
PENNSYLVANIA

Bartram's Garden
This is the oldest surviving botanic garden in the United States. It was founded in 1728 by John Bartram (1699-1777), an early American botanist, explorer, and plant collector. The garden was developed on what had been a farm and became a systematic and varied collection of North American plants over time. He initiated trade in these plants which was continued by his sons John Bartram, Jr. (1743–1812) and William Bartram (1739–1823) who expanded both the garden, nursery business and the international trade. The family sold the garden in 1850 and initiatives were undertaken to preserve the garden. In 1891, control of the site was turned over to the City of Philadelphia. It is now a 45-acre National Historic Landmark, operated by the John Bartram Association in cooperation with Philadelphia Parks and Recreation.  The garden includes the oldest ginkgo or maidenhair tree, Ginkgo biloba, in North America.  

REFERENCE:
John Bartram's House at Bartram's Garden in Philadelphia, Pennsylvania
John Bartram's House at Bartram's Garden in Philadelphia, Pennsylvania
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