eNature: Native Gardening and Invasive Plants Guide - designed to help gardeners find and recommended native garden plants in their state and lists of some of the plants that are best avoided in the garden because they can be weedy or invasive.
Interactive Key to the Flowering Plant Families of Jamaica Identifying plants can be quite a challenge. For beginners, and especially those who are learning the families, this Java-based interactive key may help. Long lists of family characteristics are at the touch of a button. Plant families listed here follow the book Flowering Plants of Jamaica,

The Flora of Northern Ireland site features species descriptions and often photographs of about 1100 plants.

Southeastern U.S. Plants Principally developed as a resource for Tennessee plants, this site features many characteristic Southeast US plants by flowering date, common & scientific name and family.

Native Plant Network Has a journal, propagation protocol database, and details of the network's partners and sponsors.

Indigenous Plants & Native Uses in the Northeast. All the plants listed here grow along the trails of Schoolhouse Brook Park in Mansfield, Connecticut. This list was compiled specifically for a trail-side tour that described the Native American uses of local plants for food and medicine, as well as some technological and ceremonial uses.

Project Muse: Native Plants Journal - allows users to sign up and subscribe to the journal for dispersing scientific information about planting and growing North American plants.

North American Native Plant Society (NANPS) Celebrating the unique botanical heritage of North America through the study, conservation, cultivation, and restoration of native plants.Native Plants Forum - from GardenWeb.

Native Plant Conservation Initiative A consortium of federal government member agencies and non-federal cooperators working together to conserve and educate people about native plants.

Pacific Northwest Native Wildlife Gardening

The Virtual Herbarium The following plants are but a few representative specimens from the Appalachians. Most of these are widely-distributed east of the Mississippi River.

US EPA Green Landscaping with Native Plants guide to green landscaping with native wildflowers and grasses in the Great Lakes environment.

The New England Wild Flower Society is the oldest plant conservation organization in the United States, promoting the conservation of temperate North American plants through key programs:

Growing Native Plants on the web In the 1970's and early 1980's the Australian National Botanic Gardens produced a landmark series of booklets called 'Growing Native Plants', issuing one volume a year for 14 years. This was at a time when little was available on this topic in commercial publishing.

Alabama

Alaska
Alaska Native Plant Society

Arizona
Arizona Native Plant Society
The Southwest Exotic Plant Information Clearinghouse is a cooperative effort among the U.S. Geological Survey, the National Park Service and Northern Arizona University to organize comprehensive information on exotic plant species in the southwest on one web location.
Northern Arizona Flora NAZ Flora A Photographic, Annotated Catalog of Northern Arizona Vascular Plants

Arkansa
Arkansas Native Plant Society
Arkansas Biodiversity - Vascular Plants

California
Back To NativesRESTORATION is dedicated to the restoration and conservation of Orange County and California wildlands, through education and restoration programs featuring native plants as a centralizing theme.

California Native Plant Society Home Page
California Native Plants Discussion Group
Growing Native California plants California native plants have adapted over hundreds of thousands of years to the very conditions that exotic plants must overcome with fertilizer, water and constant care. It makes so much sense if you think about it: They live here!
California's Coastal Plant Communities
Manual of California Native Plants Focuses on plant ecology with ideas and methods to accomplish a successful native landscape or restore a native site
Native and Introduced Plants of Southern California

Colorado
Colorado Native Plant Society
Gardening with Boulder's Native Plants Recommended species list with photos, local nurseries, and growing tips.

Connecticut
Connecticut Botanical Society

Delaware
Delaware Native Plant Society

Florida
Florida Native Plant Society and Chapters
Florida's Native Plants - shows how native plants are the trick to successful, low-maintenance, low-cost gardening in Florida.

Georgia
Plants of Georgia
Georgia Native Plant Society
The Georgia Botanical Society

Hawaii
Native Hawaiian Plant Society
Hawaii's Most Invasive Horticultural Plants This site lists Hawaii's most invasive horticultural species and goes beyond this to consider why these plants have become a threat, how to recognize a potentially invasive species when it can still be controlled, and what the most prevalent characteristics of the invaders are.
Native plants of Hawaii Forest and Kim Starr are Botanical Research Associates with the United States Geological Survey who have amassed a remarkable 6000 high resolution, copyright free images for more than 900 species of native and non-native plants of Hawaii. In addition, there are links to Insects of Hawaii, Organisms of Hawaii, Natural Areas of Hawaii, Volunteer Opportunities on Maui, Publications, Silverswords and Northwestern Hawaiian Islands.
University of Hawaii Botany Department Native Plant pages

Idaho
Southern Idaho Native Plants
Idaho Native Plant Society

Illinois
Illinois Native Plant Society

Indiana
Indiana Native Plant and Wildflower Society INPAWS is a not-for-profit organization promoting the appreciation, conservation, beauty, diversity, and environmental importance of indigenous vegetation.

Iowa
Idaho Native Plant Society
Southern Idaho Native Plants

Kansas
Kansas Native Plant Society

Kentucky
Kentucky Native Plant Society was founded in 1986 for everyone interested in the native plants and wildflowers of Kentucky

Louisiana
Louisiana Native Plant Society 

Maine

Maryland
Maryland Native Plant Society
The Horticultural Society of Maryland, is a organization where gardening enthusiasts of all levels and interests come together to expand their knowledge of horticulture through lectures, trips, workshops and shared experiences. If you want to be a better gardener, meet plant experts, visit beautiful public and private gardens, and become part of an organization that supports local community programs and horticulture education, come see for yourself what all the fun is about

Massachusetts
Massachusetts Native Plants Committee

Michigan
Michigan Botanical Club

Minnesota
Minnesota Native Plant Society

Mississippi
Mississippi Native Plant Society

Missouri
Missouri Native Plant Society
Missouri Flora Website Photographs and descriptions of the flowering and non-flowering plants of Missouri, USA. Plants are categorized by flower color and/or leaf characteristics.
Grow Native! is here to help you learn about Missouri's rich array of landscape worthy native plants and how to use them to create beautiful landscapes on your property. Explore our site to discover the many benefits of well-planned native landscaping.

Montana
Montana Plant Life Over 400 species of wild Montana plants are featured with more than 1500 images, full descriptions and identification guides. Detailed information is available on edible, poisonous and medicinal properties of the species. The picture identification system lets the user make easy choices about flower color, leaf shape etc. to arrive at a page with results containing pictures and short descriptions, and from there one can go to the detailed species pages.

Native Plants of Montara Mountain Developed by naturalist Chuck Kozak, Native Plants of Montara Mountain is just that type of website. Mr. Kozak has assembled a nice online field guide of plants native to Montara Mountain, a northern spur of California's Santa Cruz Mountains
Native Plants Montana Plant Life This web site presents a wide variety of Montana wildflowers and other plants growing in the wilds of Montana.

Nebraska
Nebraska Native Plant Society

Nevada
Nevada Native Plant Society is a non-profit corporation organized solely for educational, scientific and charitable purposes for the stimulation of interest in and the developing of an appreciation for native Nevada plants and their preservation"

New Hampshire

New Jersey
The Native Plant Society of New Jersery NJ

New Mexico
Native Plant Society of New Mexico

New York
New York Flora Association

North Carolina
North Carolina Native Plant Society

North Dakota

Ohio
Native Plant Society of Northeastern Ohio

Oklahoma
Woody Plants of Oklahoma: Descriptions and Range Maps Species are listed by common and Latin name, with each entry accompanied by detailed descriptions of plants, with images, habitat and range maps.
Oklahoma Native Plant Society

Oregon
Native Plant Society of Oregon

Pennsylvania
The Pennsylvania Native Plant Society

Rhode Island
The Rhode Island Wild Plant Society

South Carolina
South Carolina Native Plant Society

South Dakota

Tennessee
The Tennessee Native Plant Society was founded in the spring of 1978 as a botanical organization for persons interested in the native flora of Tennessee.

Texas
The Digital Flora of Texas project represents a collaboration of over 40 Texas institutions that curate plant collections. The site has links to herbarium specimen data, vascular plant images, bibliographic references, and checklists of Texas vascular plants. Eventually, identification keys and descriptions are anticipated to be included with the flora for about 1000 of the species. Eventually 300,000 specimen records will be included, with image links for 2,000 Texas species. The search engine allows searches by county, herbarium, cultivation status and name.
Native Plant Society of Texas: Austin Chapter
Native Plant Society of Texas@
Texas Native Plant Society Wichita Falls Chapter
Native Plant Society of Texas, Post Oak Chapter
The Vascular Flora of Madison County provides a detailed record of county plants, their distribution and vegetation, both for the present and future generations. This provides an excellent model and may well prove to be essential in understanding local change in the future. The plant list can be browsed by family, division/class or genus. Additional links provide county maps, geology, soils, drainage, vegetation, species endemic to Texas, invasive non-native species, plant images and more.

Utah
Utah Native Plant Society dedicated to the understanding, preservation, enjoyment, and responsible use of Utah native plants.

Vermont

Virginia
Central Washington Native Plants
Virginia Native Plant Society
Comprehensive listing of all native plants (trees, shrubs, ferns, grasses and herbaceous perennials) in Virginia:

Washington
King County Native Plant Salvage Program Washington
Washington Native Plant Society A non-profit group dedicated to the appreciation, study, and conservation of our native flora and environment.
Central Washington Native Plants Has photographs of both endemic and cordilleran species.
Native Plants of the Mid-Atlantic Region Several dozen species of globally rare plants and hundreds of state-rare plants occur in diverse habitats within 200 miles of Washington, DC.
Flora of Mount Adams, Washington - provides an extensive list of the plants found at Mount Adams, Washington, compiled by Susan McDougall and David Biek. Includes photographs and maps.

West Virginia
West Virginia Native Plant Society

Wisconsin
Wisconsin Plant of the Week Supported by Wisconsin's Department of Natural Resources, a plant is chosen weekly from among the native and introduced species found outside of cultivation in the state.
The University of Wisconsin-Madison Herbarium, founded in 1849, is a museum collection of dried, labeled plants of state, national and international importance, which is used extensively for taxonomic and ecological research, as well as for teaching and public service. It contains the world's largest collection of Wisconsin plants,

Wyoming
Wyoming Native Plant Society
Wyoming Rare Plant Field Guide From the US Geological Survey, Northern Prairie Wildlife Research Center.