Best sources to determine what plants are native to PA:

Online

PA Flora Database - Botany Dept. Morris Aboretum, U. of PA; search by species, county, or watershed. Text only, no photos.

Lady Bird Johnson Wildflower Center Native Plant Information Network - Use Native Plant Database to search for state native plants by any/all of the following: general appearance, bloom time/color, light requirement, soil moisture, common or scientific name. 

Plants Database - USDA, Natural Resources Conservation Service; search by scientific or common name; may specify state. Scroll entire page to see photos and map specific to county.

Books

The Plants of Pennsylvania - Ann Fowler Rhoads, pub. by U. of PA, detailed reference to all plants of PA. Some sketches. A 2000 edition hard copy may be checked out and is also in the Reference section at The Public Library of Union County. Newest edition published in 2007.

Trees of Pennsylvania - Ann Fowler Roads and Timothy A. Block, pub. by U. of PA Press, 2005. Excellent photos and sketches.

Read the Invasive Plants page to help identify invasive plants so you know what plants to avoid buying and to remove from your property.

Shrub ID and info:

Online

Tree Fact Sheets by Dendrology at Virginia Tech.  Excellent photos and information on trees and shrubs. Enter common or botanical name or use family names listed at site and click PA state option. Once you click on Display Matching Species be sure to scroll entire page to see all info and further info options.

Book 

The Shrub Identification Book - George W.D. Symonds, 1963; includes shrubs, woody vines and ground covers; excellent b&w photos; keyed by leaves, twigs, flowers, bark, fruit.  

Tree ID and info:

Online

Tree Fact Sheets by Dendrology at Virginia Tech. Excellent photos and information on trees and shrubs. Enter common or botanical name or use family names listed at site and click PA state option. Once you click on Display Matching Species be sure to scroll entire page to see all info and further info options.

Books

The Tree Identification Book - George W.D. Symonds, 1963; excellent b&w photos; keyed by leaves, twigs, flowers, bark, fruit.  

Trees of the Northern United States and Canada - John Laird Farrar, pub. by Iowa State Press. Excellent photos and sketches for identification of native and introduced species. Informative text.

Wildflower ID:

Online

Key by flower color with photos - Connecticut Botanical Society - photos and brief info.

Key by flower color and leaf details -  Missouri Botanical Garden and U. of Georgia. Scroll down right column of link and answer as many questions as possible to narrow search.

Books

A Field Guide to Wildflowers, Northeastern and North-central North America - Roger Tory Peterson. Organized by flower color. Sketches, some in color; brief info including bloom time, height.

Newcomb's Wildflower Guide - Lawrence Newcomb. Organized by flower parts, irregular, 3 regular parts, etc. Sketches, some in color; brief info including bloom time, height.

Wildflowers of PA - by Mary Joy Haywood, Botanical Society of Western PA. Organized by plant families, includes a few shrubs and trees. Color photos, bloom time, info.

References to help decide what plants to add to your garden: 

Online

Gardening with Native Plants - Connecticut Botanical Society; photos/brief info of suggested perennials, ferns, vines, ornamental grasses.

Native Plants for Wildlife Habitat and Conservation Landscaping: Chesapeake Bay Watershed - pub. by U.S. Fish & Wildlife Service. Click on links or scroll to bottom of page to download 5.3 MB PDF format of actual booklet. A hard copy is available at The Public Library of Union County.

Native Plants from American Beauties - photos/info of ferns, grasses, perennials, shrubs, trees, vines. Search by common or botanical name or scroll through thumbnail photos alphabetically by category.

PA Native Plants for the Perenial Garden - PSU Dept. of Horticulture Fact Sheet.

Books

Bringing Nature Home: How You Can Sustain Wildlife with Native Plants - by Doug Tallamy. The book well deliniates our need to replace suburban lawns with native plants to at least maintain our biodiversity. 96% of our song birds depend on insects to feed their young. Beneficial insects are dependent on specific native plants for various life phases.

Landscaping with Native Plants of PA - pamphlet pub. by DCNR. Pamphlet form available at DCNR and Penn State Cooperative Extension coutny offices. Brief info.

Native Ferns Moss & Grasses - William Cullina, pub. by New England Wild Flower Society. Wealth of info. Highly recommended for a native plant gardener's library.

Native Plants for Wildlife Habitat and Conservation Landscaping: Chesapeake Bay Watershed - pub. by U.S. Fish & Wildlife Service. Click on links or scroll to bottom of page to download 5.3 MB PDF format of actual booklet. A hard copy is available at The Public Library of Union County.

Native Trees, Shrubs, and Vines: a guide to using, growing, and propagating North American woody plants- William Cullina, pub. by New England Wild Flower Society. Wealth of info on woody plants. Highly recommended for a native plant gardener's library.

New York Botanical Garden Illustrated Encyclopedia of Horticulture - Thomas H. Everett. Many pages viewable online through Google Book Search; in upper right corner of link use "Search in this book" search box to find desired plant.

Wildflowers: A Guide to Growing and Propagating Native Flowers of North America - William Cullina, pub. by New England Wild Flower Society. Wealth of info on native wildflowers. Highly recommended for a native plant gardener's library. Many pages viewable online through Google Book Search; in upper right corner of link use "Search in this book" search box to find desired wildflower.

Planting Noah's Garden: Further Adventures in Backyard Ecology - Sara Stein, pub. by Houghton Mifflin Co., available through used bookstores and online. Detailed info on replacing lawn with native plants, establishing plants, all aspects of gardening, entertaining as well. A word of caution: Be sure to avoid using any invasive plants mentioned such as butterfly bush, the Buddleja species. 

Other Recommended Books

Life in the Soil: A Guide for Naturalists and Gardeners - James B. Nardi, 2007. A definitive guide to all aspects of soil. Learn what creatures create rich fertile soil. Excellent photos and sketches.

Teaming with Microbes: A Gardener's Guide to the Soil Food Web - Jeff Lowenfels & Wayne Lewis, 2006. Clearly describes the organisms that make up our soil food web and how to cultivate the life of the soil through the use of compost and mulches.

Organizations 

Center for Plant Conservation - Dedicated solely to preventing the extinction of U.S. native plants. Network of more than 30 leading botanic institutions. Founded in 1984, operates only coordinated national program of off-site (ex situ) conservation of rare plant material. Mission is to conserve and restore the rare native plants of the United States.

PA Native Plant Society - general-membership non-profit native plant organization with headquarters in State College. The Society sponsors numerous field trips and projects throughout the state and publishes a quarterly newsletter, PNPS Notes, for members.