Exploratorium: Science Of Gardening [more]
Features articles and videos on several topics, including gardening with nature, carnivorous plants, soils, fruits, and vegetables, propagation, and plant reproduction.
http://www.exploratorium.edu/gardening/

Garden Ideas [more]
Articles and advice on a variety of gardening topics.
http://www.gardenideas.com/

Garden Terms [more]
Gardening dictionary with brief definitions of a large number of terms, optionally arranged by category.
http://www.gardenterms.com/

Gardening For Nutrition [more]
Guidance for gardening and food intake with the purpose of maximizing nutrition.
http://celticbuttonbox.com/GardenForNutrition/

Hort.net [more]
Offers a collection of garden reference materials, including a plant gallery and profiles, archives of mailing lists, gardening news, and links.
http://www.hort.net/

LoveToKnow Garden [more]
Short informational articles on gardening topics, vegetables, and ornamental plants, contributed and edited by users.
http://garden.lovetoknow.com

MyGardenGuide [more]
Offers plant database with photos and summary information, questions and answers on various garden topics, and a garden weblog.
http://www.mygardenguide.com/

Ohio State University PlantFacts [more]
Collection of horticultural resources, including a search engine drawing from U.S. university factsheets, plant image database, how-to videos, illustrated FAQs, and glossary.
http://plantfacts.osu.edu/

Pioneer Thinking: Gardening Center [more]
Information and tips on outdoor and indoor gardening. Many articles grouped by topic.
http://www.pioneerthinking.com/gardening.html

Yardener.com [more]
Lawn and garden resources for homeowners whose garden is not their hobby. Includes information and care data for many plants, shrubs, and trees, and dealing with pests and problems.
http://www.yardener.com/