• Garden Guides - Great design. Find what you want fast. Simple fact pages on a wide variety of gardening topics: annuals, bulbs, perennials, vegetables, herbs, tips...
  
• Horticulture Corner - From the University of Illinois, College of Agriculture Cooperative Extension Service. Plain, fast, easy to find information on a wide variety of horticulture topics.
  
• Garden Web - Excellent resource with vast amount of information.
  
• Horticultural Myths - by Linda Chalker-Scott at the Washington State University Urban Extension Center.
  
• Brooklyn Botanic Garden - Tons of great material online for gardeners located anywhere.
  
• Ed Hune's Gardening Archive - Detailed information on growing specific vetetables, flowers, fruits, trees, etc. Also order Hune seeds online.
  
• United States Department of Agriculture - Home Gardening - Find facts on growing plants in your garden.
  

Also see: Articles on invasive plants - Curious Cat Science blog posts on plants