Urban Agriculture Notes is not set up to answer your horticulture questions, so try these sites.
GardenNet includes pages titled Garden Links, Information & Shopping Directory, Garden Guidebook, Garden Shop, What's New, E-Catalog Request, Ask Daisy!, RoundTables..
"The Garden Gate offers gardeners and nature lovers a carefully-selected and well-organized collection of links to informative and interesting horticulture sites around the world. So if you enjoy plants, either in the garden or in nature, stop on by, the gate is open!"
GardenWeb features links to The GardenWeb Forums, GardenWeb Directory, The Mystery Plant Contest, The Garden Bazaar, Sesbania's Gardening Tips, GreenLeaves, Glossary of Botanical Terms, RoundTables, Calendar of Garden Events.
Earth&Table "Moderated Email Digest & Resource Site. The list functions as a roundtable discussion for those interested in growing, preserving, and preparing healthy and delicious food. Subjects range from composting to book reviews, new seed technologies to irrigation, raised beds to casseroles."
Browsing4gardens "We aim to be the ultimate on-line resource for garden accessories and gardening related information. We currently have links to 3178 UK and worldwide sites. Our regular features are listed below. Keep checking the site regularly, more features will be added in the near future."