GardenHacker:
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Gardening Podcasts! Total immersion Technique for Gardeners

There are a couple of podcasts that I can't do without: The Alternative Kitchen Garden (AKG), and Blue Collar Gardeners (BCG). They're from England and Texas, respectively, and while their styles differ dramatically, they both offer a great deal of good information in a way that's not dry, yet not full of themselves.
Emma, aka "Fluffy Muppet" - the host on AKG, focuses on one topic, covering it quite thoroughly, yet in a relaxed manner as if you were chatting with a friend from "across the pond". Her flickr site has some nice pics of what she's up to, too.
BCG are a couple of professional landscapers and gardeners (from what I can tell) from Texas, who really know their stuff, too. They have a relaxed and informative show that generally focuses on one topic, but they also read from their mail bag and joke around a bit - though not as much as "Click & Clack", the auto call-in show from National Public Radio. I listen to both podcasts (and re-listen) during my short commute to work and back, and I'm never disappointed, though the April Fool's show from BCG I just wasn't in the mood for (I must have gotten too many people telling me my loafers were untied that day). Give them a listen at: Atlernative Kitchen Garden
Blue Collar Gardeners