Unfortunate Metaphor

My brother's White House Briefing today includes this zinger:

… at the rally in Cincinnati, Bush uncorked a possibly unfortunate image. From the transcript:

“I appreciate the grassroots people who are here. Listen, you've got to work hard to turn out the vote, and that's what we call grassroots. I want to thank you. I'm here to fertilize the grassroots today. I'm here to ask you to grow. (Applause.)”