Look for and read "The Truth About Organic Gardening" by Jeff Gillman. That will prepare you for that program better then anything else.

The topic of organic gardening is very easy to start rambling. If you are worried about missing your major points, summarize them at the beginning of your talk. Then write out a full script about each topic. Write out 20 minutes worth of text and practice it repeatedly until it sounds like you speaking normally. Stick strictly to the script or you will ramble. Don't make the mistake of fleshing out a topic with words that are not on your script or you will ramble. If you practice it enough and modify the actual script as you practice, you will have all the words you need in the script. Then at the end of your talk, repeat the major topics again.

I used to speak on technical topics and found the only way I could stay perfectly on topic was to have a full script written out beforehand. I never ever stuck perfectly to the script because my talks were interactive, but whenever I needed to get back on topic, I could find my place and get back into the groove.

In my mind a quick summary of organic landscaping would be, "organic garden care relies on nature's predators (birds, toads, lizards, and insects) along with microbes (bacteria, fungi, protozoa, and microarthropods (microscopic insects)) to manage your garden for you. All you have to do is feed them and not poison them. When you understand this and learn what foods to feed the beneficial critters in the garden, then most of your issues will take care of themselves."

There are many other ways to describe organic gardening. If you believe in organics the way I've described it, then you might pick as your main topics the following: Large Beneficial Critters, Microscopic Beneficial Critters, and Feeding your Beneficial Critters.