Sandy,
Since your students are learning about agriculture in High School, I suggest that you include actual hands-on learning activities using realia- real life objects. You might want to consider actually bringing in a live plant and having that be the basis for your English Discussion- or, having the students actually have to plant their own plant from seed, and then keep a daily journal of the progress, and also share this journal orally with the rest of the class.
If you have access to a computer with Internet Access, you also might want to make a Google search for Agriculture and Horticulture ESL Lessons, as well as check out the different ESL Sites listed under the Resources Banner on this website. I'm sure you will be able to find lots of good ideas to help motivate your high school students.
Dr. Yanni Zack