One of my favorites, which is also in our Treasures reading book (in the fall for some reason) is Eric Carle's The Tiny Seed I adore his style and even 7 or 8 year olds get a kick out of the cute story. Someone always says "It's a circle story!" at the end.
Gail Gibbons has some terrific science books. I really like From Seed to Plant. It is clear, informative and second graders can access it. I find so many science books are to difficult for the primary grades; this one is an exception.
Wanda's Roses by Pat Brisson
Wanda is an endearing little girl who cleans up an abandoned lot near her home and is determined to grow roses. Everyone is a doubting Thomas, but they become so invested in her quest they help her get her roses in the end!
How a Seed Grows by Helene Jordan
This is another great non fiction book that primary grades can access. Explains things well but is not too challenging!
Eating the Alphabet by Lois Ehlert
I love her stuff! I don't usually read this aloud but they're so fun for silent reading and buddy reads.
Time for Kids - Plants!
We have fun reading the regular issues of TFK in class. This is a great resource for the plants unit.
Magic School Bus Plants Seeds
Ah Mrs. Frizzle! These books are all super!
Green Toys Planting Kit
This is a great kit made from recycled materials. It includes sunflower seeds and a soil disc. The instructions are pretty easy, too. If I didn't already have our huge Foss kit and was just working with a few kids I would use this.
Planting Seeds: Practicing Mindfulness with Children
This is a very interesting text. It's really for parents and teachers. The author spent thirty years using plants and seeds to teach mindfulness and compassion to children, parents, and teachers. It comes with the book and a CD. There are a lot of fun and thoughtful activities. He says they will "help relieve stress, increase concentration, nourish gratitude and confidence, deal with difficult emotions, touch our interconnection with nature, and improve communication". For a teacher, the concrete activities are my favorite. Read more about it on amazon and see if it would work for you.