The people planted these plants and destroyed them thousands of years ago.
Maddy and Jack get the beans sold to them by a sketchy man who takes their car in exchange for the beans. Maddy starts to plant the beans and the plants start to grow.
Resolution
Youre mighty Jack!
The climax was when the plants started attacking them and tried to kill them, but it was not working. Maddy got hurt, and mom rushed her to the emergency room.
As giant beasts erupted out of the ground, Jack thought he had to kill the plants. Then he did it by blowing up the plants with gasoline. When Maddy found out she was devastated and laid down in the dirt for hours. Lilly slapped Jack and they were all sad and angry at one another.
For a very long time, there had been a black seed that they never planted. Maddy planted it and grew an evil garden. A beast took Maddy and the book finishes with Jack and Lilly going back into the garden.