Hi
You can 1st segment the plants from the image. To do this, green part is easily segmented by applying the suitable threshold value as it is more likely that the green pixel values tend to be [0 255 0]. Also, you can use some other color space to segment that, for more information on that you can refer this documentation.
After segmentation, you can calculate area of the green part by using regionprops function. You can refer this documentation for that.