What is a gnarl?

How can I effectively sift rocks from dirt? How do you dehull barley? How do you grow healthy roses? Do zebras have any known predators? What is the ideal temperature for orchids? How do plants help to increase oxygen levels in the environment? How often should you water vegetables to ensure they thrive and produce a bountiful harvest? What is the name of the plant that has toxic leaves? Is limestone good for any kind of trees? Why is my neon pothos not growing as expected? What are good nicknames for rose? Are dahlias perennials, meaning they will come back year after year? Can plants grow sideways or upside down? Can potato peels go in compost? What kind of root does a watermelon have? How can I effectively treat and prevent brown spots on leaves of my houseplant? Why do they call it an orange grove instead of an orchard? How do birds of paradise use light to attract mates and communicate in their natural habitat? How can I effectively treat and prevent dark spots on my aloe plant? Why are my philodendron leaves turning red?