 From VOA Learning English, welcome to agriculture in special English. I have a question for you. When is a plant considered a weed? Well, the experts at Penn State University have a simple answer. When the bad qualities outweigh the good qualities. Consider the fact that crops produce several hundred seeds from each plant. By comparison, each weed plant can produce tens or even hundreds of thousands of seeds. And some buried seeds can survive up to 40 years or longer. Eradicating or getting rid of weeds means that you have to remove all the seeds and roots so the plants will not grow back. But birds, wind and weather can bring them back to the land. A more common way to deal with weeds is to control them so that the land can be used for planting. Experts advise using two or more control methods. Chemical weed killers or natural treatments like corn gluten can suppress weed growth. Dense planting of a crop can also act as a natural control. Bill Curran is a professor of weed science at Penn State University in Pennsylvania. He says one of the most common methods for suppressing weeds is dense planting. He says a dense competitive crop that quickly shades the soil will help. The weeds need light to grow so blocking the sun will reduce their growth. Other controls include turning over the soil, pooling the weeds or covering them with mulch made of shredded wood, garden waste or other material. But even mulch has its limits. Weeds can be transported in mulch. This is also true of soil, grain, hay and animals. Animals like sheep or goats eat weeds so they can provide a biological control. Even insects and other organisms can also act as biological controls. For VOA Learning English, I'm Carolyn Presuti.