Global Picture
Wherever plants go, there's the potential for weeds. Plants grow on land, in water, and even in tiny crevices in rocks. Likewise, weeds grow almost everywhere you give them a chance. There are probably even more plants that are considered weeds today than in the past because so much land has been developed for specific uses. Once people decide to use the land for something, they don't want anything there that doesn't belong — including weeds.
Alien Invasion
Invasive plants are those that come from somewhere else in the world. These weeds are called invasive, alien, or exotic. People may have planted them originally thinking they were pretty or they may have sneaked to their new home through no conscious effort by humans. Either way, they can quickly out-compete native plants.
The reason many people do not like invasive species is that they can quickly reproduce and multiply, since they have no natural enemies. They end up using resources like water and nutrients from the soil before the native plants can. This means they may eventually wipe out the natural biodiversity of an area, which ends up harming the insects, birds, and animals that live there as they lose their food sources and natural habitat.
Most plants end up where they aren't supposed to because humans moved them, either knowingly or unknowingly. This started as long ago as ancient Roman times. Explorers found exotic and beautiful plants and decided to bring them home. Today, there is much more global travel and thus, many more opportunities for plants to find their ways to new homes.
Sometimes seeds or spores are accidentally carried in the cargo of ships and airplanes.
When ships have no cargo or a light load, they fill the ballasts with water or soil to help weigh the ship down and keep it steady in the water. When the ship arrives at its destination, the ballasts are emptied and stray plants can be inadvertently dumped in strange places.
Many exotic species are introduced because people use them as ornamental or decorative plants. Most of the plants on the World's Worst Invasive Alien Species list were introduced intentionally for decorative reasons.
More people are traveling the world and bringing plants, seeds, fruits, and flowers across borders.
It's never easy to find out exactly how a new species gets introduced, but you can find out where it originated. Choose a list of invasive species that grow in your state. The U.S. Department of Agriculture site www.invasivespeciesinfo.gov will take you to lists for your state. Assign one to each child to find out where it is from. Use a world map and chart how far the plant traveled.
Seed Banking
Climate change, droughts, floods, and human development are all placing pressures on the natural environment. One way that people are trying to protect the world's plants is by collecting and preserving seeds. People cannot always protect every natural habitat, and trying to keep and grow plants in greenhouses can become very difficult over time. Preserving seeds is much easier because they take up such a small amount of space, they don't need much attention, and they can be kept for hundreds of years.
According to the National Strategy for Invasive Plant Management, it is estimated that invasive plants have infested more than 100 million acres of land in the United States. An addition 3 million acres are lost every year, which means invasive plants are taking over 4,600 acres every day.
The U.K.-based Millennium Seed Bank project has already gathered millions of seeds from all over the world. Norway recently built the world's largest vault for storing seeds. It already has more than 100 million seeds and keeps them safe inside the vault built deep into a glacial mountain between Norway and the North Pole.
Let your students explore the beauty of seeds by making seed art. Use a variety of seeds and dried beans that are all different colors. Have students draw a picture using a pencil on a small piece of sturdy cardboard. They should glue the seeds on to fill in the picture, almost like creating mosaic. Use seeds that are common.
