Human Health
Just as some plants are considered desirable and some plants are considered weeds, some plants are good for people and some plants are bad. Simply because people live near and walk past or even pick different weeds all the time doesn't mean they are perfectly safe. Likewise, people may be picking “weeds” that have beneficial qualities.
Never eat a weed unless you are absolutely positive it is an edible plant. Also, weeds that are growing on road shoulders may have been sprayed with pesticides or may be covered with litter, so they are not safe to eat. And just because a plant is an edible weed doesn't mean that every piece of it can be eaten. Some edible weeds must be cooked first! Know your weeds.
Don't Eat Me
It's hard enough getting most kids to eat vegetables, so it's unlikely that you'd have to worry about elementary-school kids eating plants. Still, there are many weeds and plants that are poisonous to people, and children should be aware of the dangers. Most of these types of weeds are listed in each state's noxious weed list; they must be by law. You can find your state's list through the USDA at http://plants.usda.gov/java/noxiousdriver.
Perhaps the best way to impress how plants can be dangerous to health is by starting the discussion with common plants that can be harmful. We eat fruits like rhubarb and tomato, but the leaves of the plants are poisonous. We are surrounded by many beautiful plants, many of them cultivated intentionally by gardeners, which can be harmful as well. The stems of a daffodil or the flowers on a foxglove are just two examples. People need to be very careful about ever putting any part of a plant into their mouth. Children have gotten sick simply by using hollow stems like straws.
If you can, invite someone from your state university extension service to talk to your class about poisonous plants. If at all possible, have them bring specimens so everyone is very clear what they look like and what they need to look out for.
Pick invasive weeds before they go to seed. This prevents them from reproducing. In addition, make sure you throw them away in the garbage. If you put them in your compost pile or just throw them in with other brush, they still have a chance to reproduce.
Nature's Medicine Cabinet
Plants can be poisonous, but they can also have medicinal properties. For thousands of years, indigenous peoples have used native plants for their healing qualities.
The common dandelion has amazing healing qualities. Fresh dandelion juice is useful on wounds because it has antibacterial qualities. The latex in the dandelion sap can be used to remove warts and corns. Dandelion is also used for digestive disorders and skin problems.
Red clover is used as a sedative and a cough suppressant. It can also be used to treat skin problems and even cancer.
Mullein has been shown to have antihistamine and antiviral qualities.
Are there any common weeds in your region that have medicinal qualities? Maybe they are simply edible. Dandelions are used for salads. Can you find a local wildflower recipe to try in class?
