Find this: weedseeds.garden Weed seeds are the seeds of weed plants. They grow into weed plants. These plants produce the plant matter that is used for smoking, eating, or making medicine. There are many different kinds of weed seeds that grow into different kinds of weed plants. Where do Weed Seeds Come From? Weed seeds come from weeds. Weeds are any kind of plant that grows in places where people don’t want it to grow. Weeds can be found in gardens, on roadsides, and in forests. Some weeds can be poisonous to animals and humans if they eat them or touch them with their skin or fur.