Edge Effect

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Author: Tricia Edgar

In this experiment, you’ll see how sneaky weeds try to get into the forest and discover how a web of trees acts as a weed filter.


How do weeds move into forests? To find out, we'll look at the population density of weeds inside and outside the forest edge.


  • Plant identification book
  • Measuring tape
  • Camera
  • Pegs
  • String
  • Notebook
  • Pencil


  1. Before you head outside, familiarize yourself with the local weeds. Common weed species can vary a lot from place to place. If you’re not sure what a weed is, go to a roadside, your garden, or any place where nature is allowed to do its thing. Find the leaves and flowers of the area’s most common plants. Take photos of them and identify them using your plant identification book. If the plants are in your own yard, you can pick a few and bring them to a garden store if you’re not able to identify them yourself.
  2. Ask questions and do some research about the ways in which these weeds travel and reproduce. Do they move underground through roots, popping up every so often? Do they have seeds that birds eat and distribute throughout the forest? Do their seeds move by wind to land on a new patch of ground to grow in?
  3. Get familiar with native plants as well. What’s local to your area? What naturally grows in the forested areas around your home?
  4. Now, find a forest! Whether it’s in a local park or in your own backyard, you’ll need an area where the forest meets an open place. This area is called the forest edge. The more forested area you can find, the better.
  5. Create a hypothesis, your best guess about what is going to happen. Will the forest edge or the forest interior have more weeds? What about the areas in between? What percentage of plants in each area will be native species that are local to your area, and what percentage will be weed species?
  6. Set up study plots in three zones: a zone at the forest edge, a zone 10 feet or more inside the forest, and a third zone 10 feet deeper into the forest. Measure a 2-foot by 2-foot study plot at the forest edge, and mark it with pegs and string. Now, walk in a straight line 10 feet into the forest, and mark another 2 foot by 2-foot plot marked with pegs and string. Finally, walk 10 feet more (or farther if you wish) and mark another plot with pegs and string. Make sure you have permission to go off the trail!
  7. Using your notebook and pencil, take note of all of the plant species that you see in each square. Use your camera to photograph those that you don’t know, and bring your photos to a local nature center or garden store so that they can help you identify the plants. Create a bar graph of the different species that you see in each plot. Compare the graphs with each other. How are the plant populations different from one area to the next? How many weeds are there on the forest edge versus the forest interior?


At the forest edge, there are more weeds and fewer native plants.

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