Recording stem density or percent cover of your target NIS from plots within NIS patches is another good way to evaluate the abundance of your NIS. This method does not rely on GPS technology, but will require you to either purchase or construct sampling frames. Constructing sampling frames is inexpensive and easy. You will need to decide what size sample frames you wish to use. Typical sizes are 1 m by 1 m, 50 cm x 50 cm, and 25 cm x 25 cm squares. The ideal size to use is often dependent on the size of the species you are monitoring: The larger individuals of the species are, the larger the frame needs to be to capture an adequate sample.

Constructing a sample frame:

  • Required Materials and tools
  1. Meter stick
  2. Saw
  3. PVC pipe (1/2” diameter works well) long enough to cut
  4. lengths of your desired frame size (e.g. 1 m, 50 cm, 25 cm etc.)
  5. 4 PVC L joints of the appropriate diameter
  6. PVC cement
  • Construction
  1. Measure and cut 4 equal lengths of PVC pipe for your desired frame size
  2. Brush ends of frame sides and inner surface of L joints with
  3. PVC cement
  4. Join pieces together and let dry

**It is best to join pieces together on a flat surface in order to avoid warped sample frames. If you want to be sure that you select the correct frame size you could choose the size you think is correct and then use 1 larger one (e.g. 0.5 and 1 m frames). You would then sample within the same patch recording the number of individuals or cover up to 10 times. Which frame gives you more similar numbers – the aim is to have the lowest amount of variation possible in your data. There is always variation in plant numbers but one hopes that the variation (plus or minus the mean) is less than the mean value. So, a mean of 15 plants in a 0.5 m frame with a standard deviation of +/-9.6 is less desirable than a mean of 27 plants in a 1 m frame with a standard deviation of +/- 5. Generally bigger frames are better.

Sampling NIS populations with the sample frames:

  1. Randomly locate the position for your first monitoring plot within a population that you are monitoring.
  2. Place your sampling frame on the ground and mark 2 opposite corners permanently by pounding lengths of painted re-bar or 6” spikes with fiberglass stake whiskers (available through For stake whiskers, blue is the most resistant to fading, and last for at least 4 years in the field.

NOTE: You need to label each sample location within a population uniquely so that in future monitoring sessions, you are certain to record data associated with the same plot. There are several types of tags (aluminum and plastic etc) that can be attached to one of your plot corner markers for this purpose

  1. If possible, record the position of your plot with a GPS just in case your plot markers get removed.
  2. Count the number of stems or estimate percent cover (or both) for your target species within the sample frame.
  3. Repeat steps 1-4 above. In general, you want to record data in at least 3 plots in each population that you are monitoring.
  4. Optional: You can also establish monitoring plots at the edge of your population so that ½ the plot is inside the population boundary and the other ½ is outside. This will allow you to see if the population is being affected differently or behaving differently at its margins.
  5. Enter your data into a spreadsheet on a computer
  6. With plots permanently marked, you can now monitor the same plot in successive years or after successive treatments.


Sample frame assembly diagram1. Randomly locate first sample frame location1. Randomly locate first sample frame location. (sample location is shown)2. Place a sampling frame on the ground2. Place a sampling frame on the ground and permanently mark two opposite corners and label the plot3. Mark position with GPS if possible4. Count stems and/or percent cover of the target species and record on a data sheet5. Repeat steps 1-4 at least 2 more times6. (optional) Place additional monitoring plots at the edge of the population and record data7. Enter your data into a spreadsheet on a computer

Click on an image to view as a slideshow.