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Counting weed plants and estimating density Counting weed plants to estimate density • Determine the weed species whose density you want to estimate and the area covered by the weed management objective. For high - priority weed patches, the area would be the patch. For a larger area infested by a high - priority weed species, it would be the mapped area where the weed occurs in a weed management unit. • Randomly select a spot to begin to collect density data. This can be done by overlaying a grid on the arial photo, or using measuring tapes laid at right angles on the ground, and then using a random number generator to select coordinates. Randomly selected plots can be permanently marked, or new randomly located plots can be selected each time monitoring is performed. This choice will affect the type of statistical analysis used. • At each spot, orient a plot frame with the long axis of the frame running north- south. We recommend using a plot frame that is 2 feet by 4.5 feet because it covers one square yard of ground surface and because rectangular plots are more efficient than other shapes. One can make an inexpensive plot frame by purchasing a 20 -foot length of 1/2 diameter white PVC pipe and four elbow joints. Use a hacksaw to cut the pipe into two pieces each 55 inches long and two pieces each 23 inches long. Fit the pieces together and measure the size of the resulting frame (inner dimensions). Disassemble the frame and trim the pieces of pipe if needed. When the pieces are cut to the proper length, assemble the frame, check the inside dimensions, and glue the frame pieces together. • Count the number of established weed plants that are rooted within the plot frame and record this number on a data sheet. Do not count seedlings but note their presence on the data sheet. See the attached sample data sheets. Repeat the process of randomly selecting sampling locations 29 more times to sample a total of 30 spots, if you will be performing statistical analysis on the data. If the patch is small (less than 100 feet by 100 feet in size), 15 samples should give you an adequate picture of the infestation. • Back at the office or at home, calculate the average plant density by adding up all the density data and dividing by 30 (or 15). Computer spreadsheets can do this very easily. 'Visually estimating weed density • Follow the steps presented in the first three bulleted items in the box above. • Instead of counting the number of weed plants in each frame, estimate the number of weed plants according to the categories below: Category Number of target weed plants in each 2' x 4.5' plot Symbol Absent 0 A Scattered 1 S Light 2 -5 L Moderate 6 -9 M Heavy 10 or more H Enter the symbol (A, S, L, M, or H) for the appropriate category on the data sheet. • Repeat this process of randomly selecting sampling locations 29 more times to sample a total of 30 spots, if you will be performing statistical analysis on the data. • Back at the office, count the number of plots that fell in each density category and record these numbers on your data sheet.