Counting Kudzu: Milli-Acre Sampling

The surgical treatment for killing kudzu is labor intensive because it means removing each rooted node and crown, although it is a simple operation and highly effective. But it is "cost effective" only if volunteers do the work! Other methods are less effective, and it would be useful to measure their effectiveness by counting the number of plants killed. That is, count the number of crowns that survive a treatment.

Our prior work, and published research, teaches us that there are tens of thousands of kudzu crowns per acre. Therefore, an exact count is not practical. A sampling approach is necessary. Dr. James H. Miller from Auburn University shared his approach with the Coalition. He cuts four 3 foot 3 inch sections of PVC tubing, and joins them together to make a square. The square is one quarter of a milli-acre (0.25 one-thousandths of an acre). The idea is to place the square over ground with kudzu, count the number of rooted nodes and crowns within the square, multiply by four, and then multiply by one thousand. That is, multiply the count by 4,000. The result is an estimate for the number of kudzu plants per acre, based on the one milli-acre sample area. This is done at several different locations within an area infested with kudzu, which produces several estimates for the number of plants. The maximum and minimum counts in a quarter milli-acre provides an indication of how kudzu density varies in an area on a quarter milli-acre scale. The average of the calculated per-acre values is an estimate for the total number of plants in one acre of the area. Multiplying this last result by the number of acres in an area estimates the total number of plants in the entire area.

During the dormant season the sampling device is tossed onto the brittle gray kudzu vines. The leaves have then crumbled and fallen away. Tossing it provides some randomness to the measurements (improving statistical reliability), an idea from Coalition volunteer Dr. Gill Newberry. During the growing season when kudzu is waist high, one side of the device is removed so it is "U" shaped. This makes it easier to push the two "prongs" of the device into a mass of kudzu at ground level for sampling. This last tip comes from Dr. Miller.

Our measurements in unshaded flat fields of kudzu are from 30,000 to 180,000 kudzu plants (crowns and rooted nodes) per acre. Even the lower value is a huge number! We suspect that the maturity of the kudzu patch has an impact on the total, and conjecture that more mature infestations are more densely packed. Under optimal conditions a worker can remove about 50 plants per hour using the surgical treatment. So this means that a single worker, removing 50 plants per hour, would take 75 to 450 8-hour days to remove one acre of kudzu. 75 volunteers could do the job in 1 to 6 days. Faster methods are obviously desirable, even if they are not 100% effective. The skid loader treatment pulls up many rooted nodes, and even middle-sized (golf ball size) crowns, during clearance of biomass. Relatively large areas are treated quickly, so even if treatment only killed half of the kudzu plants, it would essentially halve the amount of time for volunteers to clear an area of kudzu using the surgical treatment. The sampling method provides a way to determine whether this is the case. Thanks to Coalition volunteer Paul Savko (left in October 2006 photograph below) for constructing milli-acre frames, and to Barbara Daniels and Rich Mead, kudzu counters extraordinaire!

As shade increases, and/or as the terrain becomes rougher, the amount of sunlight reaching kudzu decreases. So we expect the number of kudzu plants to fall below numbers found in open fields, and plan to determine whether this is the case. The number of kudzu plants is much smaller at a forest edge than in an open field, and drops to zero deep into a forest. (Jack Callahan reports that kudzu cannot grow when light falls below about 60 foot-candles.) Paradoxically, crown size is largest under trees: Kudzu vines that reach high up into trees are exposed to much sunlight, causing the vine to increase greatly in size (increasing crown size above that of plants not in trees), and shade-out competing kudzu plants (reducing the plant count below that of plants not in trees).

The large number of kudzu plants per unit area does not discourage us. While there are an *awful* lot of crowns per acre (pun intended), Coalition treatments are improving almost every month.

Newt Hardie & Lou Adams

May 2007

May 2007