The Effect of Hedychium gardnerianum on the Surrounding Soil and Native Flora in Volcano, Hawaii

Part of the Young Naturalist Awards Curriculum Collection.

by Mali'o, Grade 7, Hawaii - 2005 YNA Winner

Whack! Slice! Whap! The stalks of Kahili ginger fell to the ground. My older brothers and I moved on to a new patch. They had their machetes; I, being the youngest, was armed with a stick. We were on a mission to destroy the invader!

An example of dense Kahili ginger, with it large green leaves growing near one of the plots.
Example of dense Kahili ginger (H. gardnerianum) growing near one of the plots

Ever since I can remember, Kahili ginger (Hedychium gardnerianum) has been an enemy of my household. It dominates the ground cover and blocks out sunlight, preventing the regeneration of native plants, and it's difficult to kill. Having huge rhizomes makes the ginger incredibly hard to pull out by hand; a shovel is required to drive the plant out of the ground. Even then, there are always some bits of the rhizomes left behind, which makes the ginger practically impossible to entirely remove. Many of the park rangers at Hawaii Volcanoes National Park (HAVO) grimace when they hear "Kahili ginger." The park has even asked the artists who sell their work in a small gallery in HAVO not to depict Kahili ginger in their artwork. HAVO also tries to keep the ginger away from endemic, and sometimes endangered, plants, usually by digging out the rhizomes by hand, with the assistance of a pickax, or a couple of machetes and a shovel. I grew up assuming that I was doing a very good deed for the environment by depleting the ginger population. I never once wondered, Is ginger  truly  bad for the environment? Does it really affect native germinants? I never once pondered the question of whether Kahili ginger changes the soil; maybe it actually helps native plants and lives in perfect harmony with them. Now I know.

Through a summer conservation program with HAVO, I grew to ponder this question more and more. Not only for Kahili ginger but for other invasive plants as well, such as Morella faya, or Falcateria moluccana, which have both been part of previous studies. I decided that Kahili ginger was what I needed to study. To do this, I made a detailed procedure in order to collect the data required so I could analyze the problem.

First, I made four four-meter-by-four-meter plots, then divided each plot into four two-meter-by-two-meter quadrats labeled A, B, C, and D. The plots were labeled 1, 2, 3, and 4. Quadrats labeled A were those in which the ginger was dug out, and quadrats labeled D were those in which ginger was cut out at ground level. Quadrats B and C were controls, and nothing was done to alter them. Each plot was then mapped out on graph paper, with quick sketches of some of the plants in each.

Malio Plot 1
Plot 1
Malio Plot 1
Plot 2
Malio Plot 3
Plot 3
Malio Plot 4
Plot 4
Click to view enlarged drawings.
A healthy Kawa'u, show growing straight with large oblong green leaves.
A healthy Kawa'u (Ilex anomala) in Plot 4C

About four native plants in each quadrat were measured for height, from ground level. Some of the native plants whose heights I measured were 'ie'ie (Freycinetea arborea), pa'iniu (Astelia menziezii), kawa'u (Ilex anomala), and pilo (Coprosma ochracea). These four graph papers were used as my data charts. In each quadrat, I took four pH measurements in the soil. That's 16 per plot, or 64 total measurements. The rod of the pH meter was stuck into the ground and left in until the meter showed a definite number, which usually took about 30 seconds. The measurements were recorded on notepaper, and the position of the measurements in the plots was recorded on the data charts. In all quadrats A, the ginger was dug out. In all quadrats D, the ginger was cut as close to ground level as possible, so that less than six centimeters, or just the stalk, was left protruding out of the ground.


Plot 1 Dugout Malio
Plot 1
Plot 2 Dugout Malio
Plot 2
Photo of invasive ginger plant dug up and lying on the ground. The plant has large tubers.
Plot 3
photo of Kahili ginger (Hedychium gardnerianum) in study of its effect on native and enemic plants
Plot 4
Photos of dug out plots
A large pile of the ginger plants removed from all four plots.
Total ginger removed from all four plots

Digging out the A quadrats was especially strenuous because ginger roots are very heavy. The plots were positioned far back from my house, because the forest farther away from the house is in a much more "natural state" than that close by. All of the ginger that was removed from the plots had to be put inside garbage bags and then taken to the local dump. It couldn't be thrown in a hole, because it'd grow back in an instant, even if it were deeply buried; and it couldn't be used as plant mulch because it is too stringy to shred. I did this entire procedure on September 19, 2004. On November 2, all the pH measurements were taken again, and on December 22, all of the native plants that had been measured for height before were re-measured, and all the pH measurements were taken once more.

A bar chart of the soil Ph averages per quadrant in plot 1.
Chart 1: Soil pH Averages Per Quadrant in Plot 1

At the end of this process, I analyzed my results. Although at first I didn't detect any obvious trends, under close analysis the data presented an interesting result. The pH measurements taken near hapu'u pulu (Cibotium glaucum) plants were much more acidic (6.5, 5.1, 6.1, and 6.0) than those taken near the Kahili ginger (Hedychium gardnerianum), which proved to be much closer to alkaline (6.99, 6.89, 7.00, and 6.70).

A photo of the large fronds of a malnourished hapu'u pulu
A malnourished hapu'u pulu (Cibotium glaucum) growing out of the middle or a large area of ginger in Plot 4

When I researched previous studies, I found out that other invasive plants, such as Morella faya and Falcateria moluccana, actually changed the chemical makeup of the soil, making it easier for invasive plants to grow and harder for natives to grow. It is possible that H. gardnerianum is doing the same. Previous studies, especially one performed by Kolea Zimmerman, showed that hapu'u pulu is a very important nursery log for native plants, including 'ohi'a lehua. It may be assumed that native plants prefer more acidic soil, because the soil surrounding the hapu'u is more acidic. The soil surrounding the ginger is nearer to alkaline, and many fewer native plants grew next to the ginger than next to the hapu'u.

There were some uncontrolled variables that may have affected this study. The positioning of my experimental plots, for example, may have influenced my data. If the four plots had been directly adjacent to each other, which they were not, the tree cover overhead would have been more similar, as would the amount of rainfall. The rain in Volcano is very much affected by eruptions from Kilauea and therefore most likely contains some acid. If the plots had been close together, the amount of acid rain received would have been more consistent. The level of available sunlight would also have been more consistent. And if the plots had been all in one place, they would have been easier for me to access, and I might have been able to take more data samples from them.

However, studying plots that were spread apart had some important benefits. The distance between the plots meant that they included a greater variety of plant species. If all the plots had been adjacent to each other, there might have been a mass of ginger covering one entire plot, but none in any of the others. The plots may have only had 'ie'ie in them, as opposed to including pa'iniu, hapu'u pulu, and other native flora. With all of the plots scattered, the data collected addressed a bigger range of variables, including what happens if the tree cover overhead is thicker, or if one plot receives more rain than another.

Another key variable in this study was in the height measurements of the native plants. When I measured them the first time, I did not mark the place on the plant's stem from which I had started my measurements, which may account for the "shrinking" of some of the 'ie'ie and a few other plants. The meter stick may have been placed a few centimeters above or below the initial starting point, thus altering the measurement's "dependability."

The weather was another unstable variable that may have affected my project. The first measurements were done during a drought, which means the plants may have been malnourished or slow-growing for that period in time. The rain that came later could have washed away soil at the plants' bases, making it appear that the plants grew when actually their roots were just exposed. This goes back again to how the stems should have been marked when they were first measured.

In closing, my study did in fact answer many of my questions. On the basis of my pH measurements, I now suspect that Kahili ginger actually does change the soil it thrives in, making the soil more alkaline. Native seedlings seemed to thrive in more acidic soils, which then triggers the assumption that all native plants prefer more acidic soils. I found fewer native plants around the ginger.

A few ginger seedlings with green leaves shown growing out of brown ground surface detritus.
A few ginger seedlings that grew back in Plot 2D

My study also shows that digging out ginger works much more effectively than cutting it out at ground level, because the ginger that was totally removed (that means every bit of rhizome) grew back much more slowly than that cut at ground level or left undisturbed.

Thus, I have answered my initial questions. Kahili ginger really does seem to have a negative impact on its environment, and it does make it much more difficult for native species to grow. I guess that I—and many others—will be hacking away for years to come at the overpowering beast that is H. gardnerianum.



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