In 2004, Dr. Jodi McGraw wrote a 354-page Sandhills Conservation and Management Plan: A Strategy for Preserving Native Diversity in the Santa Cruz Sandhills, for the Land Trust of Santa Cruz County. One section of the report assessed various eradication techniques for invasive plants.
From the report:
A wide variety of techniques have been developed to remove plants or plant biomass by hand, with or without hand tools (manual removal), or using mechanized tools (mechanical removal).
Cutting exotic plants at their base using saws (manual or chain), machetes, loppers, brush cutters, weed whackers, mowers, and brush hogs (which twist off aboveground biomass) can sometimes effectively kill them. Many exotic species resprout when cut, and therefore require physical treatments, such as stump grinding, or chemical treatment with herbicide.
Because cutting often allows plants to resprout, pulling exotic plants out by their roots is often more effective.
The loose sand soil conditions of the sandhills render it fairly easy to hand-pull seedlings as well as adults of many species. Pulling large adult shrubs, including [French] broom (Genista monspessulanai) can be aided by “weed wrenches.”
In 2002, volunteers covered cut stumps of Acacia in the Olympia [watershed]with black plastic tarps that inhibited re-sprouting and thus killed trees in dense stands. Such “tarping,” as it is called, might be used to remove exotic plants in other degraded sandhills sites, but should not be used as a widespread treatment within intact habitat.
The porous nature of Zayante soils may increase the likelihood that herbicides used in the sandhills will be able to reach groundwater and thus potentially contaminate the public water supply.
For this reason, the San Lorenzo Valley Water District (in 2004) prohibits the use of most types of herbicides in their lands. Use of herbicides in adjacent sandhills habitat could similarly contaminate the aquifer and harm sandhills animals including the endangered insects. Proposed use of chemicals to control exotic plant species should be carefully evaluated in consultation with experts as well as officials charged with protecting the aquifer and sandhills animals.
Like all potential management techniques, chemical control methods can have both positive and negative effects via direct and indirect mechanisms, all of which should be considered in evaluating the potential use of herbicides.
In 2001, the San Lorenzo Valley Water District partnered with the United States Fish and Wildlife Service to remove Acacia from the Olympia [watershed].
The water district contracted with the Natural Resource and Employment Program to have a crew remove the Acacia trees from the center of the habitat.
The crew cut the trees using chain saws to no more than 10 cm above the soil surface, burned the cut material, and then experimented with two methods to avoid resprouting: herbicide and tarping.
Two separate herbicide treatments were used: a 2 percent solution of Roundup in 2001, and Garlon 4 in 2002. In both cases, the herbicide was applied within one minute of cutting directly to the stump. The tarped stumps were trenched,then covered with three layers of 6 mm black plastic tarp for two years.
[A study of the results] found that both tarping and Garlon 4 provided kill rates of over 95 percent.
The 2 percent solution of Roundup was not effective, although stronger concentrations may work better.
Given the loose soil of the sandhills, which are conducive to trenching and tarp installation, tarping may be more economical.