Salvinia Control

Two species of the invasive aquatic salvinia are found throughout Louisiana.

Common salvinia, Salvinia minima, which is native to Central and South America, was first identified in Florida in the 1930s. From there it has spread to every southern state in the U.S. Giant salvinia, Salvinia molesta, also native to South America, has spread to almost every temperate and tropical country worldwide. It has become known as the most noxious of all aquatic weeds.

While giant salvinia at maturity is quite a bit larger than its cousin the common salvinia, the two can be distinguished from one another through close observation of the upper leaves. Both species have many small spike-like hairs on the upper leaf surface. The giant salvinia’s hairs split into four prongs that rejoin at the tips to form a structure that resembles an eggbeater or cage. The four prongs on common salvinia do not rejoin at the tip. The giant’s young plant leaves lie flat on the water similar to the common salvinia in size and appearance, however as the giant salvinia grows and matures the leaves begin to fold upward and inward.

Giant salvinia is much more damaging than its smaller cousin because of its ability to grow into dense mats that can cover entire water bodies with a thick layer of vegetation. These mats smother native plants by blocking the penetration of sunlight into the water.

Other consequences of giant salvinia are reduced dissolved oxygen in shaded waters, causing fish kills and reduced production of submerged aquatic plants. This severely reduces the value of an area as waterfowl habitat. Solid mats of giant salvinia may also reduce or eliminate boating and fishing opportunities simply because boats are not able to push through the thick mats.

Controlling salvinia is difficult and can be quite costly. Common salvinia does not tolerate salinities and dies in a few days at levels at or above three to four parts per thousand. Giant salvinia is more tolerant requiring salinities of 7-10 parts per thousand for a week to begin killing it. In areas where salt water is not an option, such as in fishponds and freshwater marshes, herbicides can be used.

Diquat dibromide, commonly sold under the brand name of REWARD is a contact herbicide that is effective at controlling salvinia. Another option is to use an aquatic labeled formulation of glyphosate, commonly known as the Roundup family of herbicides. It is widely sold under many brand names.

When applying these two herbicides it is important to add a surfactant which breaks the surface tension of the herbicide mixture allowing it to coat the plant and stick to it long enough to work. When controlling salvinia in waterfowl hunting areas, time of application should be considered. It is most desirable to control unwanted vegetation without killing desirable waterfowl foods, if possible.

The ideal time is early in spring before the peak of growing season when infestations are light, water levels allow easy access, and it is less costly to spray small areas of growth. Spot spraying throughout the summer to maintain control should follow this.

In ponds with no flow, another herbicide option is to use Sonar or Avast, which requires a long residence time to be effective. It is quite costly, but one application gets control for up to a year. The long-term plan for control is through introduction of salvinia weevils throughout each watershed until the population grows large enough to keep the salvinia in check.

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