The Slippery Plant Pests: Algae, Moss and Liverwort By A.R. Chase

One of the toughest problems faced by ornamental plant producers is the prevention and elimination of algae, moss and liverworts that grow on pots, soil, benches, walkways and even on plant leaves and stems. The first step in controlling these pests is understanding the conditions that promote them.

The high moisture, light and temperatures necessary for successful crop production also favor these “lower plants.” Couple these favorable environmental conditions with excessive amounts of fertilizer and algae populations will explode (see bottom left photo). In the western U.S., the presence of liverworts and moss is a constant reminder that we have not managed water and fertilizer levels as well as we might (see top photo).

Plug production can be especially sensitive to algae growth when fertilizer or water is applied in excess. Managing these elements is complex and affected by weather as well as by the needs of specific plug crops. For instance, quick-growing crops such as zinnia are rarely covered with algal mats, while slow-growing crops such as lisianthus are frequently affected by algal growth, which then inhibits the crop’s access to water and fertilizer. Even when fertilizer is applied sparingly and directly to the plants, any small amount of fertilizer unused by the plants will provide food for algae, moss and liverworts.

The appropriate control of these unwanted plants is dependent upon where they are growing. Walkways, pots and benches can be treated with some chemicals Ð but these chemicals can be toxic to plants. Commercial bleach (sodium hypochlorite), bromine, copper, peroxy-acetate and quaternary ammonium compounds are available for control on nonliving surfaces such as walkways, bench tops and empty pots. Algae control in cooling pads is generally achieved with chemicals, especially the quaternary ammonium compounds.

Algae, moss or liverworts can form thick mats on the soil surface in pots. These mats are unsightly and cause problems with water penetration and drainage. In addition, these pests use fertilizer intended to promote crop growth, further limiting crop values.

Removing these mats by hand is a common solution despite the costly nature of “weeding” each and every pot in a nursery. While liverworts and mosses are more common in nursery crop production, they will grow, reproduce and spread quite efficiently once they find their way into a greenhouse. Recent reports have been made of serious problems in perennial production, both indoors and outdoors.

Because they can be toxic, most algae control products are not recommended for direct application to plants. Although a 10-percent bleach solution provides good short-term control of algae on the soil surface, visible re-growth occurs in as little as three days. Algae or moss buildup on walkways creates unsightly and dangerous conditions (footing can be slippery).

Preventative treatments should include keeping the walkway as dry as possible and making sure that runoff from fertilizer applications is minimal. Some periodic cleanup is usually necessary. A pressure cleaner with or without a chemical can speed up removal of algae buildup. Be sure to direct sprays away from plants Ð concentrations that kill algae and moss can cause phytotoxicity. Any plants accidentally sprayed should be immediately rinsed with plain water.

Control with carbamates

Research on a wide variety of fungicides and disinfectants was conducted in 1984 when I worked at the Central Florida Research and Education Center in Apopka. We evaluated control of algae on surfaces such as walkways, potting media and capillary mats. We found that a variety of carbamate fungicides (such as Dithane M45 and Dithane Z78) provided good control of algae for up to five weeks after a single preventative application.

One special cautionary statement must be made regarding use of carbamates for this purpose: Never drench plants with a carbamate product. While many nursery crops may tolerate this treatment, the majority of potted flowering greenhouse plants (i.e., poinsettia and geranium), as well as most bedding plants, are dramatically damaged and sometimes killed when carbamate fungicides are accidentally applied as a drench.

Copper hydroxide (Kocide 101) and quaternary ammonium products did not provide long-term prevention of algal growth in these tests. In addition, Captan provided a high degree of algae control when sprayed onto the soil surface.

We tested the potential toxicity of some products to foliage in both direct sprays and sprenches applied to the potting medium surface. Dithane M45 and Dithane Z78, as well as Physan 20, were safe on all plants tested, while 10-percent bleach caused severe damage to all plants tested. Seed germination of three of the four foliage plant species tested was not significantly affected by these treatments. As mentioned earlier, this does not imply that carbamates can safely be added to the potting media for most other ornamentals.

Control with bromine

Further testing conducted in Florida centered on Agribrom, a bromine compound used to treat irrigation water. Agribrom was very effective in controlling algae without causing damage to numerous bedding plants and most foliage plants. However, a few foliage plants (lipstick vine, ficus, hibiscus, English ivy) developed etching and yellow leaves that were smaller than normal after a spray with 55 to 60 ppm bromine. Applications of 25 ppm bromine were safe on all plants tested. Do not expect this product to “clean-up” an algae problem without using rates well above those labeled. Agribrom treatments should begin on clean surfaces since this will maintain a high degree of algae control.

Control in Ebb and flow

Ebb and flow irrigation systems are gaining in popularity among growers throughout the southeastern U.S. While considerations such as plant quality and disease spread are important whenever these systems are employed, the threat of algae is not viewed as a concern anywhere else in the country. Tests were conducted in 1993 at the CFREC-Apopka to evaluate safe controls for algae in an ebb and flow system. Copper at 2 ppm was found effective for algae control and safe for use in trays containing a variety of foliage plants. This treatment proved more effective than did either bleach or bromine at 30 ppm active ingredient.

Cinnamite, Physan 20, and Phyton 27

Recently, we have been testing a few products for the eradication of liverworts, algae and moss. These trials were performed with liverworts or moss transplanted from woody ornamentals into pots containing only potting media. The pots were then sprayed with Cinnamite (Cinnamic acid derivative used for control of powdery mildew and mites) with or without benefit of added wetting agents. Cinnamite was applied at 1 percent; the wetting agents tested were Silwet, Capsil 30, PsiMatric and Latron B 1956. In a series of five trials, the addition of any of these products did not consistently improve the ability of Cinnamite to kill liverworts and moss (see chart on pg. 62).

Overall, Cinnamite was very effective in killing liverworts and moss Ð obvious damage to the liverwort or moss usually was evident within 48 hours. However, the environment under which the application was made dramatically affected the speed of this reaction. Under consistently wet and cool conditions, the reaction was very slow (sometimes taking more than two weeks to kill the target pest). Under warm, dry conditions, the reaction was faster; with the obvious death of the liverwort or moss occurring within one week (see bottom right photo on pg. 60).

In one test, Physan 20 (quaternary ammonium used at 800 or 1600 ppm) was also evaluated for eradication of algae, moss and liverworts. In this trial, Physan performed very well against algae and did a good job against liverworts but did not affect the moss. Tests of Cinnamite or Physan 20 directly applied to containers of woody ornamentals showed a high level of safety on these crops (mondo grass, camellia, phormium and boxwood).

Finally, we performed a trial to evaluate the ability of Cinnamite and Phyton 27 (copper pentahydrate) to prevent algae growth during Christmas cactus propagation. The products were applied twice (one month apart). We evaluated the growth of algae, as well as damage to the cactus cuttings, one month after the second application. The products were used alone or in combination with PsiMatric or Capsil 30. Although all products were safe on these cactus cuttings, only treatments containing Cinnamite provided good prevention of algae.

Prevention: desirable but not always possible

Obviously, the most desirable strategy is to avoid problems like algae, moss and liverworts, but this is rarely possible under the conditions typically associated with ornamental crop production. Whether you are producing plugs in a high-tech greenhouse or landscape material in an outdoor field operation, unwanted plants are always a concern. Preventive measures Ð limiting light exposure, reducing fertilizer runoff and promoting rapid drying of both plants and their surrounding areas Ð are sometimes possible and helpful. However, chemical products remain the crucial means by which both these pests are prevented and eradicated.

This article was not meant as an exhaustive report on algae, moss and liverwort control products for the ornamental greenhouse or nursery. My apologies to companies whose products were not tested and therefore not included in my article. There are many more products being sold for control of these pests than the few mentioned above. Be sure to follow product labels carefully and perform small tests to determine safety on your crops under your conditions.

A.R. Chase is plant pathologist at Chase Agricultural Consulting LLC and can be reached at