Control of liverworts and mosses in greenhouses

Summary of research results from Sven Svenson

Oregon State University, North Willamette Research and Extension Center, Aurora, Oregon

 We have been studying control of Marchantia (and some other unidentified liverworts) in greenhouses since 1996. We sought alternatives for control of liverworts in container production of ornamental crops (see Svenson 1997, 1998; Svenson et al. 1997). Most of the research has been supported by the Oregon Department of Agriculture, with some additional support from the company Natural Plant Products.

Control of liverworts (and mosses) is needed because heavy infestations on the soil-substrate surface causes irrigation water (and any liquid fertilizers) to pass between the growing substrate and the side of the pot, washing out the container's drain holes before it can soak into the substrate. Heavy liverwort infestations cause growers to use more water (and subsequently more fertilizers and pesticides) to keep crops from wilting. Thus, regulating the liverwort population in container production systems reduces the potential for environmental pollution by reducing the growers need to use excess water, fertilizers and soil-applied pesticides (even if chemicals are used to control the liverworts). Finally, the liverworts provide a "house" for fungus gnats (which are proven to damage roots and spread crop diseases).


 Interestingly enough, the chemical-control side of our research has shown that available herbicides (have a container nursery label) do not provide adequate control of liverworts using typical nursery production systems. Originally, our best treatment was a cinnamon oil extract (originally sold as CINNACURE by ProGuard, Inc.). This product is now sold by MycoTech under the name CinnaMite (active ingredient: cinnamic aldehyde). Similar products have been used in hospitals for sanitation. I am not sure a nursery label that includes liverwort control is available. The product must be used with caution, as it will show phytotoxicity on nursery crops under certain environmental conditions. At the correct concentrations, cinnamic aldehyde kills mosses, liverworts, and pearlworts (Sagina sp.), without any phytotoxicity to the crop plants. It also kills ornamental crops like Scotch Moss, which are also in the genus Sagina. The status of registration for home-owner use for application to walls and roofs for moss control is unknown. Since the breakdown products are "natural," do not last for very long in the environment, and have very low toxicities, the EPA was expected to fast-track use-approvals for this product. Expect its availability soon. However, the cost associated with EPA registration may be preventing the development of this product.

We have tested cinnamic aldehyde for control of moss on brick patios. The product does kill moss, but applications must be repeated several times each year. Again, I do not know of a registered label that allows this use.


Another product we have tested that is very effective for control of liverworts is Mogeton (active ingredient: quinoclamine). Quinoclamine was originally developed by Uniroyal back in the 1960's for algae control in rice paddies. A Japanese company now manufactures this herbicide, and it is registered for use throughout Northern Europe for liverwort control in nurseries. To our knowledge, this herbicide has never been registered for any use in the United States. Mogeton has very low risk of phytotoxicity to nursery crops, but similar to CinnaMite, the risk of phytotoxicity from Mogeton can increase under certain environmental conditions.
As of May 2000, Marchantia infestations in container nurseries and greenhouse operations throughout Oregon, Washington and British Columbia have reached epidemic proportions. The cost to remove the liverwort by hand, one pot at a time, can completely consume the profits from each pot. In short, Marchantia is slowly putting many nurseries out of business. Greenhouse and nursery operations in Colorado, Michigan and North Carolina have also contacted us for information on Marchantia control.

We are preparing an emergency request of the use of Mogeton in Oregon and Washington, hoping to preserve the profitability of many businesses and the livelihood of their employees.
Another option comes from Oregon's meadowfoam seed growers. This grower cooperative (Oregon Meadowfoam Growers) has supported research that has shown that meadowfoam seed meal (the meal left over after the crop's oil has been extracted) can be used for effective liverwort and moss control. Meadowfoam, or Limnanthes alba, is native to Oregon and has been bred to produce an oil that serves as a higher-quality substitute for whale or jojoba oil in the manufacturing of lubricants and cosmetics. The cost of applying the seed meal to nursery crops is much higher than spraying an herbicide like Mogeton, but the seed meal is a natural product and may provide less environmental risk. Use of the seed meal may be safe in some locations where use of herbicides might be an environmental risk. An emergency request of the use of this seed meal for Marchantia control is also being prepared.

Another product that is routinely effective is vinegar (acetic acid). There are no products available to nursery growers that permit the use of vinegar for killing liverworts. When applied to container nursery crops, the concentration of vinegar must be adjusted to the prevailing weather conditions to avoid phytotoxicity to crop plants, and the acid usually needs to be rinsed off the foliage of crops plants soon after application. My advice with respect to vinegar is "use at your own risk." The safest use is in mid-winter on deciduous crops before new leaves have emerged from dormant stems.

Otherwise, here is my general response to nursery and greenhouse growers...

1. Do not overwater. If the nursery crop will tolerate the drying, allow the surface of the growing medium to dry between irrigation cycles. If possible, switch to subirrigation systems.

2. Do not apply excess nitrogen or phosphorus fertilizers. The fertilizer concentrations that are above the "required" amount for your crops are often optimal for liverwort growth.

3. Surface applications of slow-release iron sulfate and/or copper sulfate help prevent liverwort infestations. Fertilizers applied to the surface of the growing substrate (as a liquid, granular or slow-release product) support more liverwort growth than if the fertilizer is incorporated into the growing substrate before potting (unless the surface-applied slow-release fertilizer contains adequate amounts of iron, zinc manganese and copper). Zinc sulfate or zinc chloride fertilizers can help control liverworts, be the amount applied to kill the liverwort is often toxic to nursery crops.

4. Liverworts generally die if the crop's canopy will provide sufficient shade to the surface of the growing medium.

5. In the Pacific Northwest, liverwort infestations are more likely to start in early fall through late spring. The heat and dry air of summer will help reduce liverwort growth and establishment. In greenhouses or structures that maintain high humidity, or under production systems with frequent overhead irrigation applications, liverworts are a problem year-round.

6. Surface mulches that dry rapidly will reduce liverwort establishment. The most useful mulches we have tested are: hazelnut shells (not crushed too small); oyster shells; filter-fabric weed barriers (often treated with copper hydroxide). Consumers have stated that they dislike the appearance of the hazelnut shells, oyster shells and weed "discs," and their purchase and application reduces nursery crop profitability.

7. A combination of reduced irrigation frequency, reduced nitrogen and phosphorus application, and a surface-preventative fertilizer (slow-release iron sulfate) has provided nearly 80% control of liverwort infestations. Several nurseries have nearly eliminated their liverwort weeds by changing irrigation practices and "spot-treating" wet areas (walkways, under benches, driveways, etc.) within the production area. Use of subirrigation greatly reduces the establishment of liverworts.

8. Useful pesticides for liverwort control do not currently have a label for this use. These chemicals are usually labeled for use as fungicides, and usually contain the heavy metals manganese and/or zinc. The effective fungicides often have REI's of 48-hours, which restricts their usefulness in any ornamental plant production system.

9. Chemicals that do currently have a "label" for use on mosses (and sometimes liverworts) are usually phytotoxic to crops if applied to the foliage. Examples: DeMoss; GreenShield; Physan. These work with some satisfaction for walkways, under benches, etc., but most growers have not been able to use "over-the-top" foliar sprays without some level of damage to the crops. Similar damage risks have been observed by growers spraying vinegar for liverwort and moss control.

10. Available preemergent herbicides do provide some control of liverwort, but common nursery irrigation and fertilization practices often reduce the effectiveness of the herbicides. Reducing irrigation application frequency, and reducing fertilization, in combination with the use of peremergent herbicides does provide moderate control of liverwort infestatons.

A cooperating grower in Washington found a fungus growing on his liverworts, effectively "controlling" the liverwort infestation. This potential biocontrol agent has resisted laboratory culture (to date).

Finally, most of our cooperating nurseries and greenhouse growers use recycling irrigation systems, systems that include chlorination, bromine injections, and/or ozonation. We have always been able to grow liverworts from water samples collected after treatment (suggesting that the gemmae, spores or liverwort fragments are resistant to typical commercial-sanitation procedures used by our ornamentals industries). I have done no studies, but I suspect that air-borne spores can be wind-translocated over very long distances (so growers will "never" be completely rid of the liverwort pressure).

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