by Tamara Scully
When Debalina Saha, assistant professor with the Michigan State University Department of Horticulture, wanted to assess the primary weed issue in Michigan’s nurseries and greenhouses, she wasn’t too surprised to find that liverwort – there are somewhere between 6,000 and 9,000 species – was one of the top concerns.
Luckily, growers don’t normally contend with thousands of species of liverwort. There is one primary species which is of concern in nurseries and greenhouses: Marchantia polymorpha. Like all liverworts, the combined factors of damp substrates, low UV light levels, cool temperatures and a moist environment mean that nursery containers – and the ornamental plants growing in them – often become prime targets for this primitive plant, which forms a mat and is often mistaken for moss.
“These conditions are actually very prominent in the greenhouse and nurseries and the organism thrives and spreads rapidly once established,” Saha said.
Liverwort not only spreads rapidly, it also competes well for the nutrients, water and space it needs to thrive. The ornamental plant’s root zone isn’t able to receive adequate water or fertilizer when liverwort has spread on the container substrate, and the plant becomes weak and spindly. Liverwort reduces the quality and value of ornamental plants.
The prime conditions in which M. polymorpha thrives include temperatures from 64º – 72º F, low levels of UV light, high fertility and damp areas. Containers growing in or out of greenhouses often meet these conditions, and the risk is greatest if they are irrigated with overhead sprinklers.
Liverwort grows low and spreads across the container (or perhaps on the ground cloth or anywhere else in the nursery where drainage is poor). It has a thallus structure – its vegetative tissues are not differentiated into roots, stems or leaves. It’s bright green in color, with purplish margins. The plant’s thallus structures are dichotomously branched, with each thallus measuring between one and three inches, and individual lobes on the thallus no more than about one-half inch wide.
Liverwort has reproductive organs known as gemma cups, which are found along the middle of each lobe of the thallus. These small oval cups are about one millimeter in size. They contain numerous asexual plant buds (gemmae), each capable of producing a new plant. The gemmae spread when irrigation or rain splash on the surface, moving them to a new location, where a new liverwort plant or two will be established from each gemmae, germinating when the temperature and moisture levels are conducive to survival. Asexual reproduction can also occur via fragmentation.
But asexual reproduction isn’t the only way liverwort propagates. It also has sexual reproductive structures, with male and female structures separated on different thalli. The male structure (antheridia) is a flattened disk on a narrow stalk. The female structure (archegoniophore) produces eggs and is a stalked structure found on the underside of lobes.
The sperm from the antheridia move through water to the reach the antheridia and fertilize the eggs. Once fertilized, spores are produced. The spores are dispersed by water or wind, and germination occurs when conditions are moist and at suitable temperature.
The primary management strategy for liverwort is reducing substrate moisture. Eliminating overhead irrigation is a preventative measure.
“We need to see that the moisture is reduced,” Saha said. “Try to improve the drainage in the containers and in the greenhouse or nursery facilities.”
Over-fertilization also leads to liverwort growth, as the plant thrives in high fertility environments. “This liverwort… reproduces rapidly at an elevated level of nitrogen and phosphorous,” she said.
Topdressing fertilizer in containers promotes liverwort growth. Incorporating the fertilizer into the container substrate has been shown to depress its growth.
Sanitation is also important. Peroxide or quaternary ammonium should be used to disinfect tools, pots and greenhouse surfaces. It’s also imperative to examine any incoming plant materials for liverwort and prevent introduction into the nursery facilities.
There are few pre-emergent or post-emergent options for liverwort control, particularly in greenhouse environments. Some options are not specifically labeled for liverwort, but might provide some control outside of greenhouse use, such as oxyfluorfen or oxadiazon. Container and field production operations have more control options than do greenhouses. As a pre-emergent, flumioxizin is labeled for liverwort control, but cannot be used on greenhouse plants.
Post-emergence use of flumioxizin is also available for liverwort control outside of the greenhouse. Some studies have shown that acetic acid is able to provide post-emergent control and some products containing this ingredient are labeled for approval for nursery and greenhouse use. Some chemicals which do control liverwort post-emergence – ammonium nonanoate, diquat, sodium carbonate peroxyhydrate and pelargonic acid – can also cause significant damage to ornamental plants.
Saha’s ongoing research is examining non-chemical methods of liverwort control. She’s focused on the effect various types of mulches have in suppressing liverwort growth. It’s been previously found that certain organic materials, when used as mulch, can inhibit liverwort, including parboiled rice hulls, pine bark or hazelnut shells.
In her research, Saha will be assessing the depth and moisture-holding capacity of organic mulches and their effectiveness as pre-emergent liverwort control. The phytotoxicity of these mulches in a container plant setting will also be studied. Her research involves the use of rice hulls, hardwood chips, coco shells and pine bark. Two varieties of hosta are used as the ornamental plant.
“We are trying to assess how these mulch toppings, and the depth and the moisture, can prevent the liverwort growth,” she explained.
Further research will be devoted to examining herbicide efficacy, including organic versus chemical control. Fertilization rates and application procedures – topdressing , subdressing and incorporation – and the resulting effect on liverwort growth patterns will also be studied.
“The topdressing and the incorporation have shown more weed growth in my previous experiment,” Saha said.
Finally, Saha’s research will explore whether the organic mulches currently being studied have any allelopathic effects on liverwort.
An MSU fact sheet on liverwort is available at https://tinyurl.com/y6ee5bzk.