Liverwort is one of the major weeds in greenhouse and nursery production. Historically, liverwort has been reported as weed problem in cooler regions of the the United States including the Northeast and Pacific Northwest. It is primarily a problem in herbaceous perennials, woody ornamentals, or those crops with long production cycles with a dormant or vernalization period, where crops remain damp for long periods of time (Fig. 1). There are 6,000 to 9,000 species of liverwort with Marchantia polymorpha being the most common one prevailing in greenhouses and container nurseries.
Liverworts form dense, prostrate mats covering the container substrate surface of ornamental plants (Fig. 2). Liverwort becomes highly competitive with the ornamental crop for water, nutrients, and space. The liverwort mat can prevent the irrigation water and fertilization from reaching the root zone of the ornamental crop and can repel water when dry (Neal and Derr, 2005). As a result, the overall quality and market value of the ornamental crop decreases.
Figure 1. Liverwort is a major weed in both perennial and annual containerized crops. Photos: Heidi Lindberg
The containerized crops that are most often host to liverworts include those that are overhead irrigated, in greenhouse propagation areas, and in any other poorly drained or moist areas. Liverwort prefers cool temperatures, low UV light radiation, high fertility, and moist or damp substrate.
Figure 2. Liverwort creates a mat on the substrate surface and becomes highly competitive with the ornamental crop for water, nutrients, and space. Photo: Heidi Lindberg
The optimum temperature for vegetative growth is 64 to 72°F. Even while in the vegetative stage, liverworts can reproduce asexually. Each gemmacup can produce numerous gemmae (asexual plant buds) which can form 1-2 clonal plants after contact with moist soil/substrate (Fig. 3). Liverwort may also propagate asexually by fragmentation.
Figure 3. Liverworts can reproduce asexually; gemma cups (bowl-like structures) produce gemmae (asexual plant buds) which can form clonal plants when spread to new moist substrate. Photo: Debalina Saha.
Liverworts also reproduce sexually. When liverworts are exposed to temperatures between 50 and 59°F, they develop sexual structures or fruiting bodies (Fig. 4). Stalked, umbrella-like male and female reproductive structures are borne on separate thalli, undifferentiated plant bodies (or primitive ‘leaves’). The male reproductive organs (antheridia) and female reproductive structures (archegoniophores) are both on the top of stalks. Sperm cells from the male structures travel to the eggs underneath the female reproductive structures by splashing water (rainwater or irrigation). After fertilization, spore development takes place. Once they mature, they get dispersed by wind or water and germinate on moist substrate under suitable growing conditions.
Figure 4. Liverworts also reproduce sexually by forming stalked male and female reproductive organs. Fertilized eggs create spores that are then dispersed by water or wind. Photo: Debalina Saha.
Physical and cultural control
As liverwort thrives in damp conditions, growers should improve drainage both in containers and in greenhouse or nursery facility (walkways, floors) and avoid over irrigating containers. Research has shown that drip or micro irrigation can reduce the spread of liverwort.
Figure 5. Sanitation is important to reduce insect, disease, and weed, including liverwort, pressure in greenhouses. Photo: Heidi Lindberg
Growers should avoid introducing infested stock to crop area, however this is often not possible; therefore, growers are forced to use manual labor to reduce the infestation on the substrate surface by hand. Sanitation is very important in controlling liverwort in production nurseries and greenhouses (Fig. 5). Growers should sanitize greenhouse surfaces, pots, and tools with labeled disinfectants (quaternary ammonium, peroxides, etc.) prior to the season.
Figure 6. Woody liner producers use a thin layer of parboiled rice hulls as a mulch to suppress liverwort. Photo: Heidi Lindberg
Mulching with organic mulch such as parboiled rice hulls (PBH), pine bark, or hazelnut shells can also decrease liverwort cover. Greenhouse and nursery growers have been using PBH in production for the last few years and research shows that a layer of 1⁄2 inch or 1 inch mulch of PBH was 100% percent effective at liverwort and weed control. However, this mulching depth is more applicable to larger container sizes (gallon, 3 gallon) and proves to be too deep in smaller propagation containers or liners. The 1⁄4 inch mulch provided some suppression but not complete control (Fig. 6). Additional research is required in this area in order to determine the best mulch type, depths, and particle size for liverwort control in the nursery and greenhouse conditions.
Growers should avoid top-dressing the containers with fertilizer as incorporating or sub-dressing fertilizer in containers reduces liverwort growth. Do not over-fertilize because liverwort reproduces rapidly when exposed to elevated levels of nitrogen and phosphorus.
The chemical control of liverworts in greenhouses is extremely limited. SureGuard (flumioxazin) is a preemergent herbicide which is most helpful on greenhouse floors. Sureguard is labeled for greenhouse production but it can only be applied if no plants are present in the greenhouse and growers must then wait at least 24 hours before bringing plants inside the treated greenhouse (Marble et al., 2017).
Post emergence herbicides which have shown some efficacy in liverwort control in ornamental production in nurseries and greenhouses are listed in Table 1. There are four organic products that provide control of liverwort that are labeled for use inside greenhouses; however all of them have limited practicality in greenhouse production. They may be more useful in nursery production when crops are dormant.
Table 1. Herbicides labeled for use in ornamental plant production that have post emergence liverwort activity (Adapted from Marble et al., 2017)
For example, a limited number of studies have shown that acetic acid (vinegar) products can control liverwort. However, the low pH of vinegar can cause phototoxicity and also alter the pH of the growing substrate. Other pesticides that have postemergence activity on liverwort include: ammonium nonanoate (Axxe), diquat (Reward), sodium carbonate peroxyhydrate (TerraCyte), and pelargonic acid (Scythe). However, these herbicides have been shown to cause significant damage to the ornamental plants. Dimethenamid-P is a potential herbicide that can suppress liverwort growth, but it works very slowly and the results may vary (Marble et al., 2017).
The Saha lab at Michigan State University will be conducting trials throughout 2020-2021 to evaluate the following mulches and mulching depths for liverwort control: rice hull, hardwood chips, cocoa hull, and pine bark will be applied on top of the substrate in nursery containers at a depth of either 0.25 or 0.5 or 1 or 2 inches. Dr. Saha and her graduate student also plans to conduct research to evaluate different organic and synthetic products such as baking soda for liverwort control in greenhouse production system.
- Marble, C., M.S. Frank, D. Laughinghouse, S. Steed, and N. Boyd. 2017. Biology and management of liverwort (Marchantiapolymorpha) in ornamental crop production. University of Florida, Institute of Food and Agricultural Sciences. ENH278 1:6.
- Neal, J.C. and D.F. Derr. 2005. Weeds of container nurseries in the United States. North Carolina Assoc. of Nurserymen, Inc. Raleigh, NC. 16p.
- Newby, A.F. 2006. Liverwort control in container-grown nursery crops. Master Thesis, Auburn University, Auburn, AL. 80p.
- Svenson, S.E. 1998. Suppression of liverwort growth in containers using irrigation, mulches, fertilizers, and herbicides. HortScience33:3 S231.