Weed of the Month: Liverwort
Posted: February 16, 2012
It is one of the most difficult to control in greenhouse and nursery operations because the same conditions of moisture, fertility and temperature that favor the growth of crops also promote liverwort growth. Liverworts can reproduce both sexually and asexually, which means that large populations can develop very quickly.
These low growing, mat-forming plants reproduce sexually through spores. Spores are microscopic and are carried by air currents, so it is impossible to keep them out of propagation areas. Asexual propagation occurs through small clonal fragments known as gemmae. Gemmae collect in specialized structures on the “leaves” (thalli) of liverworts called gemmae cups. Raindrops and overhead irrigation splash gemmae onto the growing media where they germinate and give rise to a new plant. While there are a limited number of herbicides labeled to control liverwort, herbicides alone will not provide sufficient control. The following cultural practices will go a long way toward controlling liverwort:
- Covering stored planting media to protect it from liverwort spores as much as possible.
- Sanitize all greenhouse surfaces with a disinfectant labeled for greenhouse and/or nursery use, and be sure to discard containers that have been infested – they are likely to carry spores that will start the infestation over again.
- Avoid overwatering crops, and allow the surface of growing media to dry out in between irrigation cycles or sub-irrigate to provide sufficient moisture for the crop while keeping the media surface dry.
- Fertilize judiciously to avoid excessive nitrogen and phosphorus levels that favor liverwort growth.
- Research has shown that incorporating fertilizer into the planting media reduces liverwort growth compared to surface fertilizer applications.
- Applying iron sulfate and/or copper sulfate to the surface of the growing media can help prevent liverwort infestations.
- Liverworts often die when the crop canopy shades the surface of the growing media.
- Increasing airflow through the crop reduces humidity and creates a less favorable environment for liverwort growth. Consider wider spacing of plants, rolling up the sides of high tunnels and hoop houses, or increased use of fans.
Certain herbicides are labeled to control liverworts in specific sites, but many are not labeled for use in closed structures such as greenhouses, while others are not labeled for use in containers. Always read and follow label directions when applying pesticides.
- Broadstar (flumioxazin) offer pre-emergence and very early post-emergence control of liverworts in container (outdoor only) and field grown woody ornamentals and conifers.
- SureGuard (flumioxazin) is labeled to control liverwort around woody ornamental plants in landscape beds. No formulation of flumioxazin is labeled for use in greenhouses.
- Rout (oxyfluorfen and oryzalin) is also labeled for preemergence control of liverwort in container, field grown and landscape ornamentals, cut flowers and foliage crops.
- TerraCyte (sodium carbonate peroxyhydrate) is labeled for postemergence liverwort control in containers and can be used in greenhouses.