Battling liverworts and mosses

Liverworts (Marchantia polymorpha) and mosses (such as silver thread moss) have become very common weed pests of greenhouse and container nurseries throughout the United States.

Both pests thrive in the low UV-light, high humidity, and high fertility conditions associated with most greenhouses and nurseries. The flat growth of liverwort and moss results in a thick mat covering potting medium surfaces, walkways, and floors under benches. It is common to see these two pests border drainage ditches where water collects or is channeled for recycling. In pots, these pests use nutrients and water intended for the crop, impede water movement into the root-zone and reduce crop marketability. The added cost of hand removal is significant, particularly when new potting medium must be added to replace that removed with the liverworts and moss (1 inch depth is suggested).

The optimum temperature range for liverwort vegetative growth and fruiting body development is 65 to 72 degrees F and 50 to 60 degrees F, respectively. However, they do well under almost all conditions used to produce ornamentals. Liverworts produce both sexually and asexually, and their spores may live up to a year.

Both mosses and liverwort can spread with water re-circulation and the spores can be spread via splash of rainfall or overhead irrigation from at least five feet away. Conditions that promote liverwort include:

• excess available nitrogen (N),
• excess water at pot surfaces,
• high humidity,
• use of fertilizer injectors,
• high light, and low ultraviolet radiation.
   

They are excellent weeds in our ornamental nurseries and greenhouses, thriving on the same conditions we use to produce our crops.

Cultural control first relies on making the greenhouse or nursery free of these pests in the benches, walkways and drainage ditches. The next step is inspection of new plant material with rejection of materials with infestations of liverwort or thorough hand weeding. Limiting available N from the container surface can be accomplished by dibbling of a controlled release fertilizer below the substrate surface. This practice may be performed more readily in nurseries than greenhouse production.

Using bottom irrigation, such as ebb and flood, can reduce water at the potting medium surface which can promote both pests. Avoid overhead irrigation where possible through the use of drip irrigation. Allow drying of the potting medium between irrigations through water management, fans, or spacing which can reduce conditions that promote liverworts and mosses.

In some settings, coarse mulches, which can also reduce surface water, may help with control. As mentioned above, use of hand weeding near crop completion may be helpful as well (at least temporarily).
 

Chemical control

While a remarkable amount of testing has been done on the use of herbicides to control liverworts, safety to the crop is always a concern. Even where extensive research has been conducted, crop safety is never guaranteed. Many preemergence herbicides commonly used in nurseries provide significant control of liverwort although registrations for use on containers in greenhouses/covered structures must be confirmed by reading the labels.

Two of the most effective preemergence herbicides evaluated at Auburn University are oxadiazon (Ronstar) and flumioxazin (BroadStar and SureGuard provided the best control of liverwort, pearlwort, and mosses). Others found to be somewhat effective were:

• dimethenamid-p (Tower),
• dimethenamid-p and pendimethalin (Freehand),
• oxadiazon and oryzalin (Regal O-O),
• oxyfluorfen and pendimethalin (OH2) and oryzalin and oxyfluorfen (Rout).
   

Be sure to check current labels for legal use sites, rates, and intervals.

Trials conducted with preemergence and postemergence applications of flumioxazin, oxyfluorfen, pelargonic acid, acetic acid (vinegar), and oxadiazon showed acceptable preemergence and/or postemergence control of liverwort and silver thread moss. The longevity and efficacy of the preemergence control with flumioxazin, oxyfluorfen, and oxadiazon was dependent upon formulation (granular and sprayable). Results indicated control of established liverwort and silver thread moss was greater with sprayable formulations than with granular formulations.

There are also a number of “disinfestant” products that are labelled for use in liverwort, algae, or moss prevention or eradication. These products include a combination of hydrogen dioxide and peroxyacetate (OxiDate for agricultural use and ZeroTol for horticultural use), sodium carbonate peroxyhydrate (GreenClean PRO for commercial use), a combination of hydrogen peroxide, peroxyacetic acid and octanoic acid (X3 - not obviously labeled for liverworts). In one trial we performed comparing X3 and ZeroTol, we saw slightly better eradication of liverwort and moss with X3. It was also safer on the woody crops we tested.
 

Other research

Another interesting study focused on mustard seed meal applied as a postemergence control of established liverwort. Rates of 113, 225, and 450 grams per square meter did not injure container-grown Pulsatilla vulgaris up to 6 weeks after treatment. These rates of mustard seed meal controlled liverwort from 83 percent to 97 percent. Significant control of creeping woodsorrel, annual bluegrass and common chickweed at eight weeks after treatment was also found. Mustard seed meal applied to the soil surface after transplanting Rosa, Phlox paniculata and Coreopsis auriculata did not injure the flowering or growth of ornamentals. This approach may be helpful to some nurseries that may have access to mustard seed meal. Safety of use on ornamentals including annuals, grasses and other potentially sensitive crops should be extensive before adoption of this control strategy.

The sensitivity of liverwort to aqueous ozone was reported. Experiments were performed to evaluate contact time thresholds and application frequencies suitable for liverwort management applications. If water is treated with ozone to remove/kill pathogenic fungi and bacteria, it may also assist in liverwort control. Unfortunately, the margin between an effective dose for pathogen control and plant safety is not wide and may make use of ozone treated irrigation water challenging.
 

Conclusion

There are still no magic bullets out there for liverwort and moss control. But growers have options for prevention as well as eradication.

A.R. Chase is part owner of Chase Agricultural Consulting LLC and Professor Emeritus of Plant Pathology at the University of Florida. Learn more at www.chaseagriculturalconsulting.com. Photos by A.R. Chase.