September 10, 2020 – Theresa Gruninger, USGS Student Services Contractor
The non-native and invasive Phragmites australis has a very iconic aboveground structure (those large fluffy seed heads are hard to miss), but there is a lot going on belowground that often does not get much attention. The root structure of Phragmites (like most grass species) contains two major components: stolons and rhizomes. Both structures serve a similar function and have similar morphology: a horizontal stem coming from a single parent plant that produces both roots and new clone plants. As seen in Fig. 1, stolons stay above the ground and run along the surface of the soil while rhizomes are found belowground and run through the soil.
Phragmites is a perennial grass that uses its belowground rhizomes for survival and spread. Rhizomes play a key role in plant health by storing carbohydrates and nutrients, supplying oxygen to the roots, and stabilizing the plant in its environment (Graneli et al., 1992). Additionally, while Phragmites reproduces sexually through their seeds, it also reproduces asexually via stolons and rhizomes. In fact, the majority of local spread occurs through the production of clones from these structures (Gucker, 2008).
Rhizome: Key to a Successful Invader
The non-native Phragmites australis is notorious for being quick to spread and hard to eradicate, traits which stem from its successful rhizome network. As a Phragmites stand ages, it is able to produce more and more clones via its rhizome structure. Phragmites is often found in dense stands with an extensive root system, making it difficult for other species to cohabitate. By the time a single parent plant is 3 years old, it could have already produced up to 6 new plants (Gucker, 2008)! Additionally, rhizomes have been recorded to grow up to 10 feet in a single season under optimal conditions (Swearingen and Saltonstall, 2010) and grow from 1 to 3 feet below the soil surface making them incredibly stable and hard to remove. Non-native Phragmites is an especially hardy invader because of the ability for broken rhizome fragments to continue to sprout even when removed from the parent plant. So, when planning on any mechanical removal of non-native Phragmites stands, it is important to dispose of rhizomes properly and check all equipment used for stray rhizome fragments to minimize the risk of continued spread and reinvasion.
Rhizome: Key to Successful Eradication
When trying to eradicate Phragmites from a specific area, it is important to get to the root of it all. This means damaging and destroying the belowground aspect of the plant. Removing just the aboveground portion (through mowing, for example) still leaves a healthy rhizome system that can later re-sprout new growth. For this reason, all of the management combinations included in the Phragmites Adaptive Management Framework (or PAMF) take into consideration the treatment of both the aboveground AND belowground components of Phragmites.
What Does it Take to Kill the Rhizomes?
Spading is a management method where a spade is inserted at a 45-degree angle into the soil (~ 5 cm) near the base of a Phragmites stem to sever the aboveground stem from the rhizome. The plant is then pulled out, leaving the soil undisturbed. It is important to dispose of the Phragmites stems properly to prevent further spread. As this is a labor-intensive management technique, it is ideal for small patches of Phragmites or for patches where herbicide cannot be applied. So, even though the rhizome structure may be left in the soil undisturbed, this method is still effective as it removes the photosynthetic tissue of the plant and slowly starves the belowground structure (Short, 2015). The benefit of spading is that it is budget friendly and does not require extensive training or licenses to implement. You can learn more about spading by clicking here!
Flooding a Phragmites stand can be an effective management tool if managers have the ability to manipulate the water levels. Due to the low-oxygen nature of flooded areas, Phragmites has been shown to die off after three consecutive years of flooding with water depths 3 feet or greater (Shay and Shay, 1986). However, constant flooding can often be hard to maintain and even with this method permanent rhizome damage may not occur. PAMF defines flooding as fully submerging all live Phragmites stems for at least 30 days. It is very important that entire plant is fully submerged due to a survival method called “snorkeling,” where living stems that are above the waterline are still able to bring oxygen to the submerged parts of the plant and keep the rhizome aerated (Weisner and Strand, 1996). It is also important to keep in mind that even Phragmites that is fully submerged, but close to the water’s surface, is able to continue a small amount of growth to send up “snorkelers” that help keep the flooded plant alive. Therefore, PAMF suggests that Phragmites plants should be at least a foot below the water’s surface when flooded.
For Phragmites in aquatic environments that may not be able to be flooded completely, the ‘cut underwater’ technique can be a perfect solution. By cutting Phragmites below the waterline (PAMF recommends cutting at least 6 inches below the water’s surface), the rhizomes become oxygen staved and the plants will die. The cut underwater technique can be implemented by hand (using a tool like raspberry cane cutters) or with large, amphibious machinery / harvesters.
Herbicide application is the most common Phragmites management technique in North America and is utilized by 94% of land managers controlling Phragmites (Martin and Blossey, 2013). Herbicides (like Glyphosate and Imazapyr) are successful because of their ability to kill the rhizome in addition to the aboveground biomass. Glyphosate and Imazapyr are systemic herbicides that are applied to the leaves, absorbed, and transported by the plant to the rhizomes below ground. The ease of herbicide application makes it appealing for managing large Phragmites stands, but appropriate licensing/permitting and training are necessary for application. For Phragmites in aquatic environments, herbicides approved for aquatic use and without chemical surfactants are required to avoid harming wildlife. Like most management techniques, herbicide application is not a one-and-done solution. Eradicating Phragmites generally requires multiple treatments over several years (Lombard et al., 2012). While it is likely to see a dramatic decline in Phragmites abundance after one application (Breen et al., 2014), the stand likely will grow back unless the rhizome structure is fully destroyed.
Common Methods That Are Not Efficient for Rhizome Control On Their Own
When researching removal of Phragmites, prescribed burns are often mentioned as a management option. Prescribed burns (when done by a licensed professional) are an effective way to remove aboveground biomass very quickly. Unfortunately, fire generally does not destroy the rhizomes, allowing the Phragmites to grow back. For this reason, prescribed burns should always be part of a combination of management actions such as with herbicide application or flooding. In fact, fire can stimulate plant growth, and burned sites often have higher Phragmites densities afterwards than non-burned sites (Thompson and Shay, 1985).
Any kind of management that removes only aboveground structures is not an effective standalone treatment option for Phragmites. If the belowground rhizome system is left intact, the grass will continue to put up new shoots. In fact, many aboveground-only removals can stimulate additional growth. Similar to burning, mowing Phragmites has been shown to stimulate shoot production, leading to even denser stands in both tidal (Gϋsewell et al., 1998) and non-tidal habitats (Warren et al., 2001). Grazing (mainly by either cows or goats) has also been documented as a form of invasion control. While a few studies have found grazing to reduce Phragmites density and biomass (Brundage, 2010, Duncan et al., 2019), it is unclear whether grazing is effective as a long-term Phragmites management approach. Overall, while these methods are an easy way to temporarily remove the Phragmites, they should be combined with other management techniques such as flooding or herbicide application.
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