You might need to hire a certified pesticide applicator to apply herbicide in this setting (or for certain chemicals). Many landscaping companies and private individuals who work in horticulture as consultants have this license from the Maryland Department of Agriculture. Management recommendations for invasive Phragmites (make sure it's not the look-alike native species, which while rare, can occur in coastal MD) involve chemical treatment with either glyphosate or imazapyr, though the selection of the precise product using either ingredient will depend on the site conditions, such as if the area contains any temporary or permanent of open water, or is within a certain distance from open water (which is likely, given this type of weed). As for timing, systemic herbicide is best applied (to be most effective) to Phragmites between midsummer and mid-autumn.

We recommend inquiring with local or county government to make sure you're not in a Critical Area where plant management and pesticide use might be regulated. Some formulations of glyphosate were manufactured for use around open water, but at least one of those formulations is no longer made. A pesticide applicator will have the most up-to-date information about what products are available and which are labeled for use in your particular environment.

Miri