Roundup Custom 53.8% Glyphosate for Aquatic & Terrestrial Use 2.5 gallons. The Roundup Custom Aquatic Terrestrial Herbicide is a post-emergent, systematic herbicide with no residual soil activity. It gives broad-spectrum management of many perennial weeds, annual weeds, woody brush and trees. The active ingredient (Glyphosate) obstructs production of an enzyme in plants and microorganisms that are indispensable to the formation of specific amino acids.