Answer: By your description, I'd guess you're dealing with Scouringrush (Equisetum hyemale). It's a tenacious perennial plant with deep, spreading rootstocks. Stems are jointed and hollow, and easily pulled apart at the nodes. The plant produces a reproductive cone at the top of each stem in July or August. If allowed to develop, spores will be spread to adjacent areas, resulting in additional plants. As you've discovered, glyphosate (Round-Up) does not affect the plant. It's resistant to all herbicides available to homeowners, with the exception of dichlobenil (Casoron). Dichlobenil is a pre-emergent herbicide registered for use in beds with woody ornamentals but it cannot be used in vegetable gardens. So, your options are to dig the weed, roots and all, or at least keep the tops chopped down as soon as they emerge to keep them from manufacturing food for the roots. Keeping the tops cut down will eventually lead to the death of the plant.
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