Answer: Common calla (Zantedeschia aethiopica) thrives in boggy soil and sounds quite happy in the location you've given them. The plants grow from rhizomes (thickened storage organs) and it's the nature of rhizomes to elongate and produce new sprouts which is why you're finding lots of little plants in the vicinity of the parents. To completely rid your pond area of callas, you'll need to dig and dispose of all the rhizomes. A spading fork will allow you to dig the area thoroughly and unearth all of the rhizomes. There is one chemical herbicide (glyphosate) registered as safe to use around bodies of water, but if you have other vegetation you want to save, your best bet is to dig and destroy the pesty callas. Best wishes with your project!
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