I just finished an experiment with Pistia here in Michigan. Michigan State University and our local garden park called Belle Isle both list it as invasive. So, last spring (2017) I planted a 5-gallon pail with some (and one other aquatic species, which was also condemned as invasive) and included a few clay pots near the water surface so that they could seek out substrate with their roots. The pail was then "planted" such that its rim was about 3-4 inches above the soil line. Over last summer, both species definitely grew profusely. But then, when the temperatures began to slide below freezing, both species took a turn for the worse. This past spring the Pistia were already gone to mush. The other species had a few weak strands of their stems, but those decayed and disappeared over a few weeks. Now it's August and there is absolutely no trace of either.
Do they grow well when conditions are right? Yes, definitely. Are they "invasive" in climates that freeze? It does not look that way, at least not in Michigan where freezing is robust over many months. It would seem that calling a species invasive would require that it be able to survive year-round in a specific climate/location.
I do also have an Arum italicum and it survives our winters just fine. I clip off the seeds before they ripen and submerge them in boiling water. They do not survive boiling. The parent plants=C2=A0grow pretty well here, but I don't know how well they have to grow before they are judged as "invasive." In my yard, the native Jack-in-a-pulpit is a weed. I have a hundred blooms each spring on my quarter acre lot. And they pop up everywhere. Most local gardeners are jealous. Is that invasive? It is a native.
Ted Held.
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