looks like a young plant. If possible, it would be worth digging up to see if it is a regrowth (from rhizome) or a regeneration from seed (in which case, likely to have very little belowground except roots).
Thanks Michael B - I have bene away but indicated to the @ACTParks-InvasivePlantsTeam that I would be away so hopefully they have followed up - If I don't here from them on Monday I will go and dig it up cheers
No contact from the Parks Invasive Species team, which was disappointing, but I removed 208 plants today (11 April 2022). I think there are more submerged plants but difficult to find them all will need ongoing surveillance. Plants in main water pond had seed pods and flowers but did not seem to have ripe seed. I removed all the flowering emergent forms of the plant. I understand the submerged rosette form of the plan can't produce flowers or seeds but can form rhizomes and corms. So a real hard one to eradicate
thats a huge effort, Michael M. Am happy to help (and I have waders) if you do a repeat. The third photo from MichaelB looks like the the plants are coming in down a drain ? (the propagules are adapted to dispersal by water, buoyant). So there must be an upstream source, and apparently vigorous.
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