Over-wintering for tropical watelilies can be done in Italy from mid November until mid December.
It is important for the plants to feel the temperatures are decreasing and that the hours of light are also diminishing. This is going to induce the plants to go to rest naturally.
The over-wintering described below simulates the summering. It’s almost like if they became dormant, something that happens to tropical waterlilies in the summer when lakes dry up – the development of the plants stops until water comes back again.
The star of the following reportage is a waterlily called ‘Panama Pacific’. A wonderful tropical waterlily that smells of violet.

The plant is being removed from the pond:

The plant is being removed from the pot:

The plant is full of roots:

The roots are being removed:

The dirt is cleaned off:

The plant is cleaned by immerging it into water:

And rinsing it off:

Now, the roots are being cut off:

And the green stuff is also being cut off to avoid the plant going rotten:

Right now, the tuber has not been cleaned well enough and could go rotten and, therefore, needs to be rinsed off again:

And the green stuff taken off as much as possible:

The procedure is completed by rinsing off the tuber very well and by taking off all the remaining green stuff.

Here is how the tuber looks at the end of the procedure:

Some varieties have cleaner tubers.

At this point, the tuber is put into a zip-lock bag:

After wrapping around it a nonwoven fabric:

It’s a synthetic cloth (2 or 3 millimeters thick):

Water is then applied to the cloth:

Extra water is being squeezed out, so that the cloth is not dripping, but just slightly humid:

The tuber is being wrapped with the cloth:

By folding the edges of the cloth:

Then the variety of the plant is written on the zip-lock bag:

The waterlily is put inside the zip-lock bag:

The zip is being locked:

This way the bag is locked, but not hermetically, as the tuber needs to breathe a little.
When you put it away, make sure the bag is not folded up, otherwise it’s like it was sealed.
The bag needs to be put away in the dark and in a warm environment (at about 18 to 20 °C), the bag needs to stand and not lay down.
The tubers can be dormant like that even for a few years, in which case the tubers should be checked every 3 or 4 months to make sure there is enough humidity in the bag, and if there isn’t, just wet the cloth a little.
Sometimes tubers will grow even while they are dormant!