No pond would be complete without water lilies.
There are two basic types, hardies and tropicals,
each of which have numerous varieties to choose from.
Hardies are the easiest to grow and can withstand
most winters. Although, a more limited color
selection, they will live from year to year simply by
cutting them off at pot level and sinking them in the
deepest part of your pond.
Tropicals, however, will not tolerate cold weather
and must be brought inside during the winter months.
They can be stored in a green house or even a hot
water heater closet. They come in a wide variety of
colors, some of which are night bloomers. (Haven't
tried one of those yet, but looking forward to it.)
The large floating leaves of the lily are great for
pond surface coverage.