1) My lily leaves became smaller and smaller during the last summer. Now, I
am not sure if it's dead. Rhizome has many roots but no leaf this year so
far. The Rhizome is dark brown color. How could I tell if it's dead?

Sounds bad, it could be water lily root rot, a terminal fungal disease
specific to waterlilies. It takes a lot to kill a water lily and once
in a pond, it's going to disease water lilies planted therein the
future. Only a few varieties are strong enough to outgrow that
disease, for example the native white alba.

Cu drain pond. Remove and burn all soil and Lily remnants. Rinse
all surfaces with a strong chlorox bleach solution. Rinse and repeat.
Happy days...

Diseased plants are FREQUENTLY dumped on message boards for 'free' or
trades and sale as 'going cheap' by folk who know something is wrong
and have no scruples as to getting replacement plants. I've even found
infected rhisome sections inserted into potting soil of marginal
plants by unscrupulous traders

2) What's the ideal pot size for hardy lily? Is 12 inches ok? I read some
previous posting mentioned about 18 inches pot (7 inches deep).

Feeder roots on water lilies are only about 4" to 6" deep. Any deeper
is absolutely totally pointless...

3) Any suggestion where to buy hardy water lily? Good price with reasonable
quality. Online? Any suggested stores?

There are several dedicated water gardeners on e-bay selling named
varieties, with good photo's and descriptions, these you can research
on the web for their growing habits.

The 'big shed' stores sell imported boxed plants of doubtful identity,
often invasive mislabeled varieties. Lowes has carried a small
selection of fairly good varieties in recent years

Regards, andy
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