I think it helps to look at the specific nature
of each plant.
Lilies like to be down about 18 inches and can
be down further.
Marginals are plants that like to grow on the edges
of ponds, you see them in water depth varying from
damp to several inches deep.
Your barrel only has one depth. If you want to mimic
a pond in a barrel you have to prop up the marginals
so it is best to plant them in containers and rest
those containers on bricks or an overturned plant
basket.

Aquatic plants really put on a lot of growth over
the summer and where you are at, into the fall.
They are going to need to be divided. Having them
in containers makes that a lot easier job.

I have a ten ft by ten ft, shallow pond, the Frog
Bog. The plants have quickly filled the entire
bog up and are marching out into the grass.
If we want to keep the bog viable for frog breeding
in the spring we have to get in every fall and
rip plants out from the middle. It is A LOT of work.
The plants are in rock (big mistake) and it takes
teenage boy power to get them out of the bog.
I would not do that again.

The nice thing about the rampant growth of aquatic
plants is that you will have divides to start another
barrel pond or an inground pond if you want.

kathy :-) www.blogfromthebog.com
this week's entry - aquatic snails
Pond 101 page for new pond keepers ~
http://hometown.aol.com/ka30p/myhomepage/garden.html