Identifying a pond plant

Question
QUESTION: Hello Tom, I have a large pond at the back of my property which we are expanding this year. There is a plant that I would like help in identifying. It has spear shaped leaves, the roots fan out in a "weed-like" pattern. They seem to grow along the edge of the pond where the water level is between 6-12". I have not seen it produce a flower. Via www I have been able to rule out a few plants, arrowhead being one. Can you help?

ANSWER: Hi Tammie,
Thanx for your question.  It sounds like acorus.  The leaves are long and spear-like, like water iris but not overly wide.  They do not produce a flower per se but may produce a growth that is often hidden in the leaves that serves as a flower (in the spring).  The roots are like rhizomes with lots of elongations coming off of it.  They can form messy, hard to break up clumps and can be invasive.  They always grow at the end of the pond.  How long are the leaves and color?  What is you general location?  Have you seen the plants growing in the wild or is this being cultivated?
thx,
Tom

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QUESTION: Hi Tom, Thanks for the info. In answer to your questions, the leaves are approx. 10" in length by 3-4" wide and the colour from light green to a forest green. The roots are just like you describe, and yes the plant can be invasive. They prefer the outside edges of our pond. We are located in southern Ontario and they are growing wild here. I've googled acorus but my plant doesn't look anything like it. http://timberglade.typepad.com/outside/images/arrowhead_071306.jpg is an image of my plant. The description is "arrowhead". Can you verify?

Answer
Hi Tammie,
Thanx for your question and the pic.  It looks like pickerelweed (Pontederia cordata).  These plants do produce flowers.  I have some in my pond but they didn't bloom much last year.  They are hardy down to Zone 4.  Let me know.  It sure looks like it to me.  Arrowheads are generally of the Sagittaria genus.  I hope this helps.
Tom