Hi Meme how 'big' is the pond you plan on constructing?

Don't do it with a tree. If it is going to be a big pond you should get the help of a professional landscaper who has pond constructing experience because the endeavor you are about to undertake is very complex, very costly and it has to be done right the first time for if it fails there will be no second time. Im afraid your question shows me that you have no or very little experience in pond construction.

To take an island that will be able to support an alder tree you're wanting to construct a lake, not a pond. Get professional advice as the others have said.

Hello Meme, I have a big pond with island and weeping willow tree planted on it. No liner though, as the pond is clay lined.

I wouldn't have thought you could use pond liner for the size of pond Meme wants...

For large ponds, multiple liners have to be laid down with seams sealed 100%. This is almost impossible an there will always be some leakage. Did a little looking about, a 1/2 acre pond with clay bottom will cost about $ 20,000 (US) and thats base price with no frills such as an island with tree and excluding pumps, filters aerator etc. Not knowing the size of your pond, the lay of the land, and the details of your personal resources such as equipment, manpower, and funds, that's it.

Hi Moongrower .... me neither.

Thankyou for all the replies I have been thinking and re-considering. My original idea isn't realistic. The garden I have is narrow but very long. So what I intended at first what to build in some sort of pond at the end of the garden my gard is 6 metres wide and the actual height would be roughly the same so 6x6 Metres, but that is variable as I might change my mind I have looked into alternative liners, as dawn mentioned Bentonite clay can be and option but how I would go about having a tree submerged in the water and having that clay liner I am not sure I'd have to consult a professional for that answer. My second idea was to have a smaller pond but with a tree next to it but that would be limited still with a rubber or plastic liner. So I think a clay liner is the best way to go. :)

6x6 metres as in the initial pond idea size.

Hello Meme, decisions, decisions. We are on heavy clay here so nothing imported. Our pond was dug and the extracted clay was piled in the middle to create the island.

A 6 m x 6 m pond couldn't hold an island big enough for an actual tree, which would need an island the same size as the pond! Better to have attractive reeds, iris, dwarf conifers, and/or boulders. Remmber that you will have to make regular trips to the island to maintain whatever you plant there.