Forest Lake is a new location introduced on 22nd April, 2013 as springtime event. New players who missed that event will still have chance to get it. As soon as you have raised 40 plants in your own garden (those gifted to you by other players don't count), you get access to your Forest Lake.
From then on, you can get to your Atrium from "My Garden" page by clicking the Forest Lake image. It contains 15 spots for plants with one can be replaced by decorations (flower pots).
As same as for Atrium and Winter Garden, plants and decorations that you possessed will be shown underneath. You can place and replace them using drag & drop function. It is also possible to swap the flowers planted in the Atrium by using the drag & drop function (you don't have to remove the old plants first).
Any adult plant from your collection below the Forest Lake can be placed in a flower container. Flowers can be removed back to your The Greenhouse by clicking the spade image . Flowerpots can be removed by clicking the tool shed image , but they must be empty in order to be removable. Placing and removing items has been described in detail in the article about the The Atrium.
This wiki contains graphics/pictures by flowergame.net.