Seisin, which is sometimes written as seizin, is a legal term. It means the possession of land by freehold, the act of taking such possession and that which is held.
We’re fortunate enough to own our own home, which I assume means that the land on which it stands and our garden is seisin, or we’ve seisinned it, or we were seisinning when we took out the mortgage, or something.
What I’m sure of is that the garden grows some lovely flowers. Here are a few examples.