Hi Franj,
As I said in the other post (species) I would de-graft it, but not until it is showing signs of growth, i.e. the sap is rising.
There are a number of schools of thought and as they say you pays your money and takes your choice.
One school is that you make your cut just below the graft, thus leaving a small portion of the grafting stock on the Astrophytum, leave the cut to heal and then encourage roots to grow in the usual way. The plant then is to all intents and purposes still on a graft but one that is invisible. Thus you have the benefits of a graft without it being seen. Possible better growth, more immunity to cold (depending on the stock) etc.
The second school of thought and the one that I would use, certainly with an Astrophytum is to make the cut above the graft, i.e. through the body of the Astrophytum. Again allow to heal and then encourage roots in the usual way. I would cherish the stock with the small portion of Astrophytum attached and hope that it started to throw small offsets of the Astrophytum. Always a possibility.
Regarding why it was grafted in the first place. many of the Japanese growers who are obsessed with producing differing forms of Astrophytum look through their seedlings and any that are showing signs of being unique in some way are grafted to speed up the growth of the plant as much as possible. Some of the more unique forms are treated like their Koi carp and bring enormous sums of money.
Yours is a beautiful plant and I am very envious, but I wouldn't dash out and buy that big ranch just yet.
P.S. If you already own a big ranch, just send me the plant.
John
