In message , Robin Graham
writes
I have an 18" tall ribbed cactus which was being eaten away about 4" from
the top and I was afraid that the top part would simply die. So I cut the
top off above the bad bit and cut the bad bit off the main stem and tried to
graft the top onto the bottom, if you see what I mean.

I held the top bit hard to the bottom bit with a weight, hoping it would
graft itself and grow together. Unfortunately, someone hit the thing and
knocked off the top bit and I now see that the cut ends, top and bottom,
have simply calloused over without trying to grow together.

What should I do to make this work?

Use a very sharp thin blade and work quickly. Then make sure noone
knocks the thing off before it has had chance to graft. Grafting is a
bit hit and miss - even with practice not all grafts will take well.

However, since the two bits have been separate for a while you may find
that whatever you do now the scion will not be in a fit state to graft
because it is no longer well supplied with sap and a bit too limp to
cut. And there has already been time for the main stem to react to loss
of its growing point by activating another.

If you put the top bit over some damp coarse sand on loose gravel it
will probably root down at this time of year. And live for a long time
(months) without any roots at all if it has to. The base will regrow a
new main stem from one of the leading nodes (and will probably do this
now anyway).

I have had to decapitate columnar cacti before (and will shortly have to
do it again since the biggest one is out of growing space at 8' tall).

Regards,
--
Martin Brown