On Apr 11, 8:15*am, Tom wrote:
Hi, someone gave me quite a big cactus arm that broke off the main plant
about a year ago.

They say it's still possible to plant it and grow it.

Is this true? I've planted arms that have only just dropped off with
considerable success.

It is very likely to be true. How big is it?

I've planted it in cactus compost, I'm just wondering how often I should
water it. And in what quantity. I haven't watered it yet.

Any advice appreciated!

Sounds like any cut surfaces have already had plenty of time to heal.
I'd mix the compost with grit sand and gravel in about equal measures
to make it even more free draining, and then give it some water (aka a
good drenching). Don't water too frequenly or leave it standing in
water that is the fastest way to kill them. The compost should be
allowed to become almost bone dry between waterings. At this time of
year in spring almost anything will root.

Regards,
Martin Brown