Rob writes
I bought a cherry tree (not the fruit kind) in a supermarket a few
months ago, and plonked it in my small yard against a fence by lifting
a couple of the block paving stones. It's not exactly shaded, but only
gets about three hours of direct sun a day.

It seems to have taken fine, with lots of leaves on the two 'side'
branches. The problem is the central branch which seems to have gone
out of control. It's just keeps reaching for the sky, and has probably
grown 4 inches in the past week. There's lots of little branch and leaf
activity, but it's all hanging off a very spindly centre branch. I've
supported this centre branch by tying it to a bamboo cane, but now it's
gone beyond the 5 foot cane on its spindly ascent, and is lops over to
one side. The other branches are about 2 feet below the the tip, on a
tree that's about 6 feet high.

I looked on the RHS web site, and they seem to say 'don't prune the
centre branch'.

Pruning cherries is a bit dodgy as they are susceptible to disease
entering through the cut ends.

I'd be inclined to leave it to its own devices. It will slowly thicken
up, and straighten as it thickens, so you will always have a floppy bit
at the top, but the floppy bit will start higher and higher up each
year.

--
Kay