In article , David Shalita
wrote:

My almost 50 year old rose has large blossoms but the stems are so thin,
that the blossoms cannot stand upright. Located in hot valley near Los
Angeles, CA.

This rose I call a rose tree, all leaves stems blossoms grow on top of
the 4 foot stem.

This is called a rose standard or a tree rose.

I feed regularly with a systemic food by BAYER company. Sun exposure is
about 5 hours per day.

Five hours is getting on the low side of optimal, tho in a hot climate,
it could be perfect, especially if the 5 hours starts in the morning.

Are weak stems a symtom of old roses or is there
something I can do to cause stronger stems?

Dave, some varieties of old (really old) roses have thin stems. They
are best used so the plant grows taller than you, so the flowers are
allowed to "nod" down at you. This can be quite beautiful.

OTOH, some roses just suffer from "weak necks," where the stems don't
support the weight of the bloom, but you wish it would! If your roses
is well-watered and well-fed, you might just have a rose with weak
necks.