My begonia flowers don't seem to last long at all!

They fade to an unsightly 'spent' flower almost the same day they bloom and I have trouble keeping a full flower head in bloom since most buds on the same flower heads open at different rates.

I find myself wanting the trim the dead flower heads off to keep the plant looking nice, but if i do that then I will also have to cut off about half of the unbloomed buds on the same flower heads that have already spent flowers.

They are your standard zonal geraniums planted on my front porch in two clay pots. They get a good 6 hours of sun from about noon onward. I give them a thorough watering every 3-4 days once the soil drys out and there seems not to be any pests. I live in downtown Philadelphia and the only thing I can think of is the sidewalks radiate heat and may cause the flowers to wilt prematurely (?)

Anyone with any tips? HELP! I fancy myself a pretty savvy gardener but this is not working!!!

Thanks in advance! -Brad