I hunted up Bob's (dreadnought) comments that mentioned geraniums and thought the whole thing was informative for those of use who are very inexperienced. Thanks Bob, please sue me after I've made my millions, since I didn't ask your permission first. Smiles.
"Geraniums will stand being dry more than they will being wet, especially in winter when its best to keep them on the dry side as they are prone to rot if too wet. For this same reason if you cut them back I would do it in the spring or early summer that way they will heal before the rot sets in. Your Dahlia will like plenty of water through the summer, the water needs to be able to drain away though. I see you have a saucer under the Geranium which is okay to catch any stray water but lots of plants don't like to be stood in water for a long time. The Dahlia will have its foliage killed by the frost which is okay, just cut this off 4 to 6 inches above the ground, tip it out of its pot and clean the dirt from the roots which are called 'Tubers'. They will look a bit like Potatoes and feel firm to the touch. Leave them upside down somewhere frost free to dry for a couple of weeks and then they need to be put in some sort of container filled with moist peat or compost and kept in a cool, dark but frost free place for the winter, the peat/compost only needs to be very slightly damp, you're not trying to grow them just keep the tubers from drying out. If that seems too much trouble throw it away and buy a new one next spring! By the way, the spider plant looks good. Bob."
Thanks again, Bob. You are like the patron saint of gardening, or something. And next year I won't throw out my Dahlia tubers.