When the first frost blackens the foliage of dahlias (or if a hard freeze is predicted), cut off the stems about 6 inches above the tubers. Carefully dig the clumps with a spade or fork, and rinse them off. Let them dry out of direct sun and wind for a day (not too long or they'll begin to shrivel).
Store tuber clumps whole, or carefully separate the tubers from the stem, making sure to include any "eyes" (small, raised nubs near where the tubers attach to the main stem) with each tuber. These are the future sprouts. Store tubers in ventilated plastic bags filled with peat moss, vermiculite, or sawdust. Place bags in a box and keep them in a dark, 35- to 50-degree F location.
If you haven't yet done so, cover your late crops of lettuce and spinach with polyester row covers to keep them warmer as the night temperatures dip close to freezing. The covers also will keep the leaves from getting damaged by heavy rains.
Hard as it is to do, refrain from cutting any more roses and let the fruits (rose hips) form. This will signal to the plant that it's time to harden off for winter. Don't spread winter mulch around roses until after the ground freezes.
You can keep geraniums in pots growing and blooming indoors by cutting them back by about a third and then starting to fertilize them a couple of weeks later. Keep plants in a sunny window. Or to keep them dormant for the winter, move the potted plants into a dark, cool (40 to 50 degrees) location. Don't water them and don't cut them back until they show new growth in spring. Many other annual flowers can be potted before frost, and kept blooming indoors well into fall.