Mulch over the dahlia bulbs. A couple inches of mulch, such as bark or chipped wood, will help maintain soil temperatures and retain water.
Water dahlia bulbs once a week and only if it has not rained an inch the previous seven days. On hot days, water in the mornings or evenings when temperatures are cooler. Water over the roots, not on top of the flowers and foliage, especially in the evening when fungus can grow overnight.
Keep the slugs, Japanese beetles and other pests away. Remove slugs in the mornings or use a commercial slug killer. You can remove Japanese beetles by simply pushing them into a bucket of soapy water. If necessary, use a pesticide or insecticidal soap for the beetles or other pests.
Allow the foliage to turn brown before cutting it back (leave a couple inches on for easy handling). The foliage usually turns brown after the first frost. Allowing the green foliage to remain after the dahlias are finished blooming gives the plants more opportunity to absorb sunlight, convert it to sugar through the process of photosynthesis and store it in the bulbs for next year's growth.
Remove the bulbs carefully from the soil by digging about 6 inches deep around each plant with a garden fork or shovel. Then pull down on the handle in several spots to lift the bulbs out of the soil. The necks of the dahlia bulbs are very fragile, so handle them with care.
Wash off the bulbs with a garden hose. Then, dry out the bulbs before storing them for the winter. Lay the bulbs upside down for two weeks.
Store dahlia bulbs in an open container filled with dry sand or peat moss. Alternatively, you can store them in a sealed heavy-duty bag. Store them in a cool location that is between 40 to 45 degrees F, such as a garage, attic or crawlspace.
Inspect the bulbs every couple of weeks. If a bulb is partially rotted or diseased, you can generally cut off the bad part with a knife. If the whole thing is rotted or diseased, throw it out. If your dahlia bulbs are shriveling up, then soak them in water for 12 hours and then repeat step 5 before storing them again.
Divide the bulbs in the spring before replanting. You can also do this in the fall after digging them up. Use a knife and cut the bulbs so that one eye--or bud--remains on each part.
Replant the bulbs in full sun after the last frost. Till the soil about 12 inches deep and mix in a couple inches of organic matter such as compost or peat moss. Then, plant the bulbs so the eyes are facing up and only a couple inches beneath the soil. Space multiple bulbs two feet apart to give them plenty of space to grow.