Dahlias grow best in full sun to partial shade. They should get at least six hours of sunlight per day. In warmer climates, afternoon shade can help the flowers by keeping them cooler. Afternoon shade can also improve the color and reduce fading in some blooms. I find the white blooms especially like the afternoon shade...this helps prevent the brown "sunburn" look that is common in whites.
Plant tubers 4-6 inches deep in well-drained soil. I like to dig a 6-inch hole, and bury the tuber 4 inches deep. The remaining 2 inches acts like a bowl to water in, keeping the water at the tuber and not weeds! The soil pH should be around 6.5. Dahlias are heavy feeders at the beginning of the season. Dahlia plants are also foliar feeders that can take in nutrition through its leaves. Spray fertilizers can be used on dahlias. When left in the ground to over winter, tuber loss is frequently due to the tubers freezing or too much water collecting around the tuber. To avoid this, add a nice layer (4-6") of compost in the fall after you have cut down the old stalks. When digging and dividing tubers for winter, make sure they are dry before you store them and store in a cool dark spot that will not freeze during the winter.
In the Pacific North West (Note: I grow in Maryland so your methods may differ based on experience ) - Bring stored tubers into warm dark spot around April 1st to get tubers to "eye up" and sprouted. Around May 1st, go through tubers and toss those that have damage or rot. For earlier blooms, plant in pots and keep protected until last frost, then plant outside - or wait till may 15-June 1st to plant directly in the ground. Dahlias may take 90 to 120 days to first bloom but will continue to bloom until they are killed by a frost in the fall.
In the fall cut all stalks to about 6-9 inches tall after first frost or Nov 1st. Dig and divide 2 weeks later or add compost to protect against freezing if left in the ground. If you have standing water in the winter, you will want to dig in the fall or plant in well drained raised beds. Water is the enemy during the winter!
*Pictured: Our Farm Manager Cash and his picks!