Dahlias should be planted when the soil reaches 60 degrees, mid April to May. I have planted tubers as late as July in California Zone 9 and had a huge crop until late frost in October.
Choose a sunny, well-drained location. Plant tubers horizontally 4 to 6 inches deep and cover with soil. Don't water untill sprouts emerge (the exception is if you are in a hot climate.) After the dahlias sprout, water lightly.
Dahlias prefer plain dirt and don't like bagged potting soils. The pH for soil should be 6.5 to 7.0.
Don't overwater or the tubers could rot. If your soil is soaking wet, let it dry out for a few days, dahlias don't like to be soaked.
If your dahlia leaves are drooping, water. Your dahlias are thirsty.
I use slugo to get rid of slugs and earwigs, it is safe to use around children and pets.
If you see something that looks like a green ladybug kill it. This is called a diabotica beetle. They eat the petals and I have no problem squishing them.
To deal with aphids, just hose them off the plant with your garden hose. No need to put extra chemicals in your garden.
Taller varietys of dahlias will need support, a wooden stake carefully put in the ground so as not to impale the tuber works fine.
Dahlias are fun to grow, but when the frost hits your dahlias, bloom season is over. Dahlias will turn black and look pitiful, clip off the stalks and carefully dig up tubers with a broad fork. Rinse off the soil and let them dry.
When tubers are dry place in a cardboard box with peatmoss and store someplace that stays around 40 degrees (I use my art studio.)
Check on tubers monthly for signs of rot and discard any that are roting, and don't be hard on yourself if you loose some carefully saved tubers. Not many people have a 100% successful storage rate.