Early July 2018, Virginia
We all grow in our own micro-climates, so comments about the weather are not relevant beyond our own gardens. Here's what we are seeing in Virginia.
Julys tend to be a challenging month for my dahlias. Mid-90 temperatures are drying out the leaves on the potted dahlias on the patio and there is no rain in the forecast. Pots and planters need regular daily watering. Do not drench them and cut off the oxygen that the roots need. I use about a half gallon per pot and double that for a planter.
If you are growing plants in the ground, the rule of thumb is an inch of moisture per week. Most folks apply it all at once, while others spread it out daily. I have mulched my patch with pine straw to keep the roots cool and to conserve moisture.
There are all sorts of mulches. I have used straw on top of horticultural cloth to discourage seed sprouts. Pine bark mulch has its proponents. Wood chips and hardwood mulch are controversial. One reason for the controversy is that decaying wood can rob nitrogen from your soil. Whatever you use, about 4 inches, when applied, will do the job.
Watch out for red spider. The bottom leaves will be mottled and the undersides will have visible webbing. See your garden center for advice. Act quickly!
As they grow, dahlias need support. I use balls of soft twine from the hardware store. They come in tan and green. Tying tightly around the stake and loosely around the plant works well.
A booster of granular or liquid fertilizer helps in July. If the weather is dry and hot, wait until it breaks.
Most blooms do not develop well in hot weather. Patience is the name of the game.
While you're waiting check out these references: