DenverDaisy, what usda zone are you in, and what city. If you are actually in Denver, your growing season should end near Sept 30, or with luck in mid Oct. If not lucky, you can have a cold front and snow in early Sept. When living 26 miles SE of Denver, I would start cherry tomato plants in mid-May but was always ready to bring them inside if a cold front came in--if I planted them later, a cold night would hinder ripening. My experiences are colored by living at an altitude of 6,440 ft. Leaves on my deciduous trees started about 2 weeks after those in the lower altitude of Denver.
If you are growing flowers by seed or plants, I would get them in the ground now. I would place them on the south side of the house, or where they can get heat. My feeling is that 2+ months (Aug, Sept, and hopefully Oct) of flowers is worth the gamble on cold fronts.
