Enjoy the lower-key nature of high-summer garden maintenance. It's too hot (and, most likely, dry, despite a recent downpour) to be planting or transplanting. Pruning of woodies (woody ornamentals) should be on hiatus till late fall. The grass has ceased its rampant growth. So, weed, water as needed, harvest consistently, and deadhead to keep the annuals blooming. Then head for the hammock.
To save flower seeds for growing next year, don't deadhead all maturing pods on these: cosmos, tithonia, bachelor's buttons, bells of Ireland, hollyhocks, daintier salvias, moonflower vine, cardinal climber. Store dry, cleaned seeds in marked paper envelopes that are then sealed in a zipper-style plastic bag and placed in the refrigerator.
Get a double blossoming of phlox - but no more - by deft deadheading. Many varieties of P. paniculata produce a second flush of flowers from the first flower cluster. So don't jump the gun on snipping off the spent blooms. But when the second flowering fades, be prompt to deadhead since the seeds of the first flowers are ripening.
Don't be too perturbed about powdery mildew. This gray discoloration of the foliage of phlox, especially, and other ornamentals is unattractive, but usually plants are not badly harmed. It's fungal and got a head start in June with moisture, warm days, and cool nights. Good air circulation inhibits powdery mildew, so in the fall thin out dense stands. Here's a home recipe that helps a bit: Add 2 tablespoons of baking soda and 1 tablespoon of vegetable oil to 1 cup of warm water and mix well; dilute in a gallon of water and spray on affected plants. Discard rather than compost affected leaves, including those on the ground.
Though the rains seem to have returned, some perennials may look like goners, but don't assume they are dead. Top growth may dry and brown, but the root system is still taking up moisture for later revival. Best to wait till spring to remove something unless you are certain that it is really dead.
When garden centers put plants on sale, be a careful shopper. First, do you have a plan for tending them until planting season resumes in September? And a deteriorating plant is no bargain. Inspect carefully for insects and diseased foliage. Be nervy and check the root system by gently lifting the plant from the pot - even if the plant is potbound, the roots should be pale, fleshy, and healthy-looking. Ascertain the store's return policy.
Since late August is the ideal time to grow a lawn from seed, prep work should begin now to avoid a mad rush. Remove all weeds, especially crabgrass and stilt grass - your choice as to weeding or using herbicide. (Note that nut sedge is almost impossible to hand weed in dry compacted soils; the "nut" remains underground and resprouts.) Loosen compacted soil, adding compost if it's heavy. Have the soil tested – you may need to add lime. When it comes time to sow, opt for a mix of fine fescues and a little perennial rye over Kentucky bluegrass, which is coarser and creeps into flowerbeds.
Understand why some of your plants don't make it through winter by observing where water stands after a heavy rain. Now it's usually no big deal if water pools in a flowerbed, since everything is so thirsty. But in winter it is a big deal; poor winter drainage can be death for many perennials and shrubs.
Some trees (especially tulip poplars) and other woodies are dropping a few leaves. It's OK, just a way of dealing with dry conditions.
Limit fertilizing to the vegetable garden and annuals you have cut back for a second flush of bloom.
Supplies often run low, so order these fall-blooming bulbs, which should be planted as soon as possible: lycoris, colchicum, and fall crocus.