Maintain roses. Remove faded flowers, cutting them off just above a leaf node with five leaflets (nodes closest to the flower have three leaflets). Then fertilize and water deeply in preparation for the next round of bloom.
Pick flowers and veggies regularly. If you're growing vegetables or flowers, harvest daily. Overripe vegetables can rot on the vine, and that rot can spread to other plants. And many kinds of vegetables and flowers (especially annuals) produce over a longer season if you pick them before they have the chance to set seed.
Make compost. As you pull weeds and the leafy remains of early crops like peas and cabbage, throw them into a pile at least 4 feet wide and high. Alternate 5-inch layers of green and brown matter (grass clippings and straw or manure, for example), turn the pile with a hay fork or spading fork every week, and keep it as damp as a wrung-out sponge. Over a period of two or three months, the pile will heat up and transform itself into rich black compost ― the best soil amendment on the planet.
Mow cool-season grasses higher. Cool-season grasses (Kentucky bluegrass, perennial rye, and fescue, for example) send down deeper roots if you mow them a little higher than normal ― 2 1/2 inches instead of 1 1/2 to 2 inches. If you live in a cool-summer, cold-winter climate, this is probably what you're growing.
Mulch. Weeds are rampant in summer, and long hot days parch plants when they need water most. Mulch helps you deal with both problems. It smothers weeds before they can get started, and slows evaporation of soil moisture so you won't have to water as often. Organic mulches like coarse compost and rotted leaves work best when you pile them on at least 2 inches deep. Plastic mulch works too: apply black plastic mulch over soil you want to be warmer (it helps warm-season plants grow in cool-summer regions); and red plastic mulch boosts the tomato harvest in the coolest climates (Alaska and Canada, for example).
Plant iris. Dig overcrowded clumps a month after flowers fade. Discard dried out or mushy rhizomes; cut apart healthy ones, trim leaves back to 6 inches, and replant in fast-draining soil in full sun. Plant new rhizomes in same way.
Water. Like humans, plants need water more than they need food. Irrigate most things whenever the top inch of soil dries out. Pay special attention to potted plants, which dry out faster than ones growing in the ground; and to plants that grow beneath house eaves, since they don't benefit much from the rain.
Weed. Hoe small weeds on the morning of a warm, dry day and the sun will parch and kill their remains by dark. To get larger weeds ― especially ones with taproots ― water deeply at night and use a dandelion weeder or asparagus knife to pop them out of the damp, softer soil the next morning.