Fertilize three or four times a summer if using liquid, but don’t fertilize a thirsty plant. Water first, or fertilize after a rain. Time-release fertilizer, along with an organic fertilizer (slow-release), is a good alternative to liquid, or use liquid organics such as fish-seaweed.
Amend soils with compost or composted manure before planting. You do not need peat moss too.
Water annuals deeply less often – a deep soaking every five days or so, depending on your summer temperatures. DO NOT HAND WATER! You get bored long before your plants get a good, deep soaking. Use a sprinkler or soaker hoses, and water first thing in the morning whenever possible.
Pinching the growing tips off many new annuals when you plant them will make bushier, more flower-filled plants. Ageratum, marigolds, petunias, scaevola, geraniums, petunias, cleome and cosmos all will be bushier if pinched early. Many times this means snipping off that first flower – be bold! The willingness to pinch it early will produce results later on.
Use cool-weather annuals to fill out containers and window boxes early in the season – pansies, violas, and osteospermum are good for this. They will stop blooming when it gets hot, but by then the other plants you’ve chosen will fill out the planter.
If you mulch your annuals, use about 2″ of mulch and keep it away from the stems of the plants.
Other annual quirks: geraniums need to be deadheaded regularly and if you splash their foliage frequently, they’ll get leaf-spot. Geraniums also bloom better when pot-bound, so don’t be afraid to plant them closely with other annuals. Zinnias also need deadheading and hate wet weather, and cleome self-seeds like crazy.
Annuals that are dependable and beautiful: Red Riding Hood Mandevilla Vine, purple fountain grass (annual in zones 8 and below), Ageratum ‘Blue Horizon,’ Nicotiana (especially the taller white Nicotiana alata), plectranthus (some are for colorful leaves and some for blooms), Scaevola, marigolds (don’t be a snob… they’re easy and reliable!) annual salvia (red or blue), cleome, and impatiens.
For interesting annual seeds, check out Renee’s Garden: