Although petunias come in hundreds of sizes, varieties and colors, show-stopping wave, or spreading, petunias are among the flashiest of all with big, showy blooms. Once they're planted, wave petunias require very little care; they're drought and pest resistant and so fast-growing that they can expand to large, bushy plants covered in masses of 2-inch blooms very quickly. Plant wave petunia seeds indoors about three months before the last frost.
Fill a seedling tray with commercial potting mix. Scatter the wave petunia seeds sparingly over the top of the potting mix.
Water the potting mix lightly with a spray bottle set on fine mist. Because wave petunia seeds are very small, they shouldn't be covered with potting mix.
Cover the seedling tray with a sheet of clear plastic, and put it in bright but indirect sunlight. Don't put the seedling tray in direct light or in a windowsill, which can be too hot and can bake the tiny seeds. If possible, hang a fluorescent bulb 4 to 6 inches over the seedling tray for 15 to 18 hours per day.
Make sure the seeds are kept warm. The temperature should be between 70 and 85 degrees in order for the seeds to germinate.
Transplant the seedlings into cell packs or individual peat pots when they have three true leaves. True leaves are the leaves that emerge after the seedling's initial leaves, which will eventually drop off. Feed them every other week with a general purpose liquid fertilizer. Follow the directions on the package label, but dilute it to half strength.
Once the threat of frost is over, harden the seedlings off for a few days by putting them outdoors during the day and bringing them in at night.
Plant the wave petunia seedlings in full sunlight, but protect them from hot afternoon sunlight for the first two weeks until they're established. Keep the soil moist.