How to Sow Petunia Seeds Indoors

Purchase or make a high-quality seed-starting mixture that is lightweight and nutrient-dense with a water-retaining agent such as perlite or vermiculite. Such mixtures usually consist of sphagnum moss or peat moss and at least one of the volcanic glass components of perlite or vermiculite. These small pieces of ground rock keep the mix from binding together and retain water with their porous, pumice-like surfaces.

Fill peat pots to the brim with seed-starting mix. Conversely, you could use a windowsill greenhouse commercial kit that typically comes with peat pellets that must be soaked in warm water to activate.

Place peat pots in a shallow baking pan or other watertight pan that is a few inches taller than your peat pots.

Sprinkle a couple petunia seeds onto the top of the soil mixture. Do not cover. Petunia seeds are so fine and small that covering them with soil can smother them.

Mist with a water bottle set to mist so that soil is moist.

Cover with the commercial product lid or with plastic wrap to create a warm, humid environment in which the seeds can germinate. Keep covered, only removing a bit of the cover, if needed, to add more water.

Remove lid or wrap once two true leaves have formed or when the sprout can reach the lid. If the leaves rest against the top, they become susceptible to rot. Continue to water as needed until time to transplant outdoors, making sure that the seedlings receive plenty of sunlight.