You can also use a "Seed Spoon (R)" such as Lee Valley sells. Petunia seeds are so small that even these spoons don't tend to hold just one at a time, but you can pick up 3-6 seeds with one scoop, and then scatter those without worrying that your hand will twitch and drop 20 seeds all into the same spot.
http://www.leevalley.com/en/ga...
I found that I could not sow petunias on a coarse seed-starting mix. I had mixed in some shredded pine bark to make to drain better and stay aerated, but the bark was too coarse.
Seeds fell into the cracks between the bark shreds and never sprouted.
I should have used finer bark, and spread a little vermiculite on top to give a smooth surface.
I agree that 'watering" with a mist sprayer, and/or covering the tray to reduce evaporation is a great idea until the seedlings have emerged and grown at least a little. Sometimes, if I have more trays than I have clear plastic domes, I will drape clear plastic food wrap over the tray. I have an 18" wide roll, but you could use two widths of 12 inch "Saran Wrap".
I stick tiny stakes into enough of the cells to support the plastic up off the surface if the ssoil mix. I cut mini-blind slats in half or thirds the skinny way, and make them long enough to reach reach the bottom of the cells and up another 2-3 inches. Draping the plastic over that gives plenty of air pace, and it keeps the plastic from touching the seeds. (They might cling to the plastic, otherwise.)
I push the micro-stakes between the plastic side of the cell and the mix, so they are least likely to disturb a root when I pull them out. You can also write the name of the variety on the mini-slat, and label a whole row.
I lay a piece of cotton flannel in the bottom of the way so I can "bottom water". It's more like "bottom moistening" since I don't like to have standing water in the tray. I add only enough water to moisten the cotton, plus maybe enough to partly fill some of the grooves in the bottom of the tray. capillary force spreads the water evenly across the whole piece of flannel and thus to the bottom hole in every cell. As long as the flannel is moist, no cell can dry out entirely.
After small seedlings are up, I occasionally also top-water ... I'm not sure why. Convince myself that water is still wicking up to the surface? Flush the imaginary salts that have not had time to accumulate out of the soil?
A mister isn't enough, but any kind of watering can or watering rose is too harsh for tiny seedlings. So I've saved some plastic squeeze bottles for mustard or ketchup. Just a little down-and-up shake is enough to push a few drops out, and that's enough to wet the surface of one small cell. I still aim to miss the seedling.
I can top-water more heavily by squeezing steadily while moving the stream along the row. I aim the stream to hit the plastic ridge that divides one row from the next. The water runs off it onto the extreme edge of the mix in each cell, staying as far away from disturbing the seedling as possible.
You could buy a restaurant condiment squeeze bottle - those have fine, elongated tips and are even better.
I've also forced some 1/4" and 1/8" irrigation tubing into the nozzle of my ketchup bottles, to give a longer "aiming snout".
If I over-water any cells, the excess water will be wicked out the bottom into the cotton and shared with the cells that I might have under-watered, or cells with extra-thirsty seedlings.
Is any of this folderol actually necessary to the seedlings? Perhaps not, but it keeps me happy and I feel that I'm pampering them. I used to kill them with kindness (overwatering). I haven't yet killed them with silliness, but time will tell.
