Is there Spring and Summer Sowing? (seed lists included)
Yes you can do this, most certainly.
What you need to do is first assure that the soil stays moist. You need a container that has some depth to it....like a cool whip tub. The increased depth of the soil in ratio to the soil surface will decrease the amount of moisture loss through evaporation. Plain and simply, the deeper soil will hold more moisture because less is exposed to the air.
Second, because of the warmer days the temperature inside the flat will heat up faster and higher so the flats need to be in part shade. More specifically, they should be in a location where they will not get midday sun which is just too intense.
Third and last, you will need to add some extra vents in the lid to allow for rapid transpiration of the warm air inside the flat.
*Don't forget*....in the Summer you have to keep a closer eye on the watering needs of all flats.
"What seeds can I start now?"
You can Spring and Summer Sow the seeds of any perennial or biennial that says on the back of the packet "sow after last Spring frost", or "sow after ground has warmed", or anything similar to that. That information might not be mentioned at all....as long as you read on the packet any wording that indicates NO COLD PERIOD is required for germination you'll make good choices. I would think that any of the seeds that they sell at the dollar store would qualify.....all of those are very easy to grow.
Here's a list of perennials and biennials you can try with Spring and Summer Sowing:
And here's some fast growing annuals to try with the method. The cheery blooms of Spring Sown annuals will help perk up the garden during midsummer when some flowers start to wane and fizzle out from the heat. Summer Sown annuals can provide a "punch" of colorful fresh blossoms to enliven the Autumn garden.
African Daisy Alyssum
The two lists above are both...