Something that the rain can pass gently gently through might be better, but black plastic will work. You will have to check each crop daily for germination and remove as soon as it happens. Otherwise, the seedlings will be blanched, and may also cook under the plastic when the sun comes out. Overall, though, the concept would work if monitored properly.

Hmm, OK, I will consider that. I was trying to make this border with my neighbor look better sooner rather than later, as their lawn looks gorgeous and it's a bit of a dump on my side. Hoeing that clay is not a pleasant prospect when it's hot and dry in August and September. It will be like rock. In the event I have to use some herbicide, what do you think about the question of how long to wait after that before planting seed?

Couple of books Ive read on micropropogation recommend 20% dilluted bleach. Ive found it disinfects the seeds really well too. I think 50% might be a little too much bleach, but you go with what works for you. Remember to also sterilize your container and soil. Microwave oven is a great autoclave. I usally nuke the the peat in the closed container for about 45 seconds. Let it cool off before you sow your seeds. Wahlaa!! No more fungus.

Depending on where you are, the soil temperature is probably well above the air temperature. At least it now is for most of the nation. For a light overnight freeze, a thin layer of soil will be an adequate insulator. So guess what -- even summer veggie seeds still underground would probably be OK. But everything you say you planted can easily withstand a light frost even if they were germinated and out in the open. Broccoli, kale, onions, and spinach are good in even hard freezes. Lettuce will easily make it through a light freeze.