Hello Aging DK, I understand your quandry about when to sow, light and temperature. Some of it is trial and error and how much risk you want to take. I think for tomatoes and cucumbers it is too early. They will be too big to keep indoors before it is safe to put them out. But peppers do want a long growing season and don't get too big too quickly. You could sow some and bring them on , on the windowsill, ready to go out to the greenhouse when the temperatures are safe. I planted mine in December and they are lovely wee plants.
At the end of the day, if it works, you're 3 months ahead. If it fails you've lost a couple of seeds, and not even the compost as you can put it on the compost heap. I'll see if I have time to post pics of my peppers tomorrow. Definately worth a try.

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From each according to his ability, to each according to his needs.