Hi Mary, Welcome--
First of all--there NO stupid questions in this forum.
I have tried what you are asking and have been successful to a certain degree.
Make sure you find a way to give some head room for the new seedlings to grow so they don't hit the plastic (propping with sticks maybe?). I have found that if you try growing more than one type of seed in the container, that they "may" sprout at different times and may overrun other seeds that haven't sprouted yet.
Hope this helps.
Duane

For very large diameter pots I like to sow seeds and place a cut off 2L/3L bottle over them. This allows for different seed germination and growth needs. If using just one seed type I still use the 2L cover as it helps with spacing. There will be several 2Ls needed to cover the pot. Push them into the soil well to keep from blowing away.

For rectangular pots if using only one seed I remove the paper part of hangers from the dry cleaner and poke them in the pot to hold up a large plastic bag. Clothes pins work well to keep the plastic bag in place. Be sure to poke holes in the plastic bag for ventilation and moisture. If you place the entire container in the plastic bag don't forget to add holes in the bottom of the bag for drainage.

I use large pots all the time and I do write abovut it in How to Winter Sow and there is an FAQ for it. If the containers are very large and deep you don't need plastin on the top to retain moisture, but the plastic does keep out some windblown seeds. You can make some lovely combos for seeds and you're all set.

Here is a link that might be useful: WinterSown Container Gardening

Thanks Trudi for all the work you do at winter sown. I read, re-read and re-re-read and learn something new every time. Great site! Now I'm going to ws my planters.