You don't want to block the light coming in windows. You do not get direct light from windows. Unless your house is built different from most you have eaves and they block the direct sun. The light you get on window sills is indirect light even tough it may look like it is very sunny.

Window sills are not the best place to grow seedlings. If you don't have a light set-up look into winter sowing. There is a whole forum on it in this site.

Only some plants like to germinate in complete darkness and that is accomplished for the most part by making sure the seed is covered by soil or by putting something dark over the top of the container. Some seeds need the light to germinate so they are sown on top of the soil and misted to keep them moist. Read your seed packages and they'll tell you what the plant needs.

Agree. Seedlings can never get enough sunlight through windows as it is this time of year - unless you live in the southern hemisphere - so you sure don't want to block any of it. Even most greenhouses have to use supplemental lighting this time of year.

Dave

Which state or climate if not US are you located?

Your sign in shows Z9, I'm wondering why you are germinating indoors on a window sill. If you germinate your seeds indoors, you will need to harden them off by gradually exposing them to outside conditions for brief periods over a number of days. Even if your temperatures indoors are close to outside temps, the amount of actual sun and wind your seedlings can take without damage will be minimal at first if they have appeared/sprouted while in a protected room.

Thanks for all the information, and to answer your questions morz8, I am in zone 9 and did not realize germinating in cell packs designed for a window sill wasn't a good idea! I did read that I will need to harden my plants off. The reason I want to try to germinate indoors (annual flowers) is because many of my plants are in pots, which are full of plants/ flowers all the time, therefore can't really sow in my outdoor pots. I rearrange, change, and/or add to them at least 3 times a year along with whatever I have going on in the beds. I guess I just wanted to try another aspet of gardening, maybe save some money and have a bit more control over my color scheme with my tried and true favorites. I know I have a lot to learn, but thats one more great thing to love about gardening!