In my experience with open containers, you risk wind blown weed seeds getting into your containers, rain causing soil compaction and sometimes flooding and washing the seeds away, faster drying of the soil surface, squirrels and varmints digging in them, and worse slug predation when the seedlings finally emerge.

I have searched online and if you are using milk gallon containers you do not cover them or put the lids on them. The top is left open. I have had success with winter sowing in gallon jugs. In Spring you can peel back the lid as the weather warms during the days but cover again when temperatures drop at night. I always watered through the bottom and no seeds floated away. I have never had anything blow into the tiny pouring hole. You can sow seeds into open planters as well but I think you would loose seed to them washing away with a heavy snow. The slugs are more of a problem in summer for me. I think it all works well. It was a great thing to learn. I had some things that did not sprout but I am blaming human error. I do not think I had enough media in some of my containers. Good luck!

Thanks for the replies so far folks. Kind of funny- wasn't till my buddy asked me why the lid is needed that I actually thought about it. Then maybe over thought it.

Also not funny. Milk jugs and cat litter jugs are pttht for me.. the ground rodents use them as personal salad bars once sprouting season is on.

To clarify, when i wrote open container, I meant uncovered nursery pots and similar. I use milk jugs and similar without the caps on with no problem. I grow almost exclusively native plants but didnt have any trouble starting mammoth sunflowers in 2L soda bottles, nothing ate the seeds or seedlings. Then I planted them in the ground and the squirrels ate them immediately.

Aww. So sorry about the squirrels. They are all over my area too. They do love to steal a tomato or two and take a bite out and toss it aside. They are big pains in the rear when it comes to taking and running. I did my best to keep them away from my little pumpkins last year. Chased those guys everyday.

Depending on how your winter weather goes, the covers help protect early germinating seedlings, especially annuals, from dips in the general temperature, and freezes. Covers also support the weight of say a blanket or something thrown over seedlings in those wild record freezes. Like right now it's in the 70's here and everything is trying to bust out of the ws containers, but I know it can go to 18f at any point over the next two months without asking anyone for permission. ;)