The Rule of Thumb for Choosing Containers
See your thumb?
That is the dual answer to choosing containers to winter sow with.
Can I Winter Sow in containers that are not clear?
If you are using a container made with tinted plastic it is easy to tell if the plastic is too dark and will not let in sufficient light. Can you see your thumb through the plastic? If you can see your thumb then the plastic will let in enough light and won't shade seeds or seedlings.
How deep should the soil be in a container?
An adult thumb, from tip to palm, is over two inches long. You can check the soil depth by plunging your thumb straight down into the dampened soil. Pre-dampen the soil prior to sowing because the water helps settle the soil down by reducing air pockets, it is much less fluffy and you'll get a more realistic idea of how much soil is in the container.
If the soil completely covers your thumb it is sufficiently deep. The deeper the soil the more moisture it can retain. The same amount of soil in a shallow pan has a greater surface to depth ratio, meaning that more soil is exposed to sun and wind. Evaporation can occur much faster.
For more information about soil moisture please read the Winter Sowing Forum FAQ called "Determining Sufficient Moisture in a Flat and Watering Techniques."