So many possibilities!
A flat can be made from anything that will hold up to the climate without rotting away or breaking down from the effects of fluctuating winter temperatures and the sun's rays. Flats need to have drains so excess moisture can seep out....the soil should never remain muddy.
The recyclable you choose to make your flat with may come with a clear lid. Otherwise a lid can be improvised from sheer plastic. Lids need to have vents to provide air-transpiration. The vents allow for excess warmth to rise up and away from the soil. Vents also allow melting snow or rain to seep inside which helps keep the soil moist so the seeds won't dry out.
Winter Sow
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Making and Sowing Flats
To make a flat you take a recycled container (of course it's clean, washed in hot soapy water) and a paring knife. Stab a few slits in the bottom of the container, this is for drainage. Now fill the base with soil to about an inch to half inch from the top. Give it a very good drink and let it drain. I do this in my kitchen. (I have a sprayer on a hose at the sink and I use this for the watering, works well and doesn't gouge out holes in the soil). After the soil has drained of excess water sow your seeds and pat them down. Cover them with more soil to the correct depth if necessary. I like growing plants with very tiny seeds, they're really just the very most easiest to sow. Sprinkle them on top of the soil, pat them down, and that's that.
Now you need to put the lid on BUT FIRST.........and this is the very most important step.......take the knife and poke several slits in the clear plastic lid. This is for air transpiration. Think about it, you're making a little mini greenhouse. If you don't vent the air that is heated by the sun you'll cook your flat and the seeds won't germinate. You've baked them to death. Okay, put the lid on secure by snapping down the rim or sealing with tape. Now the seeds are sown.
