I'm growing some seedlings in a tray that has a plastic lid. The sun causes moisture to condense on the lid. I was wondering: is this counter-productive, as the condensation then blocks the sun?
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In my opinion, those covers are a bad idea. They create a breeding ground for fungi.
I think the idea is that they keep the soil moist while the seeds are germinating. I can see through your cover that you have plants up. I have had good luck with uncovered trays and simply keeping the soil moist during germination. (I have had fungus problems in the past when I've tried to keep them covered.)
At any rate, it's unlikely that the condensation is blocking enough sun to drop the temperature inside very far, and not below your optimum range for germination.
The covers help as they not only trap the water, but they also trap the heat inside which allows for better control of soil temperature.
Each species of seed has an optimal soil temperature at which the highest percentage of seeds sown will germinate in the shortest amount of time. Most seed distributors will print this temperature in the seed catalog or on the seed package.
In general, seed trays should be uncovered when you have full germination, or you can lift seedlings into other flats as they germinate. An anti-fungal can be sprayed or sprinkled on the seed before planting and/or just on the soil surface. I use ground cinnamon and have never lost a plant to fungus. In the case of raising orchids, cinnamon should always be used.