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?

Seedling tray

4 Answers 4


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.

  • So there is no point using the lid once all the seeds have germinated?
    – Dan
    Commented Jun 8, 2011 at 19:21
  • 1
    I think it depends where your seed tray is, if you've got it in a greenhouse, then no - don't use the cover, but if you've got them next to your desk at work, then yeah, better to keep it moist. And if you're just putting it out on your back porch only use it when the weather necessitates. Commented Jun 8, 2011 at 19:24
  • @Dan: First, the atmosphere inside the lid will be very moist, you're inviting fungus. Second, before long the plants will be bumping into the lid and you'll have to remove it anyway. I successfully started a couple hundred plants this spring without using covers. The only problem I had was with the two flats that I had covers on because I thought it would be better to keep them more evenly moist... completely lost both flats to a fungus.
    – bstpierre
    Commented Jun 8, 2011 at 19:25

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.


Normally, you want to see condensation on your seed tray. It indicates there's a moist atmosphere inside, which is just what the seeds need to germinate. It doesn't block the light in any meaningful way.

I can see that you've got nice green seedlings there, so you're doing great.


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.

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