This is usually caused by a fungal disease called "damping off". The cause is infected soil, waterlogged substrate, high humidity. any conditions that promote fungus. The fungus attacks the soft tissue in the stem which withers at the base or midway. The seedling collapses and dies. Another possible cause is that the seedlings died from lack of water or a high salt level in the substrate (which basically has the same effect of drying out the plant). Seedlings have only a small amount of roots and few leaves. They cannot retain much water and quickly dry out. High humidity in the grow space does not compensate for a desiccated substrate. Your soil/rockwool has to be moist enough to sustain healthy roots and provide water to the plant. Photo shows “damping off”.
Outdoor frost can kill seedlings overnight. Keep your seedlings indoor until there is no danger of sudden frost. See our GERMINATION GUIDE for great product tips on how to keep your seedlings warm outdoor or in the greenhouse.
b) some or all seeds are intact, but have noticeably changed colour and are darker; some may also have a whitish substance coating the rounded end, or the embryo inside has turned soft or slushy.
There are 3 possibilities:
Some or all seedlings fell over and died
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Some or all seedlings grow weak and tall
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See how the outer shell of the seed looks?? Those are the ones you want to look for.
Also, another thing you could try.. is put the seed in a glass (like a drinking glass) and put it under the sink faucet.. and blast it with a little water.. not too hard though.. just enough to try and wash off the fungus.
Oh, and like Whiteflour said, the paper towel method isn’t the best method to use. I prefer either soaking the seeds overnight in a glass of water.. until the seed swells up and pops open. But, you gotta keep checking on the seed every 12 hours or so.. because once it does pop open, it will need to be removed from the water ASAP as to keep the seeds root from getting too soft. Which will cause problems.. such as causing the seed to not sprout. As soon as you see that the seed has popped open, immediately plant the seed into your grow medium.
As far as the white hair thing you’ve described. I ran into this problem not long ago.. I went ahead and planted the seed.. and it came out just fine believe it or not. I would just go ahead and plant it.
Here, here are what good seeds look like.
Also, another way to make sure seeds are good.. and viable. Is to visually inspect the seed with your eyes. The ones that are good, will be a dark brown color.. with black stripes (kinda like tiger stripes). Those are the best ones..
If it smashes in your fingers, its no good. If its able to withstand the small amount of pressure.. its good to go.