Sounds an awfully lot like what is called 'damping off', a term we use to identify any number of fungal disorders that result in the death of seedlings. But the problem could simply be cultural.

The causes for seedling death can be complicated...sort of like putting a jigsaw puzzle together; if one of the pieces is missing, we don't get the whole picture.

Excess moisture is the most obvious explanation. Is it as simple as watering too frequently? The consistency of the potting medium can also come into play...some are so slow to drain that the medium stays saturated for long periods of time, which suffocates (literally) the root system of the seedlings.

Most seedlings grown on sunny window sills don't receive enough light, especially if the plants are full-sun species. The stems become elongated (leggy) and weak and exceedingly prone to collapse. Were your seedlings leaning sideways to reach for the sunlight? I think that you will find that most of keep our seedlings under a lighting system until they are ready to be placed outside.

Can I assume that you don't have these plants on a heat mat? Seedlings need to have the heat mat off as soon as they germinate.

If you give us some more information, perhaps we can be of more help. What kind of propagation mix, containers, lighting set-up (if any), heat (if any), etc.

Hi.
I've watered them frequently using a spritzer making the surface moist but I think underneath it isn't moist. I'm afraid to over water and don't know what the proper way of watering is for zinnias and cosmos. My questions are:

My seedlings are in small pots. How much watering is sufficient? What is the proper way of watering ? A way to find out if I'm watering it sufficiently?

What to do after the seeds have sprouted: can I place them outside for a full on sunshine or do I wean them via the windowsill? Etc

Thanks.

Where are you located? If your weather is warm enough you could start the seeds in their permanent spot outside.

Thanks. I figured I was being extremely cautious wi watering . I watered more judiciously and all my seedlings have grown well. I do belong to the almost tropical climactic zone.

Now, I'm faced with what I read as thinning out and transplanting. How do I thin out properly and how do decide which ones to thin out? I feel very sad at the thought since all my seeds have sprouted healthy. Also, when do I transplant? What does first true leaves mean?

I'm so happy with my output I wanna try chamomile and lavender.

What's your zone? Im in zone 12-13 and chamomile and lavender do amazing here.