Light and Heat Exposure & Seedlings (When to Start)

smithmalApril 9, 2012

In the past, I've noted that light exposure to seedlings is not necessary until true leaves are present (may take 3-4 weeks for this to occur) as only true leaves acutally photosynthesize light to plant energy. Others have suggested exposing seedlings to light as soon as they emerge following germination. Is there any benefits to exposing seedlings to lights before true leaves form?

Also I know that certain veggies (i.e. peppers) benefit from warm soil for increased root development. Would all my veggies (tomatoes, cucumbers, eggplants, peppers, herbs) benefit from seedling growth on top of a warm mat. In the past, I've only used the mat for germination purposes and then removed it from the process.



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Found the following information:

Keep a close eye on the first seedlings, as they need to be moved into bright light as soon as they emerge from the soil. They will explosively reach for light, and if the light is not adequate, you will get 3-inch-long (8 cm long) stems shortly after germination - this is very undesirable. If this occurs, you could try to transplant to a deeper container, or you may want to start over.

Here is a link that might be useful:

    Bookmark   April 9, 2012 at 4:30PM
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Cotyledons are green, and all green plant material has chlorophyll. How much it contributes is hard to say, but I agree that getting them under light does appear to avoid the development of spindly weak seedlings stretching for light.

As far as the warm mat, peppers do appear to benefit the most. You don't need as much heat as for germination. Unless your room is very cold (under 60F) other plants won't benefit much, even tomatoes. I use a large mat, so I tend to heat all my plants, but I try to keep to about 70F so which is a good balance for all plants. After about 4 weeks, I also remove heat altogether until I put them outside in the green house, where I use the heat mat only to prevent freezing. I am still experimenting with when and when not to use heat, so I may change how I do this, but so far this schedule seems to work well.

    Bookmark   April 9, 2012 at 4:50PM
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dowbright(z6 in Missouri)

I've never heard that light doesn't matter until true leaves. Without light, I've never had sprouting seeds do anything but damp off because the stems continue to elongate, searching for light. I think somebody mislead you. But that's just my opinion.

Are you sure you don't mean that they don't need nutrients until they have true leaves? This is true, because the seed is still feeding the cotyledons.

"Upon breaking the surface and reaching the light, the seedling's developmental program is switched to photomorphogenesis. The cotyledons open upon contact with light (splitting the seed coat open, if still present) and become green, forming the first photosynthetic organs of the young plant." I included a link here, but don't know if I did it right. If not, look up seedling at Wikipedia.

Here is a link that might be useful: Seedlings

    Bookmark   April 15, 2012 at 12:44AM
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dowbright(z6 in Missouri)

Live and learn. Just like gardening. ;)

    Bookmark   April 15, 2012 at 12:53AM
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smithmal is right.

Though some seeds do require light for germination... Most don't though.

    Bookmark   April 15, 2012 at 2:38PM
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From my limited experience, I'd say expose to light when seedlings erupt. I've noticed them grow better stronger and a better rate. If not spindly ones develop which I thought were good at the start but learned how detrimental it was.

    Bookmark   May 14, 2012 at 1:10PM
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