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Old April 4, 2019   #1
Join Date: Jul 2011
Location: Boston, MA
Posts: 138
Default Too hot?

Last year, I had a problem where my seedlings were too leggy. The general advice was that I needed better lights positioned more closely. This did wonders for my seedlings' strength.

I just planted a few days ago and as soon as the first couple broke through the soil, I turned the light on. After a day of this, I checked to discover that the temperature of the seedling soil inside the hydration dome was over 105.

First, how catastrophic is this for everybody? It was for about 6 hours last night and about 4 this morning.

I've removed the sprouted seedlings from the dome and put them close under the lights and moved the dome back into the dark.

Seedlings don't all sprout simultaneously, so if I've got a group of 6-9 together, is it ok to leave the sprouted ones in the darkened dome while their siblings sprout and then move them under the bright lights? I really don't want to have to cut away each individual cell from the group.
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Old April 4, 2019   #2
Join Date: Jan 2014
Location: Kansas 5b
Posts: 197

Take the dome off if you're available to mist them every six hours or so. Seeds have aa preferred temp rage for germination that tops out about 85F for most tomatoes.

If you use a heat mat the heating mat heats the soil, but the dome holds heat as well as moisture in. The biggest issue is that the moist air hold more heat on the plant than warm soil does. Too wet and your plants are in a sauna, too dry and the water ends up on the inside of your dome and not in the soil.

I use a dome with peppers but only until half have germinate. I still have better luck on later season flats where I skip it. Tomatoes I sometimes use a dome, but never with a heat mat. I look at flats a couple times a day and use a spray bottle to keep everything moist, but not wet. And every couple days I move flats around so I can judge weight to see if they need a better watering.

Once you have germination I'd recommend moving the whole flat under lights. Once gernimation has started I usually put the heat mat on the same timer the lights are on. You don't want to divide up the insert and the light may give seeds that are planted deeper a push to get to the surface. You can have too much heat from a heat mat, but it's not likely you're going to get too much light from home plant lights. You may need to lower the light closer to the seedlings to get enough. And if all the heat i coming from your lights they may be too close, you might need better air circulation, and the dome probably exacerbate s the heat issue.

Last edited by oldman; April 4, 2019 at 11:51 AM.
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Old April 4, 2019   #3
taboule's Avatar
Join Date: Jul 2013
Location: MA
Posts: 897

I wouldn't leave any seedlings in darkness once they've germinated. That's one of the reasons I dont use single piece flats, I prefer small tubs, or pucks, or individual small pots, so I can remove the seedlings and place under lights, while their siblings are still under ground.
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