No they do not. Rather it cooks the rootlets as they develop and can contribute to leggy, lanky growth with weak stems.

Heating mats are for germination only. Once sprouted seedlings emerge they should be immediately removed from the heat mats.

Check out all the info on this over on the Growing from Seed forum here.

Dave

I wouldn't use heat mat for onions and leeks even for germinating. Room temperature is all they need. Obviously no need for heat mat after germination for anythings.
All they need is light.

By the way, I always direct sow scallions, leeks and chives. Or I do it in the cold frame.

Seysonn

Ditto, no heat for leeks and onions.

Thanks for the information, I will remove heat mat. But, I will say that I germinated some with and some without and the added heat (about 15 degrees) sped up germination substantially. 6 days with heat vs about 22 days without so I would say it is highly beneficial to use added heat in germination. Also, the reason I am sowing under lights indoors is because the onions are Ailsa Craig Exhibition and in my area/growing season length, it was recommended I start indoors this early to get maximum size on the bulbs at harvest.

I place the seeded flats on top of the grow-light hoods for 2-3 days, where they get bottom heat from the lights. Then the onions go under the lights, no more heat with some cool-down at night.

Most seeds benefit from warmer soil temperatures. So even onions family, parsley, cilantro .. will germinate much faster when soil temps are 60F++. They have developed charts ( Temps vs. days to germinate), showing the effect of soil/air temperatures. There is an optimum temperature range for different seeds.
See chart below.

For example, ONIONS seeds eventually will germinate in soil temps higher than 35F but best range is 50 - 75F. So 65F room temperature is just fine.

Seysonn
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{{gwi:2117656}}

The table above is excellent because it gives a MINIMUM temp. I don't use heating pads at all and, in my experience, 65-70F house temps (for toms and eggplants) just makes germination take a little longer. Add a few days, maybe a week, onto what it would take with heating pads. I do cover my seedling trays with a plastic sheet, in order to ensure that the soil remains moist without having to apply water.

I have to assume that the minimum temperature is largely because below that, the seed will just rot before it sprouts.

I once tried a heating pad, and had a lot of trouble getting the temperature reasonably uniform across the tray (which was about 50% larger than the pad).

Is germination rate really the only reason to use heating pads? In my mind, germination time is a negotiable.