Lettuce and celery both need light to germinate. I do mass planting to start - just sprinkle the seeds on top of the soil mist heavily and cover tightly with a clear lid. Pop the lid and mist at least once or twice a day until they sprout. You can sprinkle soil on top of the lettuce once they've sprouted. I usually leave them dense until a first true leaf is out, then transplant to whatever density is desired. For transplanting outdoors they get a nice size in sixpacks.
Celery seems to like the heat mat off and on - I like to give them day on/night off for a couple of days and then let them chill under the lights. They take awhile so be patient. I do think presoaking celery seed might help to speed it up, but forgot to try again this year.
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