Once the seeds have germinated, sprouted, they need to be removed from the heat and placed under the lights. Heat is only needed for germination, not growing on, and can cause problems for the young plants.

They won't all germinate at the same time of course but once the majority have germinated, remove from heat and the rest will either follow or likely wouldn't have sprouted anyway.

This is the main reason why it is best to not start different types of seeds in the same tray - different germination times. ;)

Dave

what if your growing in a colder location or time of year? would it still be advisable to keep the bottom heat available?