Answer: Tomato seedlings will grow best when soil and air temperatures are warm; 50F minimum. You can safely transplant your seedlings outdoors after they have developed 2 sets of true leaves. If the weather is still cool outdoors after they've developed two sets of leaves, you can continue to grow them indoors under bright light. Or, you can plant them outdoors and protect them from cold weather by using cloches (little paper caps) or enclosing them beneath floating row covers such as "Remay".
Be sure to harden off your little seedlings before planting outdoors. Start by taking them outside to a protected spot for a few hours in the afternoon then taking them back inside for the night. Next day, take them out for a longer period of time and back indoors at night. Continue this process until they are spending more time outdoors than in. At this point they will be ready to spend the night outdoors. Plant them in your garden and shade them from hot afternoon sunshine for a few days until they get over their transplanting stress and have adapted to the bright light conditions.
Hope you have a bountiful harvest!
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