Plants that are started indoors should be hardened off first before planting outdoors. The severe change from a protected environment indoors to the wind, heat and sun outdoors can indeed kill tender seedlings. To harden off seedlings, place them in a protected location out of direct sun for about an hour the first day. Take them back inside the rest of the day. The next day, increase the length of time outside. Gradually increase the exposure of the plants over a week or more. By the end of that time they should be able to handle the harsher conditions outdoors.