Just how difficult this problem is depends on whether you think the Bible's "creation days" are literal 24-hour periods or not. On the third day, God is said to have created plants. This is Genesis 1:11:
The sun did not yet exist, we are told, but was created later, on the fourth day. This is Genesis 1:16:
The problem is Weak if you believe the days are 24-hour periods. Obviously plants can survive for a short time without the sun. On the other hand, there are infallibilists who believe that the "days" are metaphors for much longer periods of time. In this case, there is a more difficult problem, since this would put the creation of the sun millions or even billions of years after plants.
Nevertheless, we are told in Genesis 1:3 that there was some other source of light before the sun existed, so this problem is hardly fatal. But even if God was sustaining the plants through some other light source, it's certainly an odd way to do things.