I had to look up "darnel seeds" to better understand this parable. Darnel seeds, also called "tares," look so much like wheat seeds, that this is known as false wheat.
As the plant grows, it looks so much like wheat, that if you try to pull it up, you are very likely to pull up a lot of the wheat as well. In isn't until the ear appears that you can tell the difference.
This gives us a good reason for why the landowner did not want the servants to pull up the weeds until the harvest when the good wheat would be easily recognized.
God is letting us know that people are much the same. We all look alike in so many ways, and it is very difficult to distinguish the good from the bad. Very similarly to the weeds and the wheat, it is by their "fruits" that we can tell the difference.
This parable is another of those which Jesus explains to the disciples. The wheat refers to those who listen to the word of God and follow him, while the weeds are those who have been led astray by Satan. At the end of the world, the angels of God will separate the two groups.
Just as the householder allowed the wheat and the weeds to grow together until the harvest, God allows us to grow until we die, hoping that those who have chosen to go astray will have a change of heart and turn their lives over to God.
Just as parents always want their children to turn out well, God doesn't want to lose any of his children, and so gives us every chance to return to him.
For this we are grateful.
