The unfortunate thing about wheat and darnel is that they are similar when they first take root. It's not until they're established that one can tell the difference. With this in mind, Matthew 13:26 becomes clear, "But when the blade (wheat, verse 25) was sprung up, and brought forth fruit, then appeared the tares also." It has been thought, by minds better than the one writing this, that darnel would be a likely candidate for this parable.
Wheat is good, darnel is not. But the farmer said to let the two grow together for fear that wheat would be pulled up with the darnel. Later, it could be separated. This is the parable of the weeds among the wheat. Good (wheat) must be left to flourish until harvest. Bad will also coexist until harvest. The weeds here can represent all the bad things that stand in the way of good flourishing.
There is a bad that goes along with the good. Just think of the good people out there that have a heart for God. My Aunt Bessie fell out of a tree when she was five and broke her arm. This was back in the days when polio was prevalent and there was no vaccine for it. Yes, polio withered her arm and it swung limp from the right side of her body for all of her life. She was a teacher, a wife, mother of two and my Aunt Bessie that I went and lived with from the time I was three until I started kindergarten. I still hum old hymns that she taught me during those precious two years. She took me to church with her and I learned all the basics I would need for a christian life. Simple prayers, Jesus, the story of David and Goliath, and so on. Her faith was put to the test most of her life. When the day comes that God will separate the wheat from the tares, she will be wheat.
It's not for us to say whether the things that coexist in our lives are good or bad. Yes, her lifeless arm gave her grief for many reasons. She had pain in it, it was twice as hard for her to do many things. There were things she was totally incapable of doing. The thing was that even though she suffered with this, and she did have moments of real sorrow over it, she just kept growing like the stalk of wheat in the field. Her face turned toward the Son every time she felt discouraged and just kept growing. Would that we all could follow her example.