I took a bag of 10 or 20 pounds of grass seed, and seeded one of the bare areas. It didn't do a bit of good. That summer the star thistle seemed to hit that area with a vengeance. The domestic grass seed had no chance against the vigorous star thistle. I'm told, though, that persistent watering makes it difficult for the star thistle to thrive, and then other vegetation will out-compete it.
Thorns compete with the farmer's intended harvest crop.
They divert resources from the planted seeds, so that any which grow must grow in the shadow of the thorns. When I look at my own life, I can see how my busyness about good things has often distracted me from the vital things. When we allow competing desires and pursuits to remain, then God's Kingdom harvest in our lives is stunted, truncated, and at worst is unharvestable because of the dominant clutter which surrounds us. Be aware that thorns and weeds in people's lives will prevent fruitfulness. Don't be surprised or taken aback at this.