Who sowed the weeds is
The harvest is
The end of the age.
ὁ δὲ ἐχθρὸς ὁ σπείρας αὐτά ἐστιν ὁ διάβολος· ὁ δὲ θερισμὸς συντέλεια αἰῶνός ἐστιν, οἱ δὲ θερισταὶ ἄγγελοί εἰσιν.
Only Matthew has this explanation about the parable of the weeds that was earlier in this chapter, 13:24-30. Jesus, via Matthew, explained about the enemy and the harvest. The enemy who sowed the weeds was the devil (ὁ δὲ ἐχθρὸς ὁ σπείρας αὐτά ἐστιν ὁ διάβολος). The enemy was no longer nameless, he was the devil himself (ὁ διάβολος), not merely an evil one. The harvest will be at the end times (ὁ δὲ θερισμὸς συντέλεια αἰῶνός ἐστιν). The harvest reapers will be the angels (οἱ δὲ θερισταὶ ἄγγελοί εἰσιν). The devil has sown the weeds, but the angels will be the harvesters at the end times.