Then appeared the tares also - That is, then the tares were "first discovered." They had grown with the wheat, but were so much like it as not to be noticed until the wheat began to ripen.
So true piety and false hopes are not known by professions, by "blades," and leaves, and flowers, but by the fruit.

When the blade was sprung up - then appeared the tares also - Satan has a shoot of iniquity for every shoot of grace; and, when God revives his work, Satan revives his also. No marvel, therefore, if we find scandals arising suddenly to discredit a work of grace, where God has begun to pour out his Spirit.

But when the blade was sprung up,.... That is, the blade of the wheat; which designs the taking up, a profession of religion on principles of grace, called a profession of faith; and when right, it springs up from, and proceeds upon a work begun in the heart: and such a profession ought to be made by all that are partakers of the grace of God; and ought to be made both verbally, by a confession of the mouth, and a declaration of the work of God upon the heart, and by deeds, by submitting to the ordinances of the Gospel; and should be sincere, and from the heart, and be visible to men, and be held fast unto the end without wavering.
And brought forth fruit; which intends not the conversion of sinners, nor the performance of duties, nor the perfection of grace, but the first appearances of grace under a profession, such as sorrow for sin, after a godly sort, fear and reverence of God, great humility, much self-denial, ardent love to Christ, pantings and breathings after him, and communion with him, strong affection for the people of God, some exercise of faith on Christ, zeal for his cause and interest, and a concern to honour and glorify God.
Then appeared the tares also. They were not discernible for some time when they were first sown; they looked like good seed when they first appeared among the people of God; they seemed to have the truth of grace, as others had; their blade of profession, when it sprung up, looked like that of true wheat; but were now discernible both by their unfruitfulness in their lives and conversations, and by their bad principles, which they now endeavoured to spread, to the hurt of the churches where they were: they always appeared to be what they were to God the searcher of hearts; but now, through the zeal of true converts, to which these opposed themselves, and the fruitfulness of their lives, from which they were so very different, they became manifest to ministers and churches.

But when the blade was sprung up, and brought forth fruit, then appeared the tares also--the growth in both cases running parallel, as antagonistic principles are seen to do.

Whence come the tares? When the harvest was near at hand the difference was seen.

When the blade was sprung up, then appeared the darnel - It was not discerned before: it seldom appears, as soon as the good seed is sown: all at first appears to be peace, and love, and joy.