Matthew 13:24-35 represents another collection of topical teachings, explaining what the kingdom of heaven (in Matthew, a circomlocution for "kingdom of God" that reflects an original Jewish setting). There are no parallels to 24-30 (the tares and the wheat) or the leaven (33) and the ending verses (34-35) round out the collection. Only the mustard seed analogy (31-32) is paralleled anywhere (Mark 4:30-2, Luke 13:18-19). Since none of the three writers put this parable in the same order, this is a good example of why order does not tell us much of anything about who wrote first. As for a comparison:

Luke has deleted the note about the smallness of the seed, and Matthew has deleted the initial question since it is pointless didactically; no one needs the questions to learn from, especially since he has already collected them all under "this is what the kingdom is like" headings. However, it can hardly be coincidental that Luke and Matthew use "branches" and not "shade" from Mark. Rather than hypothesize with phantom documents like Q, why not say it is the original Ur-Matthew line which Peter, using the freedom of oral variation, changed?

Ur-Matthew Reconstruction

To-what like is the kingdom of God? And to what may-I-compare it? like it-is a grain mustard which taking man threw into a field of-him which lesser truly is all of seeds when but it-is-grown greater the plants is becomes into tree great and the birds of heaven roost in the branches of it.