How do Protestants who believe in predestination interpret "plant" in Matt 15:13?

"Plant" in this passage certainly seems to be referring to people in this instance, possibly even more specifically referring to people who claim to be Christian, superficially resembling those plants that God has planted.

When reading the surrounding verses, the disciples have just commented on the anger of the Pharisees towards something Jesus had said, to which Jesus is replying "So what? They're headed for destruction."

I don't think there's a link to predestination in this passage, Jesus is simply commenting on the corrupt nature of the Pharisees and what the fate of such people will be.

Imagine walking by death row and being jeered at by criminals, to which your friend says "ignore them, they're going to get what's coming to them". Not a reference to predestination, simply a comment on what the criminals have brought upon themselves.

I'm a Protestant who believes that God foreknew and predestined those of the "elect" before any creation started. I believe because that's what I read many times throughout the Bible. I do not believe because of having worked out a theology of time, or any kind of philosophy that would make me feel able to explain the matter. Whether I can explain any biblical theology to anyone else's satisfaction has no bearing on my belief of it.

This means that my understanding of Matthew 15:13 and Matthew 13:1-23 & 13:24-30 is based upon the doctrine of God's sovereign election. It should be said that there are other passages of scripture that need to be linked to those passages as well, in order to grasp the harmony of what the Bible is here teaching. For instance, see Revelation 14:14-20.

In a nutshell, the harmony of God's foreordination of those people who will be safely gathered in is grasped once it's seen that there is one harvest (at the very end) but two reapings, the one following on immediately after the 'good wheat' has been gathered into the 'barn'. Then the angels are sent forth, "for the harvest of the earth is ripe" (Rev. 14:15) All who had not been safely harvested at the first reaping will be collected for experiencing the wrath of God (vs. 19).

This fits in perfectly with the parable of Matthew 13:30. The various parables use different illustrations to depict the one, eternal truth; God knows those who are his, and who will be saved. He has known this from before creation started.

The good 'seed' of the kingdom is 'planted' in prepared hearts (the good 'soil') and it takes root. A lot of this 'seed' does not land on suitable 'soil' so any resulting plants do not reach maturity or produce good seed of their own. Likewise with the 'weeds' that the enemy plants in the field (the world) during the night. The weeds become noticed, but it won't be till the angels are sent forth that the weeds will be gathered and burned, without the good wheat being damaged in the process (Matthew 13:37-43).

Notice in that parable how Jesus gives the explanation? He says he is the one who sows good seed in the world, the good seed being the children of the kingdom, while the weeds are the children of the wicked one. The enemy who sows the weeds is the devil, the harvest is the end of the world, and the reapers are the angels. Jesus knew all this even while he was on Earth. He did not go on about predestination, but from what he says it is clear that all those children of the kingdom are known by him, as are all the children of the wicked one. Their eternal situation is clearly stated, and only God could both determine outcomes before creation and send his Son to be the only means of enabling children of the kingdom to grow and bear good fruit, to his glory.

This gives such massive assurance to believers in Jesus, they follow him no matter what others think of them, and irrespective of whether they can satisfactorily explain their faith in Jesus to others. There really are times when understanding only follows after taking steps of faith. That is why 'good soil' is needed for good seed to mature and produce more good seed - human hearts have to be prepared to receive what Jesus 'sows'.

That's how this predestination-believing Protestant interprets 'plant' in Matthew 15:13. I base it on what Jesus said in one of those parables you mention -

"It is given unto you to know the mysteries of the kingdom of heaven, but to them it is not given... they seeing see not; and hearing they hear not, neither do they understand. And in them is fulfilled the prophecy of Esaias... For this people's heart is waxed gross, and their ears are dull of heaving, and their eyes they have closed; lest at any time they should see with their eyes, and hear with their ears, and should understand with their heart, and should be converted, and I should heal them. But blessed are your eyes, for they see; and your ears, for they hear." Matthew 13:1-15

Those who do not believe in God's sovereign predestination in election take offense at such words of Jesus, or they may try to turn them around to fit in with their own ways of thinking. That only goes to prove the truth of what Jesus warned about.