Harvest starts with the planting of seed and preparation of vines just after the fall's early rain. Yahushua is the sower of good seed, His Word is the seed, He is the Word, He is the seed, and He is the vine; so it ALL starts with Him.

Matthew 13:37-38
He answered and said unto them, He that soweth the good seed is the Son of man;

Galatians 3:16
Now to Abraham and his seed were the promises made. He saith not, And to seeds, as of many; but as of one, And to thy seed, which is Yahushua.

The SEED is the "word of the kingdom".

Matthew 13:19
When any one heareth the word of the kingdom, and understandeth it not, then cometh the wicked one, and catcheth away that which was sown in his heart. This is he which received seed by the way side.

Now the parable is this: The seed is the word of Yahweh. Luke 8:11

Yahushua is the WORD so Yahushua is the seed.

And the Word was made flesh, and dwelt among us, (and we beheld his glory, the glory as of the only begotten of the Father,) full of grace and truth. John 1:14

Yahushua = Word = Seed

WORD of the Kingdom = Seed

Yahushua is the vine.

John 15:5
I am the vine, ye are the branches: He that abideth in me, and I in him, the same bringeth forth much fruit: for without me ye can do nothing.

Yahushua = WORD of the Kingdom = Word = Seed = Vine

For seed to produce, it must first be planted. When the Kingdom of Yahweh is preached, good seed is planted. But Yahushua, being the first seed, was planted in the earth when he died.

Verily, verily, I say unto you, Except a corn of wheat fall into the ground and die, it abideth alone: but if it die, it bringeth forth much fruit. John 12:24

Yahushua is speaking of Himself and His death. By His death, He, the seed, would be planted in the ground to yield a great harvest. Unplanted seed yields nothing. Planted seed yields fruit of that seed (barley and wheat) or those who believe in Him. Non-believers are NOT fruit of that seed, but are fruit of the VINE.

We, citizens of Yahweh's kingdom, also become good seed, like Yahushua.

... the good seed are the children of the kingdom; but the tares are the children of the wicked one; Matthew 13:37-38

We are also branches of the vine, yielding much fruit (grapes).

I am the vine, ye are the branches: He that abideth in me, and I in him, the same bringeth forth much fruit: for without me ye can do nothing. John 15:5

We plant seed (resulting in barley or wheat harvest - believers) and/or because of our attachment to the vine, it may result in the harvest of grapes (unbelievers), but all are eventually harvested.

And as ye go, preach, saying, The kingdom of heaven is at hand. Matthew 10:7

When we preach the Kingdom of Yahweh, we plant more good seed, increasing the harvest yield.

Then he called his twelve disciples together, and gave them power and authority over all devils, and to cure diseases. And he sent them to preach the kingdom of Yahweh, and to heal the sick. Luke 9:1,2

And this gospel of the kingdom shall be preached in all the world for a witness unto all nations; and then shall the end come. Matt.24.14

Then (after all the seed and vines are planted and ripened) the final harvests will come. These final harvests will be the ripened fruit (people with Yahushua's spirit) glorified at different times and ages and putting on immortality.