Sowing seeds merely means to plant them. The word of God uses this as a metaphor to describe the concept of working to achieve something positive. The Bible often uses symbolism associated with agricultural production because it was such an important part of ancient people's daily life, unlike the majority of people today.
In one of Jesus' parables he uses the analogy of someone sowing seeds, and what can happen to the seeds afterwards, to teach about the different responses people will have regarding the gospel message (Luke 8:5 - 15).
Seed time, in relation to firstfruits, refers to the time when the environment, temperature and season are appropriate for the placing of seeds in the ground for their best success of growing.
Growing up on a farm, we spoke of seedtime as the time when the earth was warm enough and moist enough from the early spring rains to best guarantee the "sprouting" of the seed once it was placed in the ground. Otherwise, it would simply lie buried in the soil and die.
Springtime is the time of sowing seeds. It is the time when life springs forth from the earth. We could associate "seed time" with the time of planting.
Your people also will all be righteous; they will inherit the land forever, the branch of My planting, the work of My hands, so that I may be glorified . . .
The Spirit of the Lord God is upon Me . . . To appoint to those who mourn in Zion, giving to them beauty for ashes, the oil of joy for mourning, the mantle of praise for the spirit of heaviness; so that they might be called trees of righteousness, the planting of the Lord, that He might be glorified (Isaiah 60:21, 61:1, 3, HBFV).
Spiritually, when God places his Holy Spirit in us at the time of our repentance and baptism, he is sowing a spiritual seed in us. His desire is to have it grow within us to produce righteous character. We can't, however, just let that "seed" sit there or it will die. We must water it through God's Holy Spirit by continued bible study and prayer. We must put into practice what God teaches us for his character to sprout and mature within us.