Psalm 90:6 In the morning it flourisheth, and groweth up; in the evening it is cut down, and withereth.

God is speaking of grass here and he tells us in Isaiah 40:6 "all flesh is grass".

"In the morning" speaks to God/ his provisions:
Matthew 5:45 he maketh his sun to rise on the evil and on the good, and sendeth rain on the just and on the unjust.

So grass = ALL flesh and God clothes the grass i.e. he gives the natural sun, rain, air that grass needs to live... so to, he provides the Spiritual sun, rain, air (Jesus) that "all flesh" need to lives. This is why he said:

Isaiah 40:5-7 And the glory of the Lord shall be revealed, and all flesh shall see it together: for the mouth of the Lord hath spoken it. [6] The voice said, Cry. And he said, What shall I cry? All flesh is grass, and all the goodliness thereof is as the flower of the field: [7] The grass withereth, the flower fadeth: *because the spirit of the Lord bloweth upon it*: surely the people is grass.

When the glory of God (Jesus) was/is revealed to the grass (all flesh), when his spirit blows upon flesh (his spirit blowing brings life) the grass withers, i.e. the flesh dies.. because to see God's glory, to receive God's Spirit, is a death sentence to flesh (grass). The withering grass is God saving us. Flesh/grass MUST die.