Most of the times, the effects of inaction are to destroy the new shoot. In the agricultural imagery with which Alma has begun, he contrasts the growing seed with one that is not tended and nourished. That seed does not gain root, and is therefore more easily destroyed by the same sun that nourishes the tended seed.
We all live in the same world, and many of the same cultural influences impinge upon us all. When does the world claim us instead of the Lord? When we give in to the world instead of to the Lord. If we do not actively nourish our understanding of the gospel, there are plenty of things to which we can use the scorching sun as analogy. For many, science can become an excuse to abandon the word of God, because it is seen as something with more substance.
Cross-Reference: Joseph underwent a period of time where he might not have nourished his tree as well he might. This did not deny his original knowledge and testimony, but neither did he progress:
Joseph Smith-History:28
28 During the space of time which intervened between the time I had the vision and the year eighteen hundred and twenty-three—having been forbidden to join any of the religious sects of the day, and being of very tender years, and persecuted by those who ought to have been my friends and to have treated me kindly, and if they supposed me to be deluded to have endeavored in a proper and affectionate manner to have reclaimed me—I was left to all kinds of temptations; and, mingling with all kinds of society, I frequently fell into many foolish errors, and displayed the weakness of youth, and the foibles of human nature; which, I am sorry to say, led me into divers temptations, offensive in the sight of God. In making this confession, no one need suppose me guilty of any great or malignant sins. A disposition to commit such was never in my nature. But I was guilty of levity, and sometimes associated with jovial company, etc., not consistent with that character which ought to be maintained by one who was called of God as I had been. But this will not seem very strange to any one who recollects my youth, and is acquainted with my native cheery temperament.
Joseph notes this as a time of little progress, and further progress was not made until he made further attempts to nourish his seed, as noted below.