At what point do we need to be concerned that wheat (or any of the 5 grains) becomes chametz?

Question

I am not that familiar with the science of fermentation as it applies to chimutz. My understanding is that we have a concern that if the wheat becomes wet, it starts to be chamet after a certain period of time - I gather 18 minutes?

So, my question, here, is at what point do we need to be concerned about the wheat becoming wet? After all, it rains on the wheat. Farmers use irrigation methods. So the wheat is getting wet. I gather that there is no concern of chimutz while the wheat is still attached to the ground, otherwise, we'd never be able to use it on Pesach.

So, at what point do we have to be concerned - immediately after it's cut? At the point that it's cleaned and the kernels are separated from the chaff? At the point that it's ground into flour?

Answer 1

Shulchan Arukh OC 467:5

דגן שבמחובר שנתייבש לגמרי ואינו צריך ליניקה כמאן דמנח בכדא דמי ומקבל חימוץ אם ירדו עליו גשמים:‏
Attached grain which dried completely and doesn't need to nurse [nutrients from the ground] is as if laying in a bucket and can ferment if rain falls on it.

Accordingly, wheat for Pesach is generally harvested a bit less ripe than regular wheat.

Ibid. 453:4

החטים שעושים בהם מצת מצוה טוב לשמרן שלא יפלו עליהם מים משעת קצירה ולפחות משעת טחינה ובשעת הדחק מותר ליקח קמח מן השוק:‏
The wheat used for making Matza for the obligation, it is good to guard it lest water fall on it from the time of harvesting and at least from the time of grinding, and in dire circumstances it's permitted to buy flour in the market.

Note the assumption that wheat won't generally become wet before grinding or before going to market may not still hold nowadays in your market.