Many people are just getting to bed when Roger Medina wakes up at the stroke of midnight and, after a quick coffee, heads to the US-Mexico border as his wife and infant daughter sleep.

Six hours later, having gone through US customs and immigration control and after an hourlong bus ride from the border, the 23-year-old is standing in a field, packaging lettuce at a California farm.

Medina is among the tens of thousands of laborers who legally cross into the United States daily from Mexicali, just opposite California's Imperial Valley, to harvest the fruits and vegetables.

And they have warily been watching as President Donald Trump rails against Mexican immigration, accusing migrants of crime and of taking US jobs, even as he orders the building of a border wall to prevent, he said, undocumented migrants or terrorists from entering the country.

"If he wants to close the border, he can come and harvest the fruits and vegetables himself," scoffed Jose Luis Carrillo, 35, as he methodically chopped iceberg lettuce heads, placing them with lightning speed into plastic bags before they are loaded in crates on conveyor belts.

Almost 55,000 people cross into the US from Mexicali every day. Nationwide, some 540,000 Mexicans work in the US farming and forestry sectors.

In the Imperial Valley, one of the world's most productive agricultural regions, most of those who work the land are Mexicans with green cards or dual citizenship.

They line up at the border before dawn every day but Sunday to cross over, and they make the long journey back home as the sun is setting.

"No gringo could survive this," said Medina, who has been working the fields since the age of 17.