These days, the desert plants for which Mr. Leblanc cares so passionately are in the midst of a renaissance in Arizona, their popularity bolstered by the promise of sustainability and relief from water bills driven up by drought. Many Northerners who migrated here tried to import the lush landscapes of their hometowns, only to discover that the desert — or their budgets — had other things in mind. They turned instead to cactuses and other succulents.

But homeowners’ knowledge of the plants has not kept up with the trend, leaving Mr. Leblanc and his crews with plenty of plants made sick by overwatering and overcrowding. Driven by demand and by Mr. Leblanc’s obsessive connection to cactuses, his crews work every day of the year. On New Year’s Day, he pointed out almost gleefully, they had a full 10 hours of work.

“We’re here to save your cactus, no matter what the problem is,” Mr. Leblanc said, his sales pitch interrupted by his ringing cellphone: “Hello, this is the cactus doctor.”

Known to his friends and co-workers as Frenchie, the cactus doctor is a man of few words and fewer passions: his wife of 26 years, his collection of bebop jazz albums and — of course — cactuses. His French parents fled the conflicts of colonial Vietnam for Thailand, where he was born and raised and caught what he calls “cactus fever” at age 11.

The one subject sure to draw him out is the special nature of his patients. “They’re full of so much character,” Mr. Leblanc said. “The way they’re shaped, the texture of the flesh, and the flowers are some of the most beautiful in the world. You could put a cactus here and just meditate on it for a week.”