When it comes to farm robots, fruit gets all the attention. But it looks like trees and shrubs could win the prize for first significant agricultural market for small mobile robots.

Massachusetts startup Harvest Automation is beta testing a small mobile robot that it’s pitching to nurseries as the solution to their most pressing problem: a volatile labor market.

The multi-billion-dollar industry that supplies ornamental plants to building contractors, big-box retailers and landscaping firms — $11.7 billion according to the most recent USDA figures — has been eagerly awaiting automation for d...