Final answer:
While the specific emergence of national lawn care companies is not detailed in the resource, the context suggests that post-World War I era - when national brands gained importance and companies like Sears, Roebuck & Company flourished - could indicate when such services started to become significant.
Step-by-step explanation:
The emergence and proliferation of national lawn care companies are not directly mentioned in the provided reference material. However, the context given involves the development of agriculture, the availability of farm supplies, and subsequent changes in consumer behavior, with national brands becoming significant post World War I. During this time, companies that could deliver a consistent product and protect their brands started gaining national reputations. Taking into account such consolidation of businesses and brands in the early 20th century, it would be reasonable to surmise that similar trends would influence the emergence of national lawn care companies.
As seen with the rise of department stores and mail-order companies, like Sears, Roebuck & Company, which supplied a variety of products including farm supplies, it is clear that the early to mid-20th century was a period of vast expansion for national companies. The economic growth following World War II, along with a rising standard of living and more leisure time, could have created the ideal conditions for the appearance of lawn care companies as a consumer service industry. Yet, for a precise historical timeline of when national lawn care companies began to emerge and proliferate, further specific research into the lawn care industry is required.