It's "too late" to plant potatoes when there isn't enough time for the crop to mature before the first killing frost or unfavorable weather arrives. The cutoff depends on your variety's maturity time and local growing season length.
For temperate regions, the latest safe planting dates are typically:
Early varieties (70-90 days) can be planted as late as early to mid-July in most northern climates. Medium varieties (90-110 days) need planting by late June to early July. Late varieties like Russet Burbank (110-150 days) must be planted by late May to early June in northern areas to mature before fall frosts.
According to University Extension Services and USDA guidelines, soil temperature is the key trigger — planting when soil reaches 8°C at 10cm depth. In Idaho, this means mid-April to mid-May for main crops, while Maine's frost-free date around May 20 sets their planting window from early May to early June.
Cold soil below 8°C dramatically delays emergence — taking 25-35 days versus just 12-18 days in standard 12-18°C spring soil temperatures. This delay can push harvest dangerously close to frost dates. Additionally, cold wet soil increases seed piece rot risk from Fusarium, Rhizoctonia, and Pythium, potentially causing 20-40% stand loss according to CIP and USDA research.
For subtropical regions like India's plains, the planting window is October-November for the main winter crop, making December plantings too late for optimal yields before hot weather arrives.