The topographical variations of Oregon have broken this Pacific Northwest state into four different gardening zones. Sweet corn can be grown in all, with variable start dates. Because corn is not transplanted, this crop cannot be started early in the greenhouse.
Test the earth with a soil thermometer to ensure that the ground has reached the required 60 degrees Fahrenheit for sweet corn. Make sure you have passed the last date of frost for your area, or your sprouts will freeze. In Oregon, it takes sweet corn 70 to 100 days to reach maturity.
Zone 1 and Zone 2
Plant corn in the coastal range of Zone 1 in April or May. The valleys from Portland to Roseburg comprise Zone 2, where corn can be started from April to June.
Zone 3 and Zone 4
Cool temperatures linger in Zone 3, which includes Oregon's mountain ranges, higher elevations and most the central and eastern state. May to June is the ideal time to plant corn in Zone 3. Zone 4, made up of the northern Columbia River valleys and a small pocket around Pendleton, provides for warmer temperatures that allow corn to be planted from April 15 through June.