Sweet corn is a warm-weather vegetable. Germination and growth start at 55°F and the optimum temperature range is 70 to 86°F. To avoid seed rot and to maximize germination, soil temperatures at planting time should be at least 60°F. When soil temperatures at planting are below 75°F, it is recommended to use seed pretreated with a fungicide to reduce loss of seed due to rots.
When planting, space rows 28 to 32 inches apart and plant seeds 7 to 8 inches apart within rows and about 1 inch deep. Planting at this spacing and depth results in approximately 24,000 plants (10 to 15 pounds of seed) per acre. A minimum of 30-inch spacing is recommended for machine harvesting. Additionally, spacing rows every 36 inches with seeds every 7.5 to 8.5 inches in the row maximizes ear size. This results in 20,000 to 23,000 plants per acre.
To ensure a steady supply of sweet corn through the summer, successive plantings are recommended, especially when similar varieties are planted. There are four general methods to predict timing of sweet corn maturity:
- Time plantings by the calendar (every 10 days when temperatures are warmer).
- Plant subsequent planting based on when the first leaf unfurrows.
- Use the days to maturity listed on the package as a general guide. (Note: If a variety is supposed to mature in 69 days, it may be ready to harvest sooner or later depending on temperatures during the growing season.)
- Calculate growing degree units.
Using Growing Degree Unis
Growing degree units (GDU), or cumulative heat units, is an accurate method to predict crop maturity. Most seed companies provide information for either the time from field planting to maturity, or the time from seedling emergence to maturity. Be sure that you understand the GDU value your vendor provides when you purchase corn seed. Additionally, some models monitor soil temperature (max/min) and use this because it reflects more of what the seed is experiencing.
For the GDU method to work, you need to know four things:
- The GDU of the variety.
- The average GDU during the anticipated harvest period.
- The base (minimum) temperature for sweet corn (50°F).
- The daily high and low temperatures during the growing season (from planting until harvest). Since you want a continuous supply of corn, you will be planting several times throughout the spring.
If you don’t have weather data for your farm, try accessing the Utah Climate Center’s climate database (climate.usu.edu). Growing degree units for many locations in Utah can be found on the Utah TRAPs website (climate.usu.edu/traps). Select a location and GDU (base 50) from the drop-down menus.
,The formula for GDU is as follows: GDU = [(Tmax + Tmin) / 2] – 50
Tmax = The daily maximum temperature (if the daily max exceeds 86˚F, use 86˚F as the maximum in the formula).
Tmin = The daily minimum temperature (if the daily minimum remains below 50˚F, use 50˚F as the minimum temperature in the formula).
50 = Sweet corn base temperature (F) for growth.
The daily maximum temperature was 84 and the low was 56.
[(84 + 56) / 2] – 50 = 20 GDU
The daily maximum temperature was 96°F and daily minimum was 48.
[(86+50] / 2] – 50 = 18
Note: Substitute max and min temperatures were used because the daily high exceeded 86°F and the daily low was below 50°F.
The daily maximum temperature was 54 and the low was 34.
[(54 + 34) / 2] – 50 = 0
Note: GDUs cannot be negative because you cannot have negative growth. So if GDU is less than the base temperature 50, the corn has NO growth for that day.
If a variety requires 1400 GDU to reach maturity and we know that this variety generally matures in Utah around the third week of July (say July 20 to 23), we can use historic weather data to estimate the daily GDU during the anticipated harvest window. If you live in Layton, UT, you plant corn on April 20, and you expect to harvest the corn for four (4) days, then you need to calculate the total GDUs during this four day period. The four day average daily max/min temperature for July 20-23 in Layton is 91°F/63°F.
GDU = [(86 + 63) / 2] – 50 = 25.
When should you plant your next field of sweet corn?
- Planted Field #1 on April 20.
- Expect harvest from July 20-23. Average GDU is 25 per day or 100 for the four (4) days.
- Monitor max/min temperature starting on April 21 using the GDU formula.
- Add the GDU from planting until they equal the GDU for the harvest window (100 GDU).
- Make planting #2 when spring conditions accumulate 100 GDUs.
- Repeat for subsequent plantings or other varieties.
Another option to provide mature sweet corn over an extended time period is to plant several varieties with different ripening dates at the same time. For example, ‘Early Sun Glow’ (standard) matures in approximately 63 days, ‘Honey and Pearls’ (shrunken-2) matures in approximately 76 days, and ‘Serendipity’ (shrunken-2) matures in 82 days.
How to Plant Corn