Authors: Natalie Hoidal and Marissa Schuh, Extension Educators, University of Minnesota Extension
Our biweekly crop by crop growing season prep series continues. This week we are biting into some juicy sweet corn tips.
When to plant?
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Consistent sweet corn emergence is more likely when seeds are planted when the soil is warm enough. Photo: Marissa Schuh, University of Minnesota Extension. |
While regular field corn is planted in late April or early May, sweet corn can be planted May through July. Many vegetable growers aim to grow the earliest vegetables possible to get a leg up at farmers markets, but sweet corn germination is very dependent on soil temperatures. For optimal germination, wait until the soil is:
55°F at an absolute minimum,
60-65°F for supersweet varieties
The Growing Sweet Corn in Home Gardens page has a list of sweet corn varieties and the optimal germination temperature for each. If your variety is not on the list, figure out which type of genetics it has (sugary, sugary enhanced, shrunken, or synergistic), and make a decision based on other similar varieties. Your seed packet may also tell you the optimal soil temperature for planting. We will start to publish weekly weather forecasts in our newsletter soon with soil temperatures from across the state.
For succession planting, sweet corn is typically planted every 10 days to allow for continuous harvest later in the summer.
It is relatively common for Minnesota growers to seed a small quantity of sweet corn in the greenhouse for transplanting. While the return on investment for transplanting sweet corn is far lower than other transplanted crops like tomatoes, melons, etc., it is an option if being the first vendor at the market with sweet corn is important to your business. Sweet corn is typically spends just 2-3 weeks in the greenhouse before hardening off and transplanting.
Cover crops and nitrogen credits after sweet corn
Since sweet corn is harvested early in the season (early August - late September), sweet corn growers have an opportunity to plant a cover crop relatively early in the season. Types of cover crops that can be planted after sweet corn include:
Fall cover crops that will die over the winter like oats and peas
Fall cover crops that will survive the winter like winter rye, vetch, and winter wheat
Some summer cover crops like buckwheat, cowpea, and phacelia
For a comparison of cover crops to plant after sweet corn, see our new cover crop selection for vegetable growers page.
Researchers at the University of Minnesota have also been studying nitrogen credits from sweet corn. They are finding that a substantial portion of the nitrogen taken up by sweet corn remains in the stock after harvest. As such, sweet corn provides a nitrogen credit of 20-25 pounds per acre for your next crop. (So, say you plant tomatoes next year and they need 130 pounds of nitrogen per acre; 20-25 pounds will already be available in the soil from the sweet corn residue, so you can subtract that from your total requirement). Planting a cover crop after sweet corn is a great way to retain nitrogen in the soil and reduce leaching, but if using rye, make sure to terminate it before May 10 to ensure that the nitrogen credit is available to the next crop. Learn more about this project in this recent recorded presentation.
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| Sweet corn residue in 2017 from a sweet corn residue study by Carl Rosen, Vince Fritz, and Charlie Rohwer. |
Isolation and pollination
All types of corn can cross pollinate, and so different varieties must be separated. Sweet corn can hybridize with field corn, popcorn, and even other types of sweet corn; this impacts kernel quality and flavor. There are two basic ways to isolate corn:
Isolating with physical distance
Isolating cultivars by leaving physical space between plantings is the most common way to isolate sweet corn. As a general rule of thumb, it’s best to plant your sweet corn in stands at least 4 rows wide vs. in thin, 1 row strips. Since sweet corn is wind pollinated, planting in larger clumps will help to ensure better pollination since the pollen will be more likely to land on corn plants in a thicker stand vs. blowing away.
If your goal is simply to produce high quality corn, a distance of around 300 feet between varieties should be sufficient to prevent too much cross pollination. (Make sure your sweet corn is planted sufficiently far away from your neighbors’ corn as well, including field corn). If your goal is to save seed and preserve genetic diversity, you’ll need to leave more space between plantings. Unfortunately the recommendations for this vary substantially. The most conservative recommendation of 1600 feet comes from Seed Savers; other sources recommend between 700-1000 feet. If you’re saving seed for personal use, 700-1000 feet is likely fine, but if you’re growing sweet corn for a seed company or a regional seed bank, it may be best to stick to the more conservative recommendation or ask the buyer if they have a required distance.
Isolating with timing
You can also isolate varieties by staggering planting dates by 14 days. Cross pollination can only occur when corn is tasseling. By waiting 14 days to plant a second variety, the first variety should be completely done with its tasseling phase by the time the second begins to tassel.
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| By staggering planting dates to make sure that varieties are not tasseling at the same time, you can grow different varieties without physically distancing them. Photo: John Lillis, Flickr |
Birds
There are a couple of methods of keeping birds out of sweet corn. The key to them working is having the deterrent in place before the birds discover the ripening ears. Once birds have discovered the ears, getting them to leave the field is nearly impossible.
Which deterrent is appropriate for the farm will be dependent on the farm. How are your fields laid out? What kind of neighbors do you have? How much bird damage do you typically see?
This bulletin from Cornell details the costs and benefits of air dancers, scare balloons, chemical deterrents, and detasseling. University of Rhode Island has work (and building plans) on lasers that work best on smaller plantings. If your plantings are in rural areas, propane cannons are an option.
It is important to remember that birds are smart and learn quickly, so a mix of deterrents may be needed as birds can grow accustomed to them.
Corn Earworm
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| Corn earworm enters the ear through the silks, so is often found feeding near the tip. They can be anywhere from dull brown to orangey yellow to bright green. Photo: Robert J. Bauernfeind, Kansas State University, Bugwood.org |
While there are a couple of species of caterpillars that can get into sweet corn ears, corn earworm is both the most common and hardest to manage. This is because the window for control is very narrow. Corn earworm doesn’t survive Minnesota winters, so each year new moths are carried up from Louisiana and Texas on weather fronts. Once dropped by the front, moths are attracted to fresh green silks. Moths mate and females lay eggs on the silks. These eggs hatch in 3-4 days (hotter weather=faster egg hatch), and the caterpillars crawl from the silk into the ear, where they are protected from control measures. This means the control window is confined to a few inches of silk over the period of a few days.
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| Nightly trap catches of corn earworm across Minnesota in 2020, courtesy of VegEdge. |
To protect corn during this narrow control window, it is important to know when moths are being dumped in your area. This varies on a farm-by-farm basis, and will be different every year. It is a safe bet that there will be some level of corn earworm pressure from mid-August on, but we do occasionally get flights as early as late June.
The only way to get information about what is happening on your farm is to trap. While this takes time, it comes with thresholds that allow you to spray when there is risk, saving you time and money. If your trap helps you detect corn earworm flights at the more uncommon times, whole plantings can be saved. The University of Minnesota does place traps across the state and report numbers on VegEdge, though the number of moths present can vary wildly over even small geographic areas. If you have had major corn earworm losses in the past, a trap of your own is the best bet.
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| A mesh heliothis trap placed near sweet corn with green silks. Photo: Howard F. Schwartz, Colorado State University, Bugwood.org |
Traps are baited with the female corn earworm sex pheromone, which attracts male moths into the trap. The goal of the trap isn’t to reduce the moth population, but to estimate the population, thus the risk to your corn. The more moths you are catching, the higher this risk of corn earworm damaging ears. This information can help you decide when sprays are justified.
For current spray recommendations, visit this Midwest Vegetable Production Guide. Note that pyrethroid resistance is somewhat common in corn earworm, so when populations are very high, it may be worth using a different mode of action. For organic growers, control can be very difficult. For a run-down of organic management options, see the Organic Insect Management in Sweet Corn Bulletin from SARE.
Other potential worms in ears include European Corn Borer and Fall Armyworm (though neither is something we see regularly in Minnesota sweet corn ears). Cutworms, armyworms, wireworms, and seedcorn maggot occasionally cause problems early in the season. For a full list of sweet corn pests with links to more information, visit VegEdge.
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