Organic Sweet Corn

Spring 2015
Sweet corn can not only lure customers to your farm stand but can be profitable, as well. USDA photo.


Organic sweet corn has profit potential for farms and is a great draw to farm stands and farmers’ markets – if grown well. David Handley, UMaine Cooperative Extension vegetable and small fruit specialist, and Jack Manix of Walker Farm in Vermont covered this topic at MOFGA’s 2014 Farmer to Farmer Conference.

Handley said that 300 Maine farmers grow sweet corn on 2,400 acres (30 to 50 percent of the state’s vegetable acreage, not including potatoes). The crop, worth $4 million in 2010, is essential to local markets, he said – as a draw rather than a big money maker. It requires a lot of land and soil fertility, seed is expensive, and it can require several pesticide applications, all tending to lower net profit. Organic enterprise budgets from New York and Pennsylvania put the net profit from sweet corn at $1,200 to $1,700 per acre. Manix has a system that is more profitable.

“If you have only 5 to 10 acres, is it worth growing?” Handley asked. An alternative is to buy the crop to sell, perhaps growing just a small, token plot next to your farm stand.

Types of Sweet Corn

Four genotypes of sweet corn are available:

Normal sugary (su), a mutation of dent (field) corn, is rarely grown now due to its poor shelf life;

Sugar-enhanced (se) has about double or triple the shelf life of su corn and is the preferred genotype in Maine now;

Supersweet (sh2), or shrunken 2, has a shelf life double or trip that of se but has poor germination in cold soil and must be isolated by about 500 feet or more from other genotypes so that it doesn’t cross-pollinate with them, turning kernels starchy; its shriveled seed breaks easily in the planter; and customers may not like its tough, somewhat crunchy pericarp, although those who buy supermarket corn are used to it;

Synergistic (syn) has mixed genetics (about 25 percent sh2 kernels, 70 percent se and some su); has some of the improved shelf life but not the poor germination of sh2; and must be isolated to prevent cross-pollination.

Handley suggested choosing early and midseason (65- to 80-day) varieties and asking nearby growers what they like. These have done fairly well in his trials:

Bicolor
‘Espresso’ – early
‘Temptation’ – midseason to late
‘Delectable’ – midseason to late

White
‘Sweet Ice’ – early
‘Silver Queen’ – late

Yellow
‘Sugar Buns’ – early
‘Bodacious’ – early

White varieties traditionally are not popular in Maine, except in southern Maine, said Handley. Yellow corn is not favored much, but some of the best early corn is yellow.

Growing

Handley recommended direct seeding sweet corn into soil that is 60 to 65 F for untreated sh2 seed; 55-plus for treated seed (not allowed in organic production) and in lighter soils that warm up quicker than others – unless these soils are more valuable for other crops. Use 2 ounces of seed per 100-foot row (15,000 to 20,000 seeds per acre). Sow seed 1/2 to 1 inch deep – or 1 to 2 inches for organic, once soils are 65 F, for a better stand with less lodging and bird damage. Space plants 10 to 12 inches apart within rows, with rows 32 to 48 inches apart (depending on cultivation equipment). Crowded plants set small, late ears. Seed new crops every seven to 14 days or keep planting later and later varieties within a few days of each other. Corn is wind pollinated, so plant in blocks rather than in a few long rows, to improve pollination.

Yields range from 900 to 1,400 dozen per acre. Thinking in terms of dozens is becoming outdated, said Handley, as customers tend to buy only three or four ears.

Be sure nutrients, especially nitrogen (N), phosphorus (P), potassium (K) and zinc (Zn), are in sufficient supply before planting, and maintain a pH of 6.5 to 6.8. Plants take up the following amounts of nutrients per acre:

N 100 to 130 pounds
P 40 to 80 pounds
K 30 to 120 pounds
Zn 4 to 10 pounds

Plowing down hairy vetch should provide 120 pounds of N, but is that N available when plant needs it? Handley said growers may hedge their bets by banding some alfalfa meal, soybean meal, blood meal or fish products 2 inches to the side and 2 inches below the seed at planting time. The cost of foliar feeding may be too high.

When not following a legume, apply 100 to 130 pounds of N – 30 to 40 pounds pre-plant and 60 to 100 pounds sidedressed when the corn is 12 inches tall to pre-tassel height. If using fish meal, about 1,000 pounds can supply sufficient nitrogen. Deficiency is more common in dry years and weedy fields.

Phosphorus additions may not be needed, especially if you use poultry manure. Otherwise, you may apply 30 to 40 pounds of P per acre. If rock phosphate is used, apply it the year before growing corn. Bone meal also supplies P but is expensive and attracts animals. Cold soils can induce P deficiency in crops; wait until a couple of warm days have passed to see if the purplish color on lower leaves (a symptom of P deficiency) passes before adding extra phosphorus.

Not all Maine soils need additional K; for those that do, apply 30 to 50 pounds of K pre-plant from manures, Sul-Po-Mag or wood ashes (watching soil pH carefully with the latter). Symptoms of K deficiency appear occasionally, especially in dry years.

Zinc helps corn get off to a quick start, is common in some starter fertilizers and is in some organic mixes. Have your soil tested to see if it’s needed, said Handley.

Harvest

Harvest about three weeks after silks emerge, said Hadley – the last week of July or first of August in the Portland area, at the earliest. This year growers were picking well into October – although the market usually crashes after Labor Day.

Harvest at the milk stage (when pierced kernels exude a white liquid) and when the tip of the ear feels full. Harvest early in the morning, when temperatures are cool and moisture is high. Then, if you’re holding the crop, cool it to 32 F, ideally, as soon as possible by hydrocooling (dipping corn in cold water for a few minutes and then moving it to a cooler) or placing ice on top of it in bins or boxes. This can double or triple the shelf life – or even increase it 10-fold.

Prepare and eat sweet corn as soon after harvest as possible. Standard sweet corn has a shelf life of 24 hours, se of one to three days if cooled, and sh2 of five to seven days if cooled.

Fresh retail sold for $4 to $6 per dozen this year or 50 or 60 cents per ear. Wholesale is usually sold as 5 dozen in a net bag or in a crate.

Plow down and grind the stalks after harvest to combat pests (especially European corn borer), and cover crop the land right away. When corn is in the pre-tassel stage, you can spin oats or rye between plants, said Handley. Often these won’t germinate much until you mow off the corn.

European corn borer larva, Ostrinia nubilalis. Photo by Keith Weller, USDA-ARS.
Corn earworm caterpillar, Helicoverpa zea. Photo by Jack Dykinga, USDA-ARS
Fall armyworm (Spodoptera frugiperda). Photo by Canadian Biodiversity Information Facility.

Pest Management

Sweet corn IPM (integrated pest management) often reduces pesticide use and improves marketable yield and profits and the public image of the farm but may require increased management skills, time and labor, Handley continued.

Components of IPM include monitoring pest populations, setting action thresholds (the point at which spraying is economically viable) and using alternatives to pesticides instead of spraying according to set calendar dates. Alternative management strategies may prevent reaching the action threshold. They may be cultural (sanitation, tillage, crop rotation, planting dates, row covers), biological (parasites, predators, biorationals, resistant varieties) or chemical (synthetic – not approved for organic production; or organic – some of which do a fairly good job, said Handley).

Monitoring is done by field scouting and/or trapping. For scouting, Handley recommended sampling at about five spots per acre (not at field edges or ends where moths enter fields), with about 20 plants per spot. Sampling a couple of more spots can increase confidence. The action threshold is based on pest numbers or injury levels.

The European corn borer (Ostrinia nubilalis) adult is a small, night-flying moth that overwinters in Maine as larvae in corn stalks and stubble and in potatoes and beans. Adults lay about 50 to 100 eggs per mass, usually on the undersides of leaves. Larvae eat holes all the way through leaves. Don’t confuse this with the common stalk borer, which need not be sprayed, said Handley; by the time you see its damage, it’s too late to spray.

Monitor in the whorl to pre-tassel stage. The threshold is when 30 percent of plants in the whorl stage (when four to six leaves have fully expanded) or 15 percent or more of plants at the pre-tassel stage show injury.

Eventually borers enter the tips of stalks, causing the tips to fall over. White or brown frass may be seen at the entry hole in the stalk. Larvae can chew out of the stalk, move to the ear (usually at night) and chew their way into the ear at the top or side of the ear. Apply control just as the stalk is starting to elongate, because larvae move up and down the plant then and are exposed. At the silk stage larvae can enter through the silk channel or side. This is difficult to spot when scouting, so pheromone traps are useful. You may see more than one borer in a tip. (Earworms, on the other hand, eat one another, so you see only one.)

Pheromone trapping is done in the silking stage. The two corn borer races, the Iowa strain and the New York strain, and don’t respond to the same bait. Set two Heliothis traps 100 feet apart in grassy borders of the field at the height of the weeds. Bait one trap with ECB1 (Iowa strain) and one with ECB2 (N.Y. strain). The threshold occurs when five or more borer moths are in both traps (added together), if you’re not already spraying for corn earworm. Empty traps once per week and change the bait about once per month.

Fabric row covers hasten corn emergence and growth and are a physical barrier to moths. Remove them when the corn is about 2 feet tall, but before pre-tassel. They make cultivation difficult, although you can remove them and cultivate during the day, as moths are active mostly at night.

Parasitic wasps (Trichogramma ostriniae) – somewhat expensive but effective – can be released when you first see borer eggs.

Zea-Later oil applicators deliver about 1 ml (a small drop) of oil into the silk channel. Use these every five to seven days after silk emergence. As larvae crawl through the silk channel, the oil suffocates them. The oil is more effective blended with spinosad or Dipel, but pesticide labels don’t specify that these materials can be mixed with oils. Maine allows the mixture; other states don’t. Mineral oil is not approved for organic use; soybean oil is.

The corn earworm (Helicoverpa zea) is usually less of an issue than borers. It lays eggs on silks and then larvae move down the silk channel and feed on the tips of ears. Pupae do not overwinter in Maine, but moths can blow up with weather fronts from the South. These cannibalistic caterpillars eat one another until only one remains per ear. Growers can cut off the top of the ear, where the earworm stays, if they have understanding customers.

Worry about corn earworms at the fresh silk stage, when they lay eggs. Earworms don’t chew holes in leaves, so field scouting won’t spot them. Instead place a Hartstack pheromone trap (also called a Texas trap) in freshly silking corn, about 20 to 30 feet from the field edge, with the base of the trap at the height of the silk (2 to 4 feet). Check traps weekly (or twice weekly, if pressure is high). Move them to a new field when the silks dry. Change the pheromones and kill strip in the traps every two to three weeks. If you see 20 males in the traps, then about 20 females are in the field. Spray silking corn (or put oil on – which takes about 8 hours per acre) according to this schedule:

No. moths/week Moths/night Spray interval
0.0 – 1.4 0.0 – 0.2 No spray
1.4 – 3.5 0.2 – 0.5 Every 6 days
3.5 – 7.0 0.5 – 1.0 Every 5 days
7.0 – 91.0 1.0 – 13 Every 4 days
> 91.0 > 13 Every 3 days

Fall armyworm, Spodoptera frugiperda, lays eggs on undersides of leaves of the youngest corn it can find. The egg masses look like a little bubble of lint, said Handley. Monitor armyworms with a plastic bucket trap (also called a universal trap or unitrap) in young corn, with eight or fewer leaves. The green or gray larvae, which have two racing stripes down each side and four spots on their butt, chew long strips between leaf veins and later chew ragged holes through leaves. They travel through the silk channel and through the side of corn ears.

Scout for armyworms at the pre-tassel to silk stage, looking for ragged holes in leaves and husks of 20 plants each in five locations. The threshold is 15 percent. In the silking stage, use a unitrap or Multipher trap with fall armyworm pheromone. If catching three or more moths per week, spray – unless, said Handley, you’re already spraying for corn earworm. Armyworms do not overwinter in Maine.

Manage these insects with Bt (Dipel, XenTari), which works best on corn borers and not so well on earworms or armyworms; spinosad (Entrust), which works well on all three but tends to break down under high numbers of fall armyworm; pyrethrum (PyGanic), which has some impact on borers but not much on earworms or fall armyworms; and/or soybean oil (Golden Pest Spray) as part of a Zea-Later application. Zea-Later guns are available through veterinarian supply outlets.

Weeds

Manage weeds with crop rotation, sanitation (to reduce the weed seed bank), strip tillage or no tillage, and stale seedbeds (prepping the ground in the fall or early spring, letting the first flush of weeds germinate, and killing them by burning, for example, then planting and burning again – very carefully – just before the corn emerges, if shields are used), said Handley. With strip and no-till, be sure you can kill the vegetative cover before planting; otherwise soils remain cooler longer. Generally, however, you aren’t going for the earliest market. Tillage is useful up to the eight-leaf stage.

UMaine’s sweet corn IPM program (https://extension.umaine.edu/ipm/programs/sweet-corn/) consists of a weekly newsletter from June through Labor Day week, a Web page, instructional videos and fact sheets, and the New England Vegetable Pest Management Guide.

Walker Farm

Jack and Karen Manix of Walker Farm in Dummerston, Vermont, farm 35 acres of Vermont-certified organic flowers, fruits and vegetables – some in their 23 greenhouses. They grow 10 to 12 acres of sweet corn. They market through their farm stand and CSAs, 90 percent retail, from mid-April to Christmas. (They just bought a Christmas tree farm).

Sweet corn is the number-one crop if you have a farm stand, said Jack Manix, and organic is important because of Vermont’s educated clientele. People will stop for a few key items: fresh peas, tomatoes, sweet corn and strawberries. If you have a roadside stand, you should have sweet corn every day during corn season, said Manix.

Growing is the easy part, and growing organically is not that difficult, he said. Walker Farm buys 60 to 70 13-yard loads of dairy manure, composts it for a year and spreads 10 to 15 tons per acre. Walker cover crops with ryegrass (or oats on some land where early peas will follow) and sows buckwheat after the first and second sweet corn plantings – for weed suppression, to feed bees and to build a little organic matter.

The target pH is 6.8, and Manix hasn’t limed in 30 years. By using good compost, his soil pH has stayed at 6.8 to 7.0.

They grow sweet corn in four fields, all with different enough soils to require separate soil tests. “Get a soil test report,” said Manix. “Then you begin to become a professional grower.”

In addition to composted dairy manure, they broadcast about 300 pounds/acre of ProGrow (5-3-4) initially and later sidedress with ProBooster (10-0-0) or Kreher’s 5-4-3. They used to broadcast Chilean nitrate (16-0-0), “the best for corn,” said Manix, but now this can meet only 20 percent of a crop’s N needs, so they don’t use much anymore.

They grow only synergistic varieties, as customers favor its tenderness and flavor. Signs elsewhere may advertise ‘Butter an Sugar’ corn, but no one sells that old variety anymore, said Manix, because the new varieties are so much better. They do not grow supersweets, which have a tough pericarp and are actually too sweet, or sugar enhanced.

Untreated seed of varieties are not widely available – especially of synergistic corn. Manix buys from E&R Seeds (A Mennonite company with the largest variety of organic and untreated seeds he knows of, and with very competitive prices; 1356 E 200 S, Monroe, Indiana 46772; 260-692-6071; [email protected]), Seedway, Osborne, Johnny’s and Harris to plant the following:

‘Espresso’ one sowing early
‘Synergy’ one sowing early and one second-early; Manix’s favorite
‘Providence’ midseason to late, four to five sowings; customers’ favorite
‘Montauk’ midseason to late, four to five sowings; sometimes gets flat sides in dry weather or if fertility isn’t quite sufficient
‘Cameo’ one sowing late
‘Captivate’ (white) one sowing late (for just a few customers)

He doesn’t try to get corn too early because they’re busy with their large greenhouse business. Instead, they buy early, machine-harvested corn from Massachusetts, which never looks as good as hand-harvested.

Their first seeding occurs on April 15 in 128 plug trays in a greenhouse that stays above 62 F and gets up to about 80 in the daytime. Their help hand-seeds two seeds per cell, 60 to 70 trays at a time. They also sell some transplants in six-packs to customers (their original impetus for trying transplanting themselves).

They transplant all their corn in order to get 100 percent stands, for ease of cultivation and to stay ahead of weeds. Even somewhat oversized transplants do well, set with a Mechanical 6000 Carousel Transplanter bought from eBay for about $3,200 and delivered from Kentucky (where it was formerly used to plant tobacco). With two nimble people on the transplanter and one driver, they can plant 1/2 acre (about 60 trays) in about 2 hours. They set plants 1 foot apart within rows (not bothering to thin two-plant transplants to one), space rows 3 feet apart, plant 1/2 acre at a time and water transplants in with Rootshield to help fight soil pathogens.

Weed Management

Transplanting enables Manix to get an early start on weed control. He does blind cultivation with a Lely – the most labor-saving device on the farm – on onions, peas, cole crops, beans and corn. They Lely has eliminated hoeing on a lot of crops. Then he cultivates two or three times with a Lilliston Rolling Cultivator, which also hills the corn. The Lilliston works well on a soil with few or no stones. Having a dry soil and good speed enables the cultivator to throw soil into the row.

Insect Management

Manix starts scouting before the corn tassels. He sets out Heliothis traps for borers and corn earworm traps (his most difficult corn pest), but does not trap armyworms, as their damage is so visible. His threshold is 10 percent tassel damage. “You don’t want a reputation for wormy corn,” he said. Theirs is more than 95 percent clean. He worries that earworms may become resistant to Entrust (as thrips have become resistant to spinosad in his greenhouse). His mist blower covers 10 rows (30 feet) at a time.

He uses a Zea-Later with corn oil and Dipel. A few years ago he conducted a test and found 95 percent damage in an unprotected plot and 5 percent in a plot treated with a Zea-Later. He uses the Zea-Later less since he has been using Entrust – but the cost of Entrust may make the Zea-Later more economical if spraying is required more than once or twice.

Animal Pests

Scare Eyes in the outside rows, four or so per planting just above the tassel, protect against crows (but not redwing blackbirds). Manix buys new ones every few years, as older ones become dull and less effective.

Four Have-a-Heart traps control skunks. When a skunk is trapped, he covers the trap very gently with a heavy quilt and moves the animal. Raccoons are lured with the stinkiest canned cat food available.

Harvest

In 2014, Walker Farm harvested corn from July 10 to October 26. They sell 25 to 30 bushels per day on weekends and 15 to 20 on weekdays. Sweet corn is also popular in their summer CSA.

Regarding economics, Manix provided these figures:

  • 60 128-trays (70 trays later in the season) plant about 1/2 acre.
  • 200 to 240 bushels per acre is a conservative harvest estimate (transplanting yields more).
  • With 65 ears per bushel, that’s 15,600 ears or 1,200 baker’s dozen.
  • 1,200 dozen x $7.99/dozen = $9,588 (They get this price, as they’re near the Mass. border. Manix said some of his friends sell early “pirate corn” for “a buck an ear.”)
  • 350 bushels would not be out of the question, grossing $13,982.
  • They’re adding $500 to $800 per day to their farm stand total with corn sales, and corn attracts people to buy other products.

Discussion

Asked about areas of a field where corn plants are yellow, Handley said this is often a soil compaction issue. He recommended testing compaction with a penetrometer. Rob Johanson said those areas are usually the ones that do not get enough water on his farm. He does a pre-sidedress nitrate test, which has helped. He plants corn after a rye-vetch cover crop, adding 45 pounds of N per acre at planting.

Regarding pest control, Johanson said he uses Zea-Later for everything and, for European corn borer, sprays Dipel over the top of the crop at the whorl stage. One spray per planting usually suffices. If fall armyworm is a problem, he sprays Entrust over the top of the crop. His boom sprayer covers 12 rows. Crows probably do the most damage on his farm. He tried Scare Eye balloons every 20 feet, but crows got used to them. A big Scare Eye balloon plus a dead crow hanging nearby worked for about three weeks. Now he puts 30-pound test weight fishing line 4 to 6 inches over the outside rows, and at ear height, stretched between posts that are 75 feet apart, and crows no longer bother the crop. (Crows tend not to go into the interior of the planting, said Johanson.)

About buying an early corn crop from Massachusetts, Manix said to pick a good grower. He paid $17 to $18 per bushel this year for non-organic corn and sold that for $7.99 per dozen, grossing $40 per bushel. Some of his customers won’t buy this corn, because it is not organic and/or because it’s not Manix’s. Harlow Farm, one of the largest organic growers in New England, wholesales some organic corn, said Manix.

Asked about plastic mulch, Handley said most growers use clear plastic to get early corn. (Black plastic doesn’t heat the soil so much.) They plant two rows of corn in a 2-inch-deep trench under 3- to 4-foot-wide plastic. As soon as the corn hits the plastic or the temperature reaches 80 F, the plastic is slit, and the corn grows through the slit. Because weeds will grow under clear plastic, this is not a good method for organic growers.

Regarding succession cropping, Manix said he grows about an acre of strawberries, and when they’re done and turned in shortly after July 4, sometimes he can get a late corn crop on that ground, taking advantage of the abundant organic matter left from the straw.

For harvest, Manix said to make this a competitive event for the crew. “The young guys are always trying to keep up with me,” he said, adding, “I had this woman who was running me ragged.” He picks into baskets. Johanson picks into bags, with 65 ears per bag. He has to leave at 4 a.m. for two markets, so for them he picks the night before, about twilight, puts about 52 ears in a bag, and spreads the bags out in the walk-in cooler. By morning they’re cooled and will hold for three days – although he prefers to sell the crop that day. Handley mentioned one grower who has his crew pick into the tractor bucket as he drives.

For rotations, Manix uses potatoes, cole crops or some other large crop. They do not replant corn on the same ground for two to four years. Johanson rotates two years of soil-building crops with one year of heavy feeders and one of lighter feeders (greens and small seeded crops). He doesn’t disc after chopping corn, because he wants weed seeds to stay on the surface where they experience more losses to predation. This has helped a lot with weed control on his farm. He follows an early corn crop with rye or oats.

Asked about pest control on the home scale, Handley said that row covers and Zea-Later should offer enough control. If someone is not using traps, Ruth Hazzard’s work at UMass shows that pre-tassel, when worms are moving from the tassel to the ear, is the critical time to spray – with Bt, for example; next would be a spray of Entrust at the silk stage.

– Jean English

Scroll to Top