Success with Planting Sweet Corn

Success with Planting Sweet Corn

Sweet corn at sunrise.
Photo by Wren Vile

Very fresh home grown organic sweet corn is so delicious! There’s no denying that sweet corn takes up a lot of space, so if you are really short of land, you may decide to forego corn. On the other hand, corn doesn’t take a lot of work, so if you have the space, but are short of help, corn is a good choice. This information comes from my book, Sustainable Market Farming.

This post about sweet corn include an overview of our corn-growing, dealing with weeds, and my slideshow on Succession Planting

This one Growing Sweet Corn for the Whole Summer – Organic Gardening … is one I wrote for Mother Earth News Organic Gardening .

Don’t Set Yourself up for Failure – Understand Sweet Corn Genotypes

 You can skip this section if you are only growing one variety of sweet corn, no other kind of corn, and it is at least 250’ (76 m) from anybody else’s corn. Otherwise, before you plant (hopefully before you buy seeds), read this.

With most crops, cross-pollination with other varieties or types is only a problem for those growing seed. With sweet corn, the seed is the food crop. Failure to isolate genotypes can lead to starchy unpleasant-flavored corn. Hybrids are made by crossing OPs. Making hybrids does not involve genetic engineering!

Our late corn beside sweet potatoes.
Photo Ezra Freeman

Nearly all genotypes of hybrid sweet corn include one of two recessive genes, su or sh2. Cross-pollination with other corn groups will produce the dominant genetics of field corn (that starchy stuff!)

  1. Normal Sugary (su or ns) types of sweet corn have old-fashioned corn flavor but are sweeter than OPs, although the sweetness disappears fairly rapidly after harvest. Most can germinate well in cool soil.
  2. Sugary-enhanced (se) and Sugary Enhanced Homozygous (se+ or se-se) types are more tender than (su), and slower to become starchy after harvest. Most, especially the (se+) types, are sweeter than (su) types.
  3. TripleSweet Sugary Enhanced (se-se-se) corn was created to be sweeter than se-se.
  4. Super Sweet (sh2) (also known as Shrunken) varieties are very sweet and slow to become starchy. If not isolated from all other types of corn, they will be very starchy and disappointing. They have very poor cold soil germination. The kernels are smaller than other corns, giving this type its name. The seed needs careful handling, to prevent mashing between a seeder plate and the hopper.
  5. Synergistic (se-se-se-sh2) types are combinations of genetics from the 3 previous genotypes. Each ear has 75% (se) kernels and 25% (sh2) kernels. They are flavorful, tender and sweet, but only when they are ripe. If picked too soon, they are a watery disappointment.
  6. Augmented Shrunken: these newer types contain the sh2 gene and some of the tenderness from the se types.

Isolate these three categories by at least 250’ (76 m) from each other:

  1. Super Sweet/Shrunken sweet corn varieties (sh2);
  2. all other types of sweet corn;
  3. all other types of corn (eg popcorn, dent corn, field corn)

Instead, you can isolate by time, sowing on dates to achieve at least a 12-14 day gap between maturity of the different plantings.

Your neighbor’s GMO sweet corn will cross with your corn, if it’s close enough for the wind to bring the pollen in. “Bt corn” has been genetically modified by incorporating Bt genes (Bacillus thuringiensis) so the corn includes its own insecticide. There are many reasons not to grow GMOs, including the spread of random bits of genetic material by cross-pollination with previously non-GMO crops, and the likely consequence of Bt-resistant insects, so I won’t give them more space here.

Sweet Corn Varieties

Our first sweet corn of the season. Bodacious
Photo Pam Dawling

Most Open Pollinated (OP) sweet corns are noticeably less sweet than modern sweet corn, so consider hybrids. OP varieties also deteriorate faster after harvest than hybrids, becoming starchier. Luther Hill is said to be the sweetest OP variety.

Some catalogs  indicate which varieties are suitable for certain latitudes. (Corn flowering is day-length sensitive.) Johnnys has a “Dynamic Comparison Chart” for the sweet corn varieties they sell. Many companies run cold-germination testing, and can tell you which varieties that year have good potential for early sowings. Others can rot in cold wet soil.

We plant corn six times through the season, often with three varieties in each sowing, as shown here in our late corn.
Photo Kathryn Simmons

For our location, we rely on the first three genotypes, supplemented with some of the others for variety.

  • Bodacious, 77 day (se) yellow, great flavor for one this early;
  • Kandy Korn, 89 day (se) delicious yellow workhorse;
  • Silver Queen, 96 day (su) white long-time favorite with some drought-tolerance and insect resistance;
  • Luscious, 77 day (se-se-se) bicolor (organically grown, good cold soil emergence);
  • Tuxedo, 80 day (se) yellow (tightly-wrapped, earworm resistant);
  • Sugar Pearl, 72 day (se+) white (very early, on short plants);
  • Argent, 86 day (se) white (tasty with tight earworm-defeating husks);
  • Spring Treat, 66 day (se+) yellow, one of the earliest yellow sweet corns with good cold soil tolerance.
Silver Queen white corn when mature (with bug).
Photo Kathryn Simmons

Sweet Corn Crop Requirements

A pH of 6.0-6.5 is ideal. Very fertile soil is needed, including high phosphorus. P deficiency shows up as purple leaf tips and margins; N deficiency as pale spindly stalks, yellow leaf tips and shriveled kernel tips; Mg deficiency as white-yellow striping between veins, with older leaves reddish-purple, perhaps with dead tips. Corn is sensitive to deficiencies of zinc or copper, but less so to low levels of boron. When looking for deficiencies, it helps to know what is normal for that variety, and to also consider water shortage.

If you used legumes in the winter cover crop preceding your later sowings of corn, a good stand can provide all the nitrogen the corn needs (100-125 lbs/acre; 112-142kg/ha). When the legume reaches its flowering point, the nitrogen nodules on the roots contain the maximum nitrogen.

The number of rows of kernels on the cob is set five weeks after emergence (although each variety has a number that is usual, under good conditions). Ear length and number of plants with double ears is established nine weeks after emergence. There’s the feedback on your farm’s fertility plan.

Sweet corn needs warm soil. Catalogs usually indicate the soil temperature (measured at 9am) recommended for each variety. 50°F (10°C) is the absolute minimum, and applies to treated seed and OP or (su) varieties only. 60°F (15.5°C) is better for most, and 65°F (18°C) or higher is required by some varieties. Common phenology signs for the season being advanced enough to sow corn are that white oak leaves are the size of squirrels’ ears and that ragweed is germinating. For us the first corn sowing date is usually around April 26, which is also our average last frost date.

Corn has no tolerance of frost, but escape from a late spring frost is possible if the seedlings are less than two weeks old and not yet very tall, as the growing point may still be underground. Thus, in a spring that promises to be warm and dry, it is possible to risk an early planting as much as 2-3 weeks before the last frost date. Having some transplant plugs for a back-up helps reduce the risk level.

Emergence takes 22 days at 50°F (10°C), 12 days at 59°F (15°C), 7 days at 68°F (20°C), and 4 days at 77°F (25°C).       

Our third planting of sweet corn on the left, fourth in the middle, 5th (barely emerging) on the right.
Photo Pam Dawling

Sowing Sweet Corn

2oz/50’ (55gm/15m) or 1lb/400’ (370gm/100m) are generally required. The (sh2) types have more seeds for the weight, because the shrunken seeds are lighter than other types – 200/oz (7/gm), 2500-5500/lb (5600-12300/kg); 1lb/1250’ (118gm/100m).

Corn is usually grown in rows 36” (0.9m) apart. We sow fresh seed 6” (15cm) apart, and if using seed from the year before, we sow at 4” (10cm) apart.  Depth of sowing can vary with the soil temperature, being a very shallow 0.5” (1cm) in spring, to 1” (2.5cm) in summer when soils are warmer lower down and seeds benefit from the extra moisture.

Seedlings are thinned to 8-12” (20-30cm) apart. 8” (20cm) spacing is usually optimal in terms of using available light and maximum yield for the area.. Upper leaves get 7-9 times the light of lower leaves, therefore it is important that the upper leaves are in good condition, to photosynthesize well. The lower ones get much less light, so may be broken off for easier harvesting.

Young sweet corn plants after imperfect hoeing.
Photo Pam Dawling

Corn is wind pollinated (though you will find plenty of bees collecting pollen, regardless). For best pollination, plant in patches at least 4 rows wide. Inadequate pollination leads to ears with flat undeveloped patches among the kernels.

One of our sprinkler tripods, in a broccoli patch.
Photo Pam Dawling

Corn seed must have moisture to germinate. If you use a push seeder, irrigate after sowing. Because we sow small areas of many different varieties, and because people love to plant corn, we sow by hand. Our method has the advantage of delivering water right where the seed needs it. We measure and flag the rows, put out ropes on stakes along the rows, and make furrows (drills). The rope and its shadow make useful guides for keeping the rows straight. Next we have someone flood the drills with water from a hose, and we hand sow into the mud. After covering the seed and tamping the soil, we ignore the patch until the seed germinates. The watering in the furrow reliably provides enough moisture to get the plants up out of the ground. The ropes (about 12” (30 cm) above the ground) deter the crows, making it hard for them to land near the buried seeds. In recent years we have started using a seeder again, as the number of crows has worryingly declined.

We use overhead irrigation for corn. If you use drip tape, you might set out the tape, turn on the water for long enough to mark the soil with damp spots, then sow those spots with a jab planter.

Transplanting Sweet Corn

 It is quite possible to transplant sweet corn, so those in marginal climates don’t need to give up hope. We usually prepare some plugs the same day we sow our first corn outdoors and use these to fill gaps at the first cultivation. We use 200-cell Styrofoam Speedling flats (1”, 2.5 cm cells). We float these in a tank of water until we set them out. Some vegetable seedlings would drown if continuously in water, but corn does not. It is important to transplant the corn before the plant gets too big, and the taproot takes off. 2″-3” (5-7.5 cm) plants seem OK. The plugs transplant easily using butter knives.

I’ll write about Growing Great Sweet Corn in about a month.

 

Sweet corn, potatoes and okra

Our first sweet corn of the season (Bodacious).
Photo Pam Dawling

Sweet Corn

We sowed our first sweet corn on 4/18, eight days earlier than usual this year, because we had auspicious weather. We look at the leaves of the white oaks to decide when it is warm enough for corn planting. The oak leaves should be as big as squirrel’s ears. Phenology signs like this are especially useful when the weather is extremely variable, which we are getting more of as climate disruption has got us in its grip.

Our first sweet corn sowing is always a bit of a risk. In fact we often prepare for this by sowing some corn seeds in styrofoam Speedling flats, the same day we sow the first corn planting outdoors. Read more about this seedling technique and transplanting corn.

Our third planting of sweet corn on the left, fourth in the middle, 5th (barely emerging) on the right.
Photo Pam Dawling

We’ve been harvesting sweet corn since July 2, which is two days earlier than our target start date, so we’re very happy. Initially, of course, we got smaller amounts, but we are now harvesting three times a week and getting good amounts. We are back up to 6 sowings this year, after we had to cut back for a couple of years. For a hundred people we sow an average of 1100 ft (335m) each time we plant, with each planting intended to last us 15 days (7 or 8 pickings). Our goal is sweet corn from July 4 to mid-October (our average first frost is October 20, a 13 year average from our own records).

Three varieties of sweet corn in one planting. On the left Bodacious; in the middle, red-flowered Kandy Korn; on the right, Silver Queen not yet fully grown.
Photo Kathryn Simmons

Each sowing includes several varieties (with different numbers of days to maturity) on the same day, so a planting will give us at least two weeks of delicious corn. In this photo from a previous year, you can see (from left to right): later-maturing Silver Queen, not yet at full height; red-flowered Kandy Korn; fast-maturing shorter Bodacious. Each plant is only going to provide two or three ears, so to have enough, and to have a continuous supply, it is necessary to plan ahead. See the Succession Planting section ahead.

Sowing sweet corn

We have switched to sowing corn with the EarthWay seeder, with a homemade next-row marker. This is much quicker than sowing by hand, but does rely on providing overhead irrigation consistently until the seedlings emerge. Another trade-off is that we get more weeds germinating in the (watered) aisles than we did when we only watered the furrows at planting time. Drip irrigation would be another approach.  So far we have not lost corn to crows, which was the reason we took to the handsowing-under-ropes method some 20 years ago. Back then we also sowed our fist corn with a tractor seeder, but we had to follow that with putting up ropes, or we lost it all to birds. Details of our hand-sowing method are in this post from May 2016.

Staying on top of weeds in the sweet corn

Once we get to late June, more of our time in the garden is taken up by harvesting (a sign of success), leaving less time for weeding. We have a system I like that helps us stay on top of sweet corn weeds. Each time we sow sweet corn, we hoe the previous planting (about two weeks old), thin the plants to one every 8″-12″ (20-30cm) in the row, and wheel hoe or till between the rows. We have two Valley Oak wheel hoes that we really like. The handle height is adjustable and they are available with different width hoes (and other attachments). Our tiller is a BCS 732 from Earth Tools BCS

Don’t let this happen to you! Weedy young corn.
Photo by Bridget Aleshire

We also hoe the rows of the corn planting before that one (about 4 weeks old), and till between those rows and broadcast soybeans (which we till in lightly). Soybeans will grow in partial shade, handle the foot traffic of harvesting, and provide some nitrogen for the soil. For the last corn of the season, we undersow with a mix of soy and oats. After the harvest ends, we leave the patch alone, and the oats survive the first frosts (which kill the corn and the soy) and go on to be our winter cover crop for that plot. When it gets cold enough, the oats do die, and the plot becomes an easy one to bush hog and disk in the early spring for our March-planted potato crop. I like that opportunity to eliminate one round of disking, and to get a winter-killed cover crop established

As we harvest corn we pull out any pigweed that has somehow survived our earlier efforts. I learned at a Sustainable Weed Management workshop, that pigweed puts out its seeds in one big bang at the end, so pulling up huge pigweed is worthwhile, if it hasn’t yet seeded. (Actually you can see it for yourself, but before the workshop I hadn’t noticed!) Our soil has improved over the years, so it is now possible to uproot 5ft (1.5 m) pigweeds. Sometimes we have to hold the corn plant down with our feet, but we do almost always succeed in getting the weeds out, without damaging the corn.

Succession Planting for Sweet Corn

In Sustainable Market Farming I have a chapter on succession planting, and my slideshow on Succession Planting is one of my most popular ones. You can watch it right here.

I posted on Mother Earth News Organic Gardening blog about Growing Sweet Corn for the Whole Summer. You can read it here:

Growing Sweet Corn for the Whole Summer – Organic Gardening

Our sweet corn sowing dates and harvests from those plantings are

  1. 4/26, harvest 7/9 (with a few ears from 7/4)
  2. 5/19, harvest 7/24
  3. 6/6, harvest 8/8
  4. 6/24, harvest 8/23
  5. 7/7, harvest 9/7
  6. 7/16, harvest 9/22

Avoid mixing types of corn.

There’s a confusing aspect of hybrid corn varieties: There are several genotypes, and if you inadvertently plant a mixture of different types, it can lead to starchy unpleasant-flavored corn. Also don’t plant Indian corn, popcorn or any kind of flint or dent corn within 600′ (180 m) of your sweet corn. For this reason we grow only sweet corn in our garden. Ignore those cryptic catalog notes at your peril!

Dealing with raccoons, skunks and curious cats in the corn

We have trapped (and then killed) raccoons in our corn most years, and the past few years we’ve tried deterring them with nightly radio broadcasts. We have the large live mammal traps, and we found for raccoons, we needed to stake the trap down to the ground. I followed suggestions from Joanna Reuter of Chert Hollow Farm, staking the traps down and smearing peanut butter high on the stake in the back of the cage. Well, the first morning I caught a small, very white skunk! I let it out carefully. The next morning I caught the same skunk again. And the third morning, again. You can read more about how I let the skunks out. I’ve also caught some of our cats by accident.

Raccoons don’t seem to like Silver Queen as much as Kandy Korn, maybe because the husks are tighter and harder to rip off. Actually I like Kandy Korn better than Silver Queen too.

Okra

Cow Horn okra flower and pod.
Photo Pam Dawling

We have started harvesting our okra. It’s a little later than it might have been because we had to replant. The first planting had leggy seedlings due to not enough light soon enough in their lives. Then we planted it out with novice helpers, and they didn’t plant them deep enough. The feeble stems couldn’t take it. We made an attempt to hill them up to give more protection to the stems. Another mistake we made was over-watering. When we pulled up some of the dead plants for our postmortem, they reminded me of retted flax stems – the fibers were still there, with the soft tissue rotted away. This is why I think the problem is that we over watered rather than under-watered.

This year we are trying Carmine Splendor, a red okra from Johnny’s, as well as our usual favorite Cow Horn from Southern Exposure.

Cow Horn okra pods.
Photo Bridget Aleshire

We like Cow Horn for its tall plants, high productivity and the fact that the pods can get relatively large without getting fibrous. They are still tender at 5-6″, which is the size we used to harvest at. Some years we have attached a card to the handles of the pruners we use for this job, with a life-size drawing of a 5″ pod. This helps new crew get it right.

We do find it very hard to convince our cooks that we have specially chosen this “commune-friendly” variety so they don’t have to deal with fiddly little okra pods when cooking for 100. We’ve had to compromise and harvest at 4″. Hence the venturing-out to another variety, accepting people are hard to convince about Cow Horn!

We grow a 90′ row, with plants about 18″ apart in the row. This is enough for the hundred of us (some people never eat okra despite the cooks’ best efforts!)

Carmine Splendot okra plant.
Photo Pam Dawling

Carmine Splendor is a 51 day (from transplanting) F1 hybrid, with sturdy 5-sided pods that are deep red when small. I haven’t tasted them yet.

Potatoes

June-planted potatoes emerging from mulch in mid-July.
photo Pam Dawling

Our June-planted potatoes are starting to come up through the hay mulch, and we need to walk through and free the trapped shoots. This means we walk through investigating the spots where we expect there to be a potato plant but we don’t see one. If we find a trapped shoot, we open up the mulch to let the plant see the light. We do this same job with garlic in late November or early December.

This week we are harvesting our March planted potatoes. I just dug 30lbs for tonight’s dinner. Yes, lots of diners! The rest of the crop will be harvested using a digging machine on Thursday.

We harvested our March-planted potatoes 21 days ago, and we are in the process of sorting them and managing conditions in our root cellar to cure the potatoes and help them store well.

Crates of potatoes in our root cellar.
Photo Nina Gentle
Click here for our Root Cellar Warden” instructions from last year.

Dormancy Requirements of Potatoes

We are researching the dormancy requirements of potatoes in an effort to store ours so they don’t sprout when we don’t want them to!

What I know so far about dormancy is that potatoes need a dormancy period of 4-8 weeks after harvest before they will sprout. So if you plan to dig up an early crop and immediately replant some of the potatoes for a later crop, take this into account. Get around this problem by refrigerating them for 16 days, then chitting them in the light for 2 weeks. The company of apples, bananas or onions will help them sprout by emitting ethylene.

To avoid sprouting, keep the potatoes below 50F (10C) once they are more than a month from harvest, avoid excess moisture, and avoid “physiological aging” of the potatoes, caused by stressing them with fluctuating temperatures, among other things.

Tobacco Hornworm pupa

Just had to add this, which I dug up yesterday. These brutes are about 2″ (5 cm) long. They are heavy and they move slightly. Also, check out the comments on the last post and be sure to see SESE’s Ken Bezilla’s Instagram of a hornworm in black light.

This is the pupa of the tobacco hornworm.
Photo Pam Dawling