Illini Xtra Sweet
Zea mays 'Illini Xtra Sweet'
A beloved yellow supersweet corn that has earned its reputation as one of the sweetest varieties available to home gardeners. This reliable performer produces ears packed with incredibly tender kernels that burst with concentrated sweetness and maintain their quality longer than standard sweet corn. A must-grow for corn lovers who want maximum sweetness and eating pleasure.
Harvest
85-90d
Days to harvest
Sun
Full sun
Zones
2–11
USDA hardiness
Height
5-8 feet
Planting Timeline
Showing dates for Illini Xtra Sweet in USDA Zone 7
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Illini Xtra Sweet · Zones 2–11
Growing Details
Zone-by-Zone Planting Calendar
| Zone | Indoor Start | Transplant | Direct Sow | Harvest |
|---|---|---|---|---|
| Zone 1 | — | — | July – August | October – August |
| Zone 2 | — | — | June – August | October – September |
| Zone 11 | — | — | January – March | May – June |
| Zone 12 | — | — | January – March | May – June |
| Zone 13 | — | — | January – March | May – June |
| Zone 3 | — | — | June – July | September – October |
| Zone 4 | — | — | June – July | September – October |
| Zone 5 | — | — | May – June | September – October |
| Zone 6 | — | — | May – June | August – October |
| Zone 7 | — | — | April – June | August – September |
| Zone 8 | — | — | April – May | July – September |
| Zone 9 | — | — | March – April | June – August |
| Zone 10 | — | — | February – April | June – July |
Succession Planting
Direct sow Illini Xtra Sweet every 14–18 days from your last frost date through early June, planting each succession in a block of at least 2–4 rows for adequate wind pollination. Staggering dates matters more than staggering distance — if two successions tassel simultaneously, you'll get cross-pollination between them, and that muddies the sugars this variety is specifically bred to hold.
Stop sowing by late June in most zones. NC State Extension IPM guidance notes that sweet corn planted early and harvested before July 15 avoids the worst of corn earworm migration from the south; later successions run straight into peak pressure and need more active intervention. UGA's vegetable calendar recommends a second planting within 2–3 weeks of the first and a third in May — that three-planting window gives you a solid 3–4 week spread of harvest without pushing ears into the high-earworm window.
Complete Growing Guide
Illini Xtra Sweet's exceptional sweetness comes from its shrunken-seed genetics, which means these seeds require warm soil (at least 70°F) to germinate reliably—plant after all frost danger passes and soil has warmed thoroughly. This cultivar maintains its peak sweetness for only 2-3 days after peak maturity, so succession-plant every 10-14 days for continuous harvest rather than planting all at once. Unlike standard corn varieties, supersweets are prone to cross-pollination problems that can reduce sweetness if planted near field corn or other sweet corn types, so isolate plantings by at least 250 feet or stagger bloom times. This variety shows moderate susceptibility to common corn borers and Stewart's wilt, so monitor regularly and consider preventative netting. The practical advantage: harvest ears in early morning when sugars peak, immediately refrigerate, and cook within hours for maximum sweetness that justifies the cultivation effort.
Light: Full sun (6 or more hours of direct sunlight a day). Soil: Clay, High Organic Matter, Loam (Silt). Soil pH: Acid (<6.0), Neutral (6.0-8.0). Drainage: Good Drainage. Height: 5 ft. 0 in. - 8 ft. 0 in.. Spread: 1 ft. 0 in. - 2 ft. 0 in.. Spacing: 12 inches-3 feet. Growth rate: Rapid. Maintenance: Medium. Propagation: Seed. Regions: Coastal, Mountains, Piedmont.
Harvesting
Harvest Illini Xtra Sweet when silks turn dark brown to black and ears feel full and firm when gently squeezed, with kernels that leave a milky fluid when punctured with a fingernail. The kernels should reach full plumpness covering the entire cob from base to tip, and ears typically reach 8-9 inches in length. This variety produces ears in a concentrated flush rather than over an extended period, so plan to harvest most ears within a 7-10 day window for peak sweetness. For maximum sugar content, pick ears in early morning after temperatures have cooled overnight, as the plants will have converted more starches to sugars during the cool night hours. Once harvested, use or refrigerate immediately, as supersweet varieties like Illini Xtra Sweet begin converting sugars to starch more rapidly than standard corn.
Color: Gold/Yellow. Type: Caryopsis. Length: > 3 inches. Width: 1-3 inches.
Garden value: Edible, Showy
Harvest time: Fall
Edibility: Edibile
Storage & Preservation
Illini Xtra Sweet's supersweet genetics help it maintain sweetness longer than standard varieties, but still harvest and process quickly for best quality. Store unhusked ears in the refrigerator immediately after harvest—they'll maintain good eating quality for 5-7 days versus 2-3 days for standard sweet corn.
For freezing, blanch whole kernels for 4 minutes or corn on the cob for 7-11 minutes depending on ear size, then ice bath and freeze. The high sugar content makes this variety excellent for creamed corn preparations that freeze well. You can also cut kernels from raw ears and freeze directly for cooking later. Dehydrating works well for this sweet variety—blanch briefly, cut kernels, and dry until leathery. Avoid canning due to the low acid content requiring pressure canning methods.
History & Origin
Illini Xtra Sweet emerged from the University of Illinois sweet corn breeding program, which became renowned for developing supersweet varieties during the latter half of the twentieth century. The variety represents the institution's commitment to advancing sh2 (shrunken-2) gene technology, which produces the exceptionally high sugar content characteristic of supersweet corn. While detailed records of the specific cross and exact introduction year remain limited in widely available sources, the "Illini" designation clearly anchors this cultivar within the university's broader legacy of corn improvement. This lineage connects it to a prestigious tradition of Midwestern breeding that prioritized flavor intensity and kernel tenderness for home gardeners.
Origin: Mexico
Advantages
- +Exceptional sweetness and tenderness make Illini Xtra Sweet a premium eating experience.
- +Extended quality retention means harvested ears stay delicious longer than standard varieties.
- +85-90 day maturity provides reliable, consistent production for most growing regions.
- +Proven track record as a beloved heirloom variety trusted by experienced gardeners.
Considerations
- -Vulnerable to multiple serious diseases including northern corn leaf blight and rust.
- -Susceptible to multiple pest pressures including corn earworm and European corn borer.
- -Moderate difficulty level requires proper growing techniques and pest management attention.
- -Disease resistance limitations may necessitate fungicide applications in humid climates.
Companion Plants
Bush beans and pole beans are the most useful neighbors — they fix nitrogen, and Illini Xtra Sweet is a heavy feeder that can pull close to 1 lb of actual nitrogen per 1,000 sq ft over a season. Winter squash planted beneath the stalks shades out weeds and holds soil moisture without competing for vertical light. Marigolds (Tagetes spp.) and nasturtiums attract predatory wasps and other beneficials that keep aphid and corn earworm populations from building unchecked. Keep fennel well away — it's broadly allelopathic and will stunt whatever is planted near it. Tomatoes are a bad pairing too; they share corn earworm (Helicoverpa zea) as a common pest, and putting the two crops side by side just gives that insect a bigger buffet.
Plant Together
Bush Beans
Fixes nitrogen in soil that corn can utilize, classic Three Sisters companion
Winter Squash
Large leaves suppress weeds and retain soil moisture, completes Three Sisters planting
Pole Beans
Uses corn stalks as natural support while providing nitrogen fixation
Marigolds
Repels corn earworms, aphids, and other harmful insects with natural compounds
Nasturtiums
Acts as trap crop for aphids and cucumber beetles, attracts beneficial predatory insects
Sunflowers
Attracts beneficial insects and birds that control corn pests
Dill
Attracts beneficial wasps and ladybugs that prey on corn aphids and other pests
Sweet Alyssum
Attracts beneficial insects like hoverflies that control aphids and other corn pests
Keep Apart
Black Walnut
Releases juglone toxin that severely stunts corn growth and can kill plants
Tomatoes
Both attract corn earworms and share similar pest problems, increasing infestation risk
Fennel
Releases allelopathic compounds that inhibit corn germination and growth
Nutrition Facts
Per 100g serving. % Daily Value based on 2,000 calorie diet. Source: USDA FoodData Central (FDC #168538)
Pests & Disease Resistance
Resistance
Moderate resistance to northern corn leaf blight and common rust
Common Pests
Corn earworm, European corn borer, corn flea beetle, wireworms
Diseases
Northern corn leaf blight, southern corn rust, gray leaf spot, crazy top
Troubleshooting Illini Xtra Sweet
What you'll see, why it happens, and what to do about it.
Silks turning brown and chewed, kernels missing or damaged at the tip of the ear at harvest
Likely Causes
- Corn earworm (Helicoverpa zea) — larvae follow the silks straight into the ear tip
- European corn borer (Ostrinia nubilalis) — secondary tunneling into the ear
What to Do
- 1.Apply a few drops of mineral oil to the silks 3–5 days after they emerge — this suffocates young larvae before they burrow in
- 2.NC State Extension IPM guidance recommends timing early plantings so ears are harvested before July 15, which cuts earworm pressure significantly
- 3.At harvest, just cut off the damaged tip; the rest of the ear is fine to eat
Long, tan or gray lesions running parallel to leaf veins on upper leaves, appearing mid-season
Likely Causes
- Northern corn leaf blight (Exserohilum turcicum) — favored by cool nights in the 65–80°F range and sustained wet or humid conditions
- Gray leaf spot (Cercospora zeae-maydis) — similar lesion shape but more rectangular; thrives in high humidity and poor airflow from dense plantings
What to Do
- 1.Strip infected leaves off the lower stalk once you spot lesions — trash them, don't compost
- 2.Space plants the full 8–10 inches and plant in blocks of at least 4 rows rather than a single long row; the airflow difference is real and slows both pathogens
- 3.Rotate this bed out of corn for at least 2 seasons — both pathogens overwinter in crop debris
Seedling leaves peppered with small round holes, or ragged and chewed, in the first 2–3 weeks after germination
Likely Causes
- Corn flea beetle (Chaetocnema pulicaria) — tiny black beetles that jump when disturbed; pressure is worst in warm, dry springs
- Wireworms (Agriotes spp.) — attack seeds and roots below ground, causing patchy or uneven germination in the same bed
What to Do
- 1.Cover newly direct-sown beds with row cover until plants reach 6 inches — established corn is much less attractive to flea beetles
- 2.If germination is patchy and you find hollowed-out seeds, avoid replanting corn in any bed that was in sod or grass the prior season; wireworm populations peak in recently converted turf
- 3.For heavy flea beetle pressure, a kaolin clay spray on seedling leaves gives physical deterrence without harming beneficial insects
Frequently Asked Questions
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Growing Guides from Wind River Greens
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Sources & References
External authority sources used in compiling this guide.
See the Methodology page for how this data is sourced, what's AI-assisted, and known limitations.