Gardening in Iowa: zones, frost dates and when to plant

North, central and south Iowa

Iowa State University Extension and Outreach builds its planting chart, Vegetable Planting and Harvesting Times, around central Iowa. Southern Iowa can move sowing, planting and harvest dates about a week earlier, and northern Iowa about a week later. Within those regions, use the frost dates for your city above to fine-tune.

NOAA’s state climate summary puts annual precipitation at about 38 inches in southeastern Iowa and only about 26 inches in the northwest. Much of the year’s rain falls in summer. Because no mountains shelter the state, Arctic air sweeps in during winter and humid Gulf air settles in during summer, which gives Iowa its wide temperature swings.

What limits Iowa gardeners

Wet springs. NOAA notes that spring rainfall has run above average since 1990, and that the April-to-June planting season has been especially wet in recent years. Saturated spring soil can make it hard to plant on time.

Heat and humidity. Intense heat waves can hit Iowa, often with high humidity. Summer heat makes cool-season crops such as broccoli, cauliflower and lettuce bolt or turn bitter. Tomatoes suffer too: Iowa State’s tomato guide explains that red pigment doesn’t form well above 90°F, so red tomatoes ripened in hot spells often come out yellow-orange. Overhead watering splashes soil onto leaves and spreads diseases such as septoria leaf spot.

Floods and drought. NOAA describes repeated floods along Iowa’s rivers and occasional serious summer droughts, including very dry Julys and Augusts in 2011 through 2013. Thunderstorms can bring hail.

Iowa State Extension’s planting advice

The Vegetable Planting and Harvesting Times chart gives a range of safe sowing or transplanting dates for each crop. Some crops can be sown every couple of weeks within their range for a steady harvest. Others need only one planting somewhere in the window.

Crop guides on Iowa State’s Yard and Garden site add Iowa-specific detail:

• Tomatoes. Transplant once frost danger has passed, typically early to mid-May. June 20 is the last practical date for a good yield. Set plants deep, up to the first true leaves, because roots form along the buried stem. Aim for 1 inch of water a week applied to the soil, not the leaves. Mulch with 2 to 3 inches of straw, shredded leaves, or grass clippings from lawns not recently sprayed with broadleaf herbicide. Iowa State considers plastic mulch not cost-effective for home gardens.
• Sweet potatoes. Often thought of as a southern crop, they do well in Iowa’s hot, humid summers. Plant slips in mid-to-late May or early June after frost danger, 12 inches apart in rows 36 to 48 inches apart. They need well-drained sandy or loamy soil, because heavy clay produces misshapen roots. Recommended cultivars for Iowa are Beauregard, Centennial, Covington, Evangeline, Georgia Jet and Jewell. Dig them before the soil cools below 60°F.

Plan a fall garden

Iowa State’s Fall Planting Vegetables guide presents a fall garden as a way to extend the harvest or make up for spring setbacks. Count back from the first fall frost: add the crop’s days to maturity plus extra time for slower growth in cool weather and for harvest. Planting too late risks a hard freeze before harvest, while planting too early exposes cool-season crops to summer heat.

The guide groups fall crops by cold tolerance:

• Tender crops. Green beans, cucumbers, okra and zucchini can’t take frost but mature fast enough to sow in midsummer.
• Semi-hardy crops. Beets, carrots, leaf lettuce, kohlrabi, radishes, spinach and Swiss chard tolerate the upper 20s°F.
• Hardy crops. Brussels sprouts, broccoli, cabbage, cauliflower, kale, rutabagas and turnips handle the mid to low 20s°F and are often picked well after the first frost, some into November.

Start broccoli, cauliflower and Brussels sprouts indoors six to eight weeks ahead and set them out in mid to late August. Hot summer soil can keep seeds from sprouting. A light mulch cools the soil, and roughing up the surface helps seedlings break through.

Soil testing in Iowa

Iowa State University does not run soil tests for home gardens. Its Extension recommends university labs in Minnesota, Wisconsin and Michigan, or private labs that give lawn and garden recommendations. Avoid labs that report only field-crop recommendations.

To sample, mix soil from 10 to 15 spots taken 4 to 8 inches deep. Leave out the surface layer, mulch and compost. Sample in fall or early spring, when the soil isn’t frozen, soggy or bone-dry, and not right after you’ve added lime, fertilizer or manure. Retest every three to five years, or whenever plants struggle. Iowa State considers home test kits much less accurate than a lab analysis.