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

Three Illinois gardens in one state

NOAA’s state climate summary notes that Illinois has no mountain barrier, so Arctic air pours in during winter and humid Gulf air settles in during summer. Average annual temperature differs by about 10°F from north to south. Rainfall follows the same gradient: more than 50 inches a year in the far south, under 35 inches in the north. Snowfall runs the opposite way and is heaviest in the northeast. Along the lakeshore in northeastern Illinois, Lake Michigan keeps summers cooler and winters milder.

University of Illinois Extension’s When to Plant in Illinois gives separate planting tables for northern, central and southern Illinois, and the differences are large:

• Northern Illinois has the shortest season, about 180 days in the far north. Tomato transplanting in the northern table starts close to a month later than in the southern table, and fall garlic goes in around mid-September.
• Central Illinois falls in between. Garlic is planted in October.
• Southern Illinois has the longest season, around 215 days at the southern tip. Potatoes and tomatoes go in earliest here, and garlic waits until late October.
• The Chicago area. Because of the urban heat effect, the metro area runs warmer than the rest of northern Illinois. Extension says its planting dates may be closer to central Illinois.

The sources differ slightly on how far apart the regions are. A central Illinois chart in Extension’s Good Growing blog suggests planting about two weeks earlier in the south and two weeks later in the north. The regional tables in When to Plant in Illinois show larger gaps for some crops. Use your own region’s table and the frost dates for your city above.

What holds Illinois gardeners back

Cold, wet springs. NOAA reports that spring and summer rainfall has run above average since the mid-1990s. Wet springs help soil moisture but can delay planting, and heavy downpours of 2 inches or more have become much more frequent.

Warm crops planted too early. The Good Growing warm-season article splits summer crops into two groups. Tender crops such as snap beans and sweet corn can take cool weather but not frost. Very tender crops such as tomatoes, peppers, cucumbers, squash, pumpkins and watermelons can also be injured by cool weather and barely grow in cool soil. For the very tender group, wait until days reach the 70s and nights stay above 50°F, or protect plants with floating row covers.

Soil drainage. Extension’s soil management article points out that clay holds water well but can stay waterlogged, while sand drains fast and dries out. Find out how long your garden stays wet after a heavy rain before deciding where to put beds.

What Illinois Extension recommends

The Cool-Season Vegetables fact sheet sorts spring crops by hardiness:

• Hardy crops can go in as soon as the ground can be worked, typically early March to early April. Examples are broccoli, cabbage, kale, kohlrabi, leeks, onions, peas, spinach and turnips.
• Half-hardy crops usually go in from late March to early May, once soil is above 40°F. Examples are beets, carrots, cauliflower, leaf lettuce, chard, radishes and potatoes.
• Fall crops. Many of both groups can be planted again from mid-July to early September. Garlic is best planted in fall.

Other tips from the fact sheet:

• Use transplants for most cabbage-family crops, because they establish faster and mature sooner.
• Sow leafy greens every two to three weeks for a longer harvest.
• Water deeply, at least 1 inch a week, and mulch with shredded leaves or seed-free straw.

For warm-season crops, the Good Growing article recommends hybrid tomatoes for beginners because they usually resist disease better than heirlooms, and notes that cherry types tend to have fewer disease problems.

To push the season at either end, Extension’s season-extension article describes cloches, cold frames and low tunnels. Use spun row cover as nights start to approach freezing in fall, then add greenhouse plastic as it gets colder. Row cover also keeps early squash bugs and squash vine borers off young plants until flowering. Small cloches don’t hold enough heat to carry crops through a typical Illinois winter.

Soil testing and fertility

Illinois Extension suggests testing garden soil about every three years. Most vegetables do best between pH 6.0 and 7.0, with 6.5 ideal. Lime raises pH and sulfur lowers it, but either can take a year or more to have an effect. Nitrogen isn’t part of a basic test because it changes so quickly in the soil. Plan on extra nitrogen for sweet corn, cabbage-family crops and leafy greens.

If your soil tests high in phosphorus or potassium, skip those nutrients. Calcium, magnesium and boron are not known to be short in most Illinois soils, so add them only if a test or a problem points to them. Your local Illinois Extension office can help with sampling and with reading your results.