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

Indiana’s growing regions

Indiana is a mid-continent state with no mountains to block weather. NOAA’s state climate summary describes bitter Arctic air in winter and humid Gulf air in summer. Average annual temperature differs by about 10°F from the Ohio River to the Michigan line, and precipitation does too, from about 47 inches a year in the south to about 37 in the north.

The state falls into three broad gardening areas:

• Northwestern Indiana near Lake Michigan. The lake gives this area cooler summers and milder winters. It also drives lake-effect snow as far inland as Elkhart.
• Central Indiana. Purdue Extension’s Home Gardener’s Guide uses Indianapolis as the baseline for its planting table.
• Southwestern and northeastern Indiana. The same guide says to plant about 20 days earlier than the Indianapolis dates in the far southwest and about 10 days later in the northeast. The latest sensible planting dates are about the same statewide.

Purdue’s Indiana Vegetable Planting Calendar takes a different approach. It skips the fixed offsets and has you time crops by your local spring and fall frost dates. Its frost maps show dates with only a 1-in-10 chance of frost afterward, and about two weeks earlier the odds are roughly even. Check the frost dates for your city above, then decide how much risk you’re willing to take.

What limits Indiana gardeners

Cold, wet soil in spring. This is the main limit Purdue stresses. Many summer crops won’t grow well until the soil warms. In years when cool air and rain keep the ground cold, the calendar says to hold off on beans, tomatoes, squash, sweet potatoes and sweet corn until the soil reaches at least 60°F. Planting sooner invites seed rot, root rot and related problems.

NOAA notes that spring and summer rainfall has generally run above average since the 1990s. That has meant both good soil moisture and planting delays. The state has also had periodic floods and droughts.

Late frosts. The Home Gardener’s Guide recommends covering tomatoes, peppers and eggplant on frosty nights with cardboard boxes, blankets, hot caps, row covers or newspaper tents. Keep covers off the leaves and remove them in the morning. Don’t use metal or plastic sheets, because they conduct cold to the plants.

Soil and site. Purdue advises avoiding heavy clay, very sandy soil and shade. If your soil is poor, work in 2 to 3 inches of topsoil, compost, strawy manure, peat or leaves. Black walnut is a particular Indiana hazard. Its roots release juglone, which is toxic to tomatoes and other sensitive plants, so Purdue recommends keeping gardens at least 50 feet from walnut trees.

What Purdue Extension recommends

The Indiana Vegetable Planting Calendar (HO-186) groups crops by how much cold they can take:

• Hardy crops. Plant four to six weeks before the last spring frost. These include asparagus, broccoli, cabbage, kale, onions, peas, potatoes, radishes, spinach and turnips.
• Semi-hardy crops. Plant two to four weeks before. These include beets, carrots, cauliflower, chard, lettuce and parsnips.
• Tender crops. Beans, tomatoes and sweet corn go in after the average last frost, once air temperatures stay at 50°F or warmer.
• Very tender crops. Cucumbers, eggplant, melons, okra, peppers, pumpkins, squash and sweet potatoes go in at least two weeks after the average last frost, when air temperatures stay at 60 to 65°F or warmer.

Spreading several sowings across the planting window improves your odds and stretches the harvest. If you plant only once, aim for the middle of the range.

Experienced gardeners can warm the soil early with black or clear plastic mulch, which lets heat-loving seeds go in ahead of the usual dates. Be ready to protect seedlings from a late frost. The Home Gardener’s Guide adds that black plastic suits tomatoes, eggplant, peppers and vine crops. Organic mulches such as straw, shredded bark or compost keep soil cooler and are best applied in late May or early June, once the soil has warmed. In rainy seasons or wet spots, any mulch can keep the soil too damp.

For fall, plant cool-season crops at least four to eight weeks before the first fall frost. Expect slower growth for anything sown after September 1, and choose faster-maturing varieties. Late-summer soil is often too dry for good germination, so water often until seedlings are established. During the season, give the garden 1 to 1½ inches of water a week in one thorough watering unless rain does it for you.

Soil pH and testing

Purdue’s guide says most vegetables do best at pH 6.0 to 6.8. Most Indiana soils already fall in that range or run more alkaline, so lime is rarely needed and can upset plant nutrition. Sulfur lowers pH for crops that need more acidic soil, such as potatoes.

Purdue does not test soil for homeowners. Purdue Extension in Marion County recommends a commercial lab over a home kit, a basic test for phosphorus, potassium, pH and organic matter, and retesting every three to five years. Testing in autumn gives you time to get results and buy amendments before spring.