University of Illinois Extension offices always know when spring is on the way based on kind of questions emailed to our Master Gardener Help Desk. As the weather warms up, the messages increase, and here are two common questions this time of year:

Q: How early is too early to start vegetables indoors for planting outdoors later?

A: I hate to start an answer with “it depends,” but in this case, it’s true. You have to consider the kind of vegetables you are going to grow. When to start the seed revolves around that somewhat mysterious average frost-free date (see more on that below in FAQ No. 2). Vegetables are often classified as cool season or warm season, but they also fall into four categories by how much bad weather they can tolerate:

• Very hardy – those that can survive freezing temperatures. This would include onions, peas, potatoes, and broccoli.
• Frost tolerant – those that would be killed by a freeze yet survive frosty weather. Good examples may be beets, cabbage and cauliflower.
• Tender – those that are planted on that average frost-free date but would need protection from frosty weather. These include tomatoes, snap beans, sweet corn, and some squash.
• Warm loving – those that absolutely need warm soil and air temperatures. This last category is for peppers, cucumbers, melons, other squash, and pumpkins.

When you start the seeds depends on when you expect to transplant the vegetable plants outdoors. The seed packet is typically going to suggest indoor planting about 4 to 6 weeks ahead of the outdoor planting date. Find more information on when to plant where you live in Illinois.

Q: What is the difference between the “average” frost-free date and the “absolute” frost-free date?

A: Average frost-free dates are a collection of recorded weather data over decades to suggest that on a particular range of dates, there still is a 50/50 chance of a heavy frost or light freeze. The absolute frost-free dates are typically considered to be two weeks later. In Illinois, that average frost-free date can be as early as April 5 in southern Illinois and as late as May 5 in far northern Illinois. For DuPage, Kane and Kendall counties, our range is April 30 through May 5. DuPage gets a bit of a head start being closer to Lake Michigan and the northwest corner of Kane is closer to the May 5 date. You now may see why those gardeners who don’t want to gamble will wait until the middle or end of May! Find a handy chart by vegetable and section of the state.

Other Considerations

Experienced gardeners often know the unique characteristics of their yard and can better gauge when sowing vegetable seeds or setting out transplants works best. For instance, is the soil a dark color that will warm up sooner that a lighter soil? Is the garden going in on the south or west side of the yard where the sun warms it earlier? Does the home shade the garden spot, delaying the time of sowing or planting? One of the biggest influencers will be the favorite family vegetables. If no one likes cabbage rolls, then gardening in April may be off the table.

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About the author: Richard Hentschel’s expertise extends across several subject areas with specialties in lawn care, fruit tree production, woody ornamentals, and home and community gardening. During his 45-year career in horticulture and agriculture, Hentschel became a well-known and respected expert for commercial and homeowner audiences, industry organizations, and media. He retired from University of Illinois Extension in April 2022 with nearly 30 years of service as a Horticulture Specialist and Educator in northern Illinois.