Some of our earliest vegetables can be sown as soon as you can carefully work the garden soil and once soil temperatures reach 45 and 50 degrees. You can place spinach and lettuce in the 45-degree group, and peas, cabbage, Swiss chard, radish, and beets in the 50-degree group. Other vegetables can go out as well, and they are planted as a root and will be protected by the soil, such as asparagus, onion sets, potatoes and rhubarb. From there, the garden soil continues to warm until we hit the upper limit for good germination, and the remaining vegetables can be sown or transplanted.
The days to germination can be as short as three to five days upwards of 10 to 15 days. Root crops can be some of the slowest, so do not panic. One garden trick for rows that will be slow to show is to plant a crop like radish with it to "mark the row." Radish being a short crop anyway will be long gone and will not interfere with the other vegetable sown.
We consider those very early crops to be very hardy and able to survive very cold temperatures, planted in a range of four to six weeks ahead of the average frost-free date. As the soils continue to warm, the next group known as frost tolerant (not able to actually be frozen) can go out from seed or transplants between two and three weeks ahead of the average frost-free date. Beet, carrot, radish, and summer squash are examples for this group.
The second to last group, known as tender, is planted on the average frost-free date. These crops will need to be protected if we get a late frost or a threat of low temperatures. The last is the warm-loving vegetables that absolutely need warm soils without any threat of cool temperatures. They are planted out as seeds or transplants two weeks after the average frost-free date. This last group to think of is the vine crops, mainly cucumber, pumpkin, muskmelon and watermelon.
In the last two groups, there some vegetables that we think of planting together, yet could be handled differently. Most gardeners will plant tomatoes and peppers at the same time. We lose one to two weeks on our tomatoes (a tender vegetable) if planted as a warm-loving vegetable or run the risk of seeing our peppers (warm-loving) damaged if planted at the same time as our tomatoes. The same goes for summer squashes and the rest of the vine crops. Summer squash falls into the tender group, while the other squashed require that warmer soil.
The hard part of scheduling your sowing or transplanting is deciding for your yard what you will be using as the average frost-free date, since all planting dates are determined from there. The best I can offer is a range for each of the four groups:
- Very Hardy – between April 10-25
- Frost Tolerant – between April 25-May 10
- Tender – between May 10-25
- Warm Loving – between May 25-June 1
Last point is there is a difference between the "average frost-free date" and the "absolute frost-free date." Be careful as you read seed packet labels or the tags with vegetable transplants, and sow or plant accordingly.
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.