K-State Research and Extension Douglas County

Ready for Radishes

Niki K., Agriculture and Horticulture Program Assistant

Officially, spring may only be days away, but deciding when to plant cool season vegetable seeds or when to move out transplants depends on more than just a date on the calendar. It is important to consider the date of last frost for the area, soil temperature of the intended garden and to be aware of any upcoming extreme weather as these can all impact those early season plantings.

hand holding radishes covered in dirt

Many cool season crops can tolerate a frost or light freeze. According to the K-State Research and Extenion's Kansas Garden Guide there are many vegetables that can tolerate frost, including, asparagus, peas, radish, spinach, arugula and most of the plants in the brassica family, such as broccoli, cabbage, and kale.

According to the National Weather Service, the average last frost in Douglas County, Kansas, from 2000-2024 period ranges from April 10th to April 17th. Reports from the Lawrence, Kansas Airport shows the earliest last frost on March, 29th, 2010 and the latest last frost occurring on May 16th, 2014, so make sure to keep an eye on the forecasts as weather is quite variable and covering plants is advised if a hard freeze is expected.

Soil temperature is another important factor to consider when determining when to get those seeds or transplants into the ground. Soil temperature is different than air temperature and is a better indicator of readiness for those cool season seeds and transplants. Air temperature can fluctuate greatly throughout a day, whereas soil temps are slower to rise. The K-State mesonet site provides a soil temperature map detailing soil temperatures across the state at depths of both 2 and 4 inches. https://mesonet.k-state.edu/agriculture/soiltemp/

A metal soil thermometer can be used to determine the soil temperature more accurately in any garden space and can be purchased at local garden centers or hardware stores. When taking a reading using a soil thermometer, collect and document the soil temperature in late morning for a few days at a depth of 2.5 inches.

Many gardeners look forward to those tender snow peas and crunchy radishes as the temperatures finally warm each spring. By watching local conditions and tracking soil temperatures, gardeners should enjoy the spring bounty before the warmer temps settle in.