Proper planting dates for crops and gardens can be challenging

Every spring it is inevitable we will get a week of above normal temperatures followed by cooler temperatures. This warm spell always gets everyone in the mood to work in fields, gardens, and landscapes (myself included), but we need to remember the calendar date, what temperatures lay ahead, and the crop growth requirements. Let’s look at these in more detail.

Calendar date: In Knox County the average spring frost free date varies slightly between the northern and southern half of the county. In the north those dates are between April 16-25 and in the south those dares are April 6-15. After these dates there is a 50% chance of frost, and two weeks later that chance decreases to 10%. This is why we typically recommend May planting dates, especially for the warm season crops.

What temperatures lay ahead: At the time of writing this article, the National Weather Service is projecting evening air low temperatures for the next seven days that range from 38-45 degrees. It’s also optimal to look at 8-14 day outlook. On the evenings that are predicted to get around 38 degrees there is generally a potential of frost in low lying areas. So, if you have warm season plants that have emerged they could suffer some injury on those cold nights unless you can cover them or bring them inside.

Soil temperatures: Soil temperatures this time of year are critical for seed germination and emergence. Typically, we recommend soil temperatures to be 50 degrees at the seed planting depth. One problem we can have this time of year is those soil temperatures can go well above this 50 degree mark, but then swing down below that. You can check local soil temperature data at iclimate.org.

Crop growth requirements: Some of our plants, such as onion, broccoli, cabbage, lettuce, Brussels sprouts, and many other cool season crops, can tolerate lights frosts and can be planted as early as mid-March. Crops such as tomato, green bean, soybean, squash, corn and other warm season crops will likely be injured if we get a frost, so waiting to plant those until May or June is typically best. If you are planting seed many need to accumulate a certain number of growing degree days to germinate and grow. This time of year we are slow in getting those good warm days to accumulate growing degree days so this may slow the rate that seed germinates and could make it more susceptible to disease and hurt its overall health.

The take home message today is to make sure you understand what the plants you are planting in gardens or fields and what they can withstand. It is important to check what air and soil temperatures are so you have successful germination and vigorous plants. You also need to evaluate what risk you are willing to take if you want to plant early.

For additional information and resources visit https://extension.entm.purdue.edu/newsletters/pestandcrop/ or https://tinyurl.com/PurdueVegPlantCalendar or contact the Purdue Extension-Knox County office by calling 812-882-3509 or by emailing clingerman@purdue.edu.

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