Soil temperature is the key to planting time

Cool season crops such as lettuce, spinach, peas, onions, potatoes, radishes, cabbage, carrots, and beets are the first vegetables that can be planted this spring. Is it safe to plant them now in your garden? The following information will help you be sure.

Soil temperature is the key

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The temperature of your garden soil is one of the most important pieces of information you can have as a Kansas gardener. By tracking soil temperature you’ll know exactly when it is safe to start planting your garden.

Cool season crops are cool season because they can germinate and grow well in cooler soils, and must be grown before the summer heat arrives.  The crops mentioned above are able to germinate and grow in a soil temperature of 45 degrees.  Right now soils in our area are averaging around 42 to 43 degrees.

Cool season seed can be planted a little before the optimum temperature of 45 is reached, and depending on the crop some may even germinate early.   The seeds that do not germinate will simply sit in the ground until the temperatures warm to the correct levels and then proceed to grow.

Warm season crops are not this forgiving.  Seeds or transplants of crops such as cucumber, squash, watermelon, tomato, pepper, eggplant or other warm season vegetables should not be planted into the ground until soil temperatures reach at least 55 to 60 degrees.  Warm season seed will not ‘sit’ and wait for correct temperatures.  Seed will rot and fail to grow if planted too soon. Likewise, transplants of warm season crops such as peppers may be set back for the entire season if they are planted into soil that is too cold.

Track soil temperatures

soil thermometer

One way to track soil temperatures in your garden is with a soil thermometer.  A soil thermometer can be purchased through many garden suppliers and I’ve seen several in the $15-$20 dollar price range that will do the job just fine.

Take soil temperatures at about 2-3 inches deep at 10:00 to 11:00 am to get a good average soil reading for your garden.  Take the temperature for 2-3 days in a row and if temperature is consistent at the desired temperatures (45̊ for cool crops, 55̊- 60̊ for warm crops) it is safe to plant the seeds.

Another way to monitor soil temperature that is not quite as precise for your particular soil, but is still helpful is the Kansas Mesonet soil temperature webpage for current readings from around the state.  This is a good and quick way to keep track of where soil temperatures are in general around the state. 

A snapshot from the KSU Mesonet Soil Temperature Page

Protect young plants if needed

Don’t forget that we live in Kansas, and even though soil temperature is a great clue for proper planting time, we still face the uncertainties of late spring freezes that can arrive even as late as the end of April that can injure plants.  Providing some protection for less hardy plants might be necessary if a cold snap is forecast.