Soil Temperature & Garden Plants

Iowa Backyard Farmer

This up and down weather has been crazy!?!?  Shorts on Monday, snow on Sunday!  I think last year might have been even a little worse...  This 2023 weather app screenshot from Iowa Backyard Farmer really shows how crazy... and this year seems to behaving similarly.

It just goes to show that we need to resist the desire to rush into planting our gardens and pay attention to those last frost date charts and old-time gardening practices.

"Potatoes and onions after Easter"
"Tomatoes after Mother's Day"
"Sweet Potatoes after Memorial Day or Father's Day" 

                                            -depending on where you grew up

Corn farmers pay close attention to soil temperatures when they are making planting decisions.  Their rule of thumb is to wait until the top several inches of soil are at least 50° F and even then, with a warming trend in the forecast.

For gardeners who love to grow tomatoes, we need to wait even longer - ideally 60°F or more for those big, juicy red fruit.  Soil temps around 50° or below can stunt tomato plants, and even after it warms up those stunted plants will struggle to keep up with those planted later in warmer soils.

So how do we know what the soil temperature is?  The Iowa Environmental Mesonet (IEM) collects temperatures throughout its observation network. This data is made available especially for farmers on their website, but we can use it too!

IEM - 4 Inch Soil Temperature Maps

Their webpage has soil temperature maps for the past few days. The maps combine observations from the Iowa State University Soil Moisture Network and corrected National Weather Service forecast models to produce a high-resolution map of soil temperatures.

Recently they have added forecasted temps as well, like this one for One Week out-

The forecasted soil temp maps will really help you plan your gardening activities.

"Should I get the starts ready to go, or can I sleep in Saturday and 

let them grow on the seed starting rack a few more days?"

Watch how I built my Seed Starting Rack HERE

or

Get your plants at the Iowa Backyard Farmer

This web tool is great for planning, but actually taking the soil’s temperature with a thermometer is a more accurate way to know when to plant.  Also, seeds planted in soil that's too cold won't germinate and just sit there, they may even rot or be eaten.  At the optimum temperature seedlings emerge quickly and grow into bigger, stronger and healthier plants.

You can buy a soil thermometer like this one on Amazon for about $10-15

or look for one at your favorite Garden Center.

Besides tomatoes, many other warm season crops are dependent on soil temperature for growth and health.  This chart from the University of Wisconsin Extension Service shows optimal growing temperatures for many common garden vegetables.  This handy chart along with the above soil temperature maps can help you plan the best targets for planting your favorite crops.  Then just confirm with your soil thermometer and plant away!

https://kenosha.extension.wisc.edu/files/2021/03/Soil-Temp-Planting-V2docx.pdf

Wishing you a great and productive gardening season!