Growing Your Best Garden Yet: Why Planting at the Right Soil Temperature Matters

Growing Your Best Garden Yet: Why Planting at the Right Soil Temperature Matters

The calendar says we're behind with our planting schedule this year! The soil temperature tells a different story. I've been testing the soil temps in the garden. Here's why!

Remember last year?  We were in a similar situation. It just wasn't warming up. But the calendar said it was time to plant! We waited well past our normal planting dates and started to get nervous.

And so we planted! The peas came up fine-- not great but decent. After they came up I did poke in some additional seeds where it was a little thin to ensure the stand we wanted.

But beans were another story. The bean stands were disappointing at best. We replanted last year! You too? 😖

Here's the lesson-- planting at the right soil temperature is crucial for the success. The calendar does NOT dictate this detail! Ignoring soil temps with both eyes on the calendar will bite ya! Planting too early in soils that are too cold can have disastrous effects on both growth and the size of the harvest.

First and most important, planting at the correct soil temperature makes sure the seeds have the best conditions for germination. Different plants have different optimal soil temperatures for germination. Some germinate better in cools soils-- like radish, lettuce, spinach, onions, carrots, and peas. Some need warmer soils-- like peppers, tomatoes, beans, cucumbers and pumpkins.

Bean seedling growing at Prairie Road Organic Seed

Knowing those minimum soils temperatures for seed germination will help you know where the threshold for planting is-- not based on the calendar but the actual soils you're planting into.

Minimum soil temperatures for planting

(Pathak et al., 2012)

But minimum temperature doesn't mean ideal. Read on!

Measure to KNOW for sure! 

With our erratic spring conditions the past few years, it's important to know what's going on in order to make the right planting decisions. One strategy is to employ your meat thermometer and use it as a soil thermometer. (Or buy an inexpensive soil thermometer.)

Insert the thermometer in the soil 1-2 inches deep. Leave it inserted for a minute or two to get an accurate reading. Take a reading in the morning, another in the heat of the day, and calculate the average.

If you have multiple plantings areas — especially if some areas have more sun exposure than others — don’t assume that they'll be the same temperature. Or if the slope is different, so the angle of the sun heats one spot of the garden more than another.  Record the temperature of each so you gain an understanding of what's happening with those soil temperatures.

The minimum temp for germination doesn't mean it's the optimal temp. Download our Start Charts complete with 'Ideal Soil Temps' for each crop. I created these charts in Excel for those of you using a Windows PC and in Numbers for you Mac users.  Choose your spreadsheet fav!

After downloading the spreadsheet, look for the column on the right half of the page titled, 'Ideal Soil Temperature in ℉'. This is your target soil temperature range for each crop at planting.

Why does it matter SO MUCH! 

Soil temperature impacts seed germination by affecting the metabolic activity inside the seed as it's working to sprout. If the soil temperature is too low, the seeds may not germinate at all! Or they may germinate real slowly, causing stunted growth and reduced production. 

Warmer temperatures speed up chemical reactions and, conversely, cooler temperatures slow them down. Those chemical reactions help break down the protective seed coat and tell the seed that it’s time to wake up and start growing.  

On the other hand, if the soil temperature is too high, it can also cause the seeds to shut down. Lettuce seed will do that-- it's like the seeds have an internal heat protector that tells it it's too hot-- don't grow! The seed will just sit and wait for cooler temps. Worse yet-- excessive soil temps can cause the seeds to literally dry out and die. 

Also, pay attention to long range forecasts and their potential impact on soil temperatures. For instance, a cold rain could dramatically affect soil temps. If this is in the forecast, you may want to hold off on planting those beans, for instance, until it has passed. 

How temperature affects disease and pest pressure 

Proper soil temperatures also helps to prevent pest and disease problems in the garden. Anytime plants are stressed due to poor growing conditions, they're more susceptible. 
For example, if the soil temperature is too low, it can encourage fungal diseases, like damping-off.

If the soil temperature is too high and the plant is stressing, it can attract pest insects such as spider mites and aphids. By planting at the best soil temperature, you can reduce the risk of pest and disease problems. 

Green beans in a row at Prairie Road Organic Seed

Factors affecting soil temperatures 

In addition to seasonal conditions, soil temperature can be affected by a variety of factors-- the time of year, the location of the garden, the slope of the land in relationship to the position of the sun, soil moisture or lack of, ground cover, and the type of soil.  

Monitoring the soil temperature throughout the season and taking steps to maintain it with strategies like using mulch, landscape fabric or row covers, can help increase overall plant health, extend the growing season, and maximize production. 

Happy Planting! 

Our best to you,