The Importance of Soil Temperature in Your Lawn

The Soil Temperature in your lawn, not calendar dates, is what should be used to determine your lawn care schedule.

If it’s one question I wish I heard more from Lawn DIY’s it would be… When? Most questions are usually ‘what product should I use’, or ‘how to apply this product?’ Those are all great questions, but if you don’t have the ‘when’ figured out you’re just wasting time and money.

Put yourself to the test and ask yourself this one simple question: ‘How do I determine when I fertilize my lawn?’

I bet for a lot of DIY’s the answer is simple. When you get a free weekend. Or maybe it’s when you get the ‘bug’ or get the time to actually go to the store and purchase product.

You may know that you should apply fertilizer sometime in early spring. You may even know that a pre-emergent herbicide should get put down around then too. But that’s all the thought that goes into it, the rest is really based on convenience.

The problem is, your lawn doesn’t care about calendar dates, and it certainly doesn’t care about when it’s convenient for you to apply treatments. When it’s time for weeds to germinate, weeds will germinate. When it’s time that your lawn needs to be fertilized, it needs to be fertilized.

So what is the driving force behind the ‘when’?…. Soil Temperature!!!

That’s right, soil temperature is what we need to focus on to know when we need to be doing the what to our lawns. (that sentence sounds ridiculous but I think it makes sense if you read it twice).

Instead of calendar dates, let’s take a look at the most important soil temperature thresholds and what’s happening to your lawn when soil temps hit those thresholds.

Since cool season lawns and warm season lawns have slightly different things happening at different soil temperatures, this post is divided into a Cool Season section and a Warm Season section.

Also you may be wondering how to get soil temperature data. This will be covered at the bottom of the post where I discuss the 2 simple tools I use to obtain soil temperature data for my lawn.

Important Cool Season Lawn Soil Temperatures

55 Degrees

Think of 55 degrees as being go time. This is when cool season grasses start to wake up in the spring. It’s also when broadleaf weeds such as crabgrass begin to germinate. Since 55 degrees is go time, you want to time your first fertilizer application, as well as pre emergent application so that these products are already in the soil by the time it hits 55 degrees.

To achieve this, you need to apply your pre emergent and fertilizer when soil temps cross 50 degrees. It should be about 10-14 days before it hits 55.

If you’re looking for an easy lawn care program to follow check out this post: 3 Product Lawn Care Program. You’ll learn more about which products to use.

While 55 degrees might be go time in the spring, it means the opposite in the fall. When soil temps head below 55 it means the season is winding down and chances are there’s probably another couple of mowing days left. This is when you want to apply a Winterizer Fertilizer.

65 Degrees

This is around the time when your cool season grass is happiest. It has fully woken up and is pushing green blade growth like crazy. This is when you want to mow as much as possible! It might sound like a lot of effort but mowing at least twice a week during this time is so much better than once a week. This is also a good time to start spot spraying for dandelions which tend to pop up at this time.

70 Degrees

70 degrees is an important soil temperature in the spring, and also in the fall as soil temps begin to cool after the summer soil temperature peaks.

In the spring, 70 degrees is when you want to apply a second round of pre-emergent herbicides and also another shot of fertilizer. Think of this as your ‘step 2’.

Since crabgrass and other broadleaf weeds continue to germinate strong at this temperature I recommend applying another round of pre emergent herbicides. This means instead of putting a super strong dose of pre emergent down at the 55 degree threshold, you instead do 2 moderate apps. One at 55 and the second at this 70 degree threshold.

This gives you much more even, long lasting protection. Also, continue to spot spray for any weeds that have already germinated. No pre emergent program is perfect, so there could still be plenty of weeds that need to be spot treated.

After the heat of the summer, 70 degrees is once again an important threshold to look for. This is when fall broadleaf weeds such as henbit and chickweed germinate. It’s also when Poa Annua germinates. That’s why I recommend applying pre emergent for a third time as temps head back down to this 70 degree range.

It’s important to point out if you plan on doing any fall seeding in your lawn then don’t apply any pre emergent when soil temps come back down to 70 degrees.

Instead start preparing your lawn for fall renovations such as Core aeration and Overseeding, Top dressing, or Dethatching. It’s not too long after this 70 degree threshold at the end of summer to pull the trigger on your seeding projects.

80 Degrees

80 degree soil temperatures are a rough time for your cool season grasses. If they don’t receive adequate water at this point then they will likely head into a period of dormancy.

At this point, soil temps are too hot for weed seeds to germinate, however any weeds such as crabgrass that have already germinated will thrive during these hot weeks. That’s why it’s so important to do the work before soil temps hit 80. At this point you want to really back off on fertilizing especially if you don’t have an irrigation system. You should also increase mowing height and decrease mowing frequency.

Important Warm Season Lawn Soil Temps

55 Degrees

Just like cool season lawns, 55 degrees is when things start happening for warm season lawns. Even though warm season lawns are in regions where the winter is way less harsh, we’re still assuming that in these regions there is some kind of winter and soil temps dip below 55 for a good portion of the winter.

As soil temps approach 55 in late winter you’ll want to apply your pre-emergent herbicide. At 55 degrees, crabgrass as well as other broadleaf weeds will begin to germinate.

But unlike cool season grasses, it’s not quite time to fertilize your warm season grass yet. These grasses like the heat, so they don’t get going until higher soil temps. So for now, lay off the fertilizer.

65 Degrees

When soil temps hit 65 degrees, this is when warm season grasses start growing. Now is when you’ll be mowing, probably pretty often (twice a week). This is when you should apply your first application of fertilizer. It’s also when you want to apply your second application of pre-emergent.

70 Degrees

Heading into spring/summer, once the soil temperature hits 70 degrees your warm season lawn should be fully awake and actively growing.

The next time you want to look for the 70 degree threshold is after the heat of the summer when soil temperatures start to head back down to 70. That’s when a third round of pre emergent herbicides should be applied to prevent fall germinating lawn weeds such as Poa Annua, Hairy Bittercress, and Chickweed.

80 Degrees

Once soil temps hit 80 degrees, warm season grasses thrive. Unlike cool season grasses, warm season grasses like the heat. At this temperature there’s no need to worry about pre emergent herbicides since most weed seeds will not germinate at such hot soil temps.

How to Find Soil Temperatures

I use a combination of 2 tools to determine soil temps in my lawn. For starters, I check out Greencastonline. This is a great website where you can put in your zip code and get the latest soil temp averages in your region. I usually look for the 5 day average.

The other tool I use is a soil thermometer. I simply stick this down about 3” and check on it daily when I know its getting to be the time to take action.

It’s important to realize though, these thermometers do spike during the heat of the day and cool during the night. This is where you have to play detective. I don’t rely solely on the soil thermometer, nor do I rely solely on Green Cast Online. I gather information from both and use that determine when it’s time to get things started.

You’d be surprised at how we all live in little micro climates. For example, I live at the base of a mountain and can have very different weather than a couple miles down the road. I like knowing that I can take soil temps in my own yard and compare it to soil temps reported in other areas.

The last thing I’ll say as it relates to timing is always error on the side of too early. Meaning, you’re better off applying pre emergent herbicides, or fertilizer, or any other application(ie. insects or fungus) too early than too late. Most products take some time to get into the soil and hang around for awhile once there.