Does Mulch Keep Soil Warmer in Winter? A Real-World Soil Temperature Experiment

By Tim the Techno-Gardener

Gardeners and growers in cold climates are always looking for ways to extend their growing season. One method that’s often mentioned is using mulch as insulation to keep soil warmer during the winter. But how effective is this approach in practice?

I set out to run a simple but informative experiment to find out. My goal was to measure whether mulch has a measurable impact on soil temperature during the coldest part of the year, and if the results could support the idea of building micro-geothermal greenhouses or cold frames for winter gardening.

Why Soil Temperature Matters

Soil temperature plays a critical role in plant growth, particularly for seed germination and root development. In winter, especially in high-altitude or high-desert climates like the Rocky Mountains where I live, cold soil can halt growth entirely—even for cold-hardy crops like lettuce.

If mulch could keep the soil just a few degrees warmer, it might make winter gardening more feasible without expensive infrastructure.

Experiment Setup: Tools and Methodology

To test this, I used the following equipment:

I buried the two sensors 4 inches deep in clay soil. The only variable? One sensor was left in bare soil, while the other was covered with 12 inches of wood chip mulch.

I also placed a third sensor above the ground, near the house, to monitor ambient air temperature for comparison. The data was collected continuously and sent to VegeCloud.com, which plotted temperature trends over time.

Location and Timing

This experiment took place in mid-February in the high desert of Utah, with daytime temperatures swinging between 20°F and 50°F, and snow still on the ground. These extreme conditions made it an ideal time to test mulch’s insulating effects.

Initial Setup and Monitoring

Installing the sensors and VegeHub took just 10 minutes. The Wi-Fi connection from the elevated PVC mount worked perfectly, and the sensors began logging data immediately.

I originally planned to run the experiment for one week. However, due to some distractions—like overseeing manufacturing operations at Vegetronix—the experiment ended up running for several weeks, which gave me a rich data set covering mid-February through mid-March.

What the Data Revealed

The graphs generated on VegeCloud.com told a fascinating story:

  • Air Temperature: Showed extreme swings—up to 40°F within a 12-hour period.
  • Bare Soil (3" deep): Displayed similar fluctuations to the air, but the variations were smoothed and exhibited a clear phase lag.
  • Mulched Soil: The real standout. Temperatures remained remarkably stable, almost like a moving average of the air temperature.

Takeaways and Implications

So, does mulch keep soil warmer in winter? Not exactly. The average temperature under the mulch was similar to that of the bare soil. However, the mulch dramatically reduced temperature fluctuations, providing a more stable environment for soil biology and potential root systems.

This is important for two reasons:

  1. In Winter: Stable soil temperatures can help prevent freeze-thaw cycles that damage roots.
  2. In Summer: Mulch can prevent overheating and reduce water evaporation.

Cold Frames and Geothermal Greenhouses: Is It Viable?

If you're thinking about building a cold frame or geothermal greenhouse, soil temperature sensors like the THERM200 are invaluable. They allow you to map the thermal profile of your soil and make informed decisions before investing time and money.

In my case, even with mulch, the average soil temperature in early spring hovered around 45°F—not quite warm enough for most plants. But with a transparent cold frame to trap solar heat, that number could rise.

Bonus Insight: Temperature vs. Soil Depth

One final chart from VegeCloud shows temperature over time at various soil depths. As you go deeper, the fluctuations diminish. This doesn’t mean the soil gets warmer—it means it becomes more stable. Deeper soil acts like a thermal buffer, which is why geothermal systems often bury heating tubes below the frost line.

Final Thoughts

This simple experiment highlights the power of data in gardening. Understanding your soil’s temperature trends can help you optimize planting times, choose the right crops, and design better garden infrastructure.

If you're interested in tools like the THERM200, VegeHub, or other smart gardening tech, check out Vegetronix.com for more ideas and products.