WarmDirt keeps plants' roots frost-free


March 28, 2012

Dr Craig Hollabaugh has created a system that keeps the roots of his plants warm and cozy during the coldest months using electric heat cables running under sand on which the pots are placed

Dr Craig Hollabaugh has created a system that keeps the roots of his plants warm and cozy during the coldest months using electric heat cables running under sand on which the pots are placed

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After a somewhat unsuccessful and rather expensive attempt at warming a greenhouse, electrical engineer Dr Craig Hollabaugh rigged up a system that keeps the winter chill away by warming the roots of his plants. The WarmDirt system has already helped his plants survive the coldest of Colorado's cold months, and is now getting ready to provide warmth to seedlings during the expected April freeze. This past season's survivors were all flowers but next winter, the setup will be used for growing veggies.

Dr Hollabaugh began the project by constructing a coffin-sized wooden box and lining the bottom with the kind of electric heat cable that prevents pipes from freezing. He covered the heat source with four inches (101.6 mm) of sand and then placed some plant pots on top. At first, the heat was either switched on or off courtesy of an AC timer, but a desire for more control and much more data led to the creation of the WarmDirt system.

Inside the box, Dr Hollabaugh installed some probes to measure the temperature of the heated sand and the soil inside the pots, the air both inside and outside the box, as well as humidity and ambient light. A prototyping PCB control board sporting an Atmel ATMEGA328P microcontroller was mounted in a waterproof NEMA-4 outdoor electrical enclosure. The system architecture for control and monitoring is a mix of Arduino, XBee, MQTT with Mosquitto, Python, replicated MySQL and jQuery. The (almost) real-time system information is accessed wirelessly via a browser.

All the source code, board design, layout, and so on are available via Dr Hollabaugh's github project page under Creative Commons BY-SA 3.0 (link at the end).

"The WarmDirt board was designed for temperature control but can be used for general purpose motor (stepper or dual servo at 5+ amps) control with encoder support, AC power control, analog voltage sensing, wireless communications and Arduino bootloader," Dr Hollabaugh told us. "It's quite a little board for a variety of projects."

A DC motor was connected to a suitably-branded, 3D-printed spool to the side of the unit that was to take care of the automatic lifting and lowering of the transparent coffin lid. Unfortunately, the stepper motor chosen couldn't quite manage to keep the lid open, so a linear actuator was recently added to raise the lid when the outside air temperature rises above 43 degrees Fahrenheit (6°C) and lower it when it falls below 41.5 degrees Fahrenheit (5.2°C). A switch has been included to indicate when the lid is closed.

The PID controller calculation has been tweaked to ensure that the potted soil temperature never falls below 47 degrees Fahrenheit (8.3°C), and the WarmDirt system uses a triac-based solid state relay for AC power-switching to allow for tighter control of the temperature of the heating element.

The WarmDirt system is still a work in progress with a few niggles to iron out but as it stands, it works very well on its own. Dr Hollabaugh just needs to visit the box from time to time for watering.

Sources: Dr Craig Hollabaugh via Hackaday, and the WarmDirt github project page

About the Author
Paul Ridden While Paul is loath to reveal his age, he will admit to cutting his IT teeth on a TRS-80 (although he won't say which version). An obsessive fascination with computer technology blossomed from hobby into career before the desire for sunnier climes saw him wave a fond farewell to his native Blighty in favor of Bordeaux, France. He's now a dedicated newshound pursuing the latest bleeding edge tech for Gizmag. All articles by Paul Ridden

The hydroponic dope growers and the outdoor growers have known about the seasonal issues surrounding ambient temperature, and soil temperature and plant growth rates.

It makes more sense the keep the dirt a little bit warm, than it does to warm the entire hot house.

The finer points not withstanding.

Mr Stiffy

How much power did he use this winter? Is there any data available?

Carlos Grados

Its a 100 watt pipe heater on about 50% for 3-4 hours a night.

Craig Hollabaugh

My father was doing something similar 20 years ago in a demo unit he built for radiant floor heat.

Douglas Renfro

Why not just do what country house estate gardeners used to do. They would put pots in a built pile of manure and put straw around it.. The heat produced from the manure would keep pots frost free and 3-9'C warmer than outside temp.

My dad did this for early strawberries/veg but he used old bubble wrap and padded envelopes. Use a big pot or box first thick layer of manure, then plastic and bury pot with compost and plant. Wrap bubble wrap around the big container and use plastic cover.

Chocolate cakes often have a nice clear cover you can use.

Karsten Evans

If I used manure, I couldn't wear a tux to do my gardening? :-)

Craig Hollabaugh

Is this guy saying that he is inventing a thermostat-controlled seedling heat mat? Sorry to burst his bubble but it's already been done.

Richard DelPivo

No, Richard, he has created a system to control the heating to keep it in the right range, and he is making the detailed knowledge available to you if you want it. Thanks are in order.

I sure wish more people would read all the details in an article -- with understanding (and follow related links), before making critical comments.

Dennis Brown

Richard, does the thermostat-controlled seedling heat mat you mentioned power 2 heaters, monitor six temperatures, light level, humidity, control 2 motors, relay data wirelessly to a database then on to your phone with 24 hour temperature plots and data display? I don't think so.

Dennis, thanks for the comment. My PID temperature control loop keeps the potted dirt temperature within 1 degree F of the set point.

Craig Hollabaugh

This article reminds me of the space race story. America spent millions developing a pen that could write in space, the Russians apparently used a pencil. Why not just use passive solar heating heating to solve the problem?

W Truter
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