Geothermal heat sink greenhouse

Geothermal heat sink greenhouse build

We are excited to be starting the planning process on a geothermal heat sink greenhouse build.  We will build this geothermal heat sink greenhouse to allow us to grow our own food year round. It would be wonderful to be able to grow frost intolerant crops like tomatoes during the dead of our winters. Without question, we will be able to grow hardy and moderately hardy crops all year inside this greenhouse.

More details about our green house plans

For the heat sink we will use 48 inches of rock that will be placed under 24 inches of topsoil beneath the greenhouse.  We will use a 4 inch thick Styrofoam insulator 6 feet tall around the perimeter of the rock.   The foam board will prevent the heat held in the rocks from being drawn out into the cooler soil outside the foot print of the greenhouse.

During the day, the warm/hot air from the peak of the greenhouse will be drawn down through duct work and a fan and be pushed out near the bottom of the heat sink.  The 4 inch pipe that is used below the ground will be perforated to allow the air to escape.  The air will move up through the rock to a second set of pipes, transferring the heat from the air and into my thermal mass.  The air will enter the second set of perforated pipes and be pushed into the greenhouse.

During the night when the green house temperature drops below that of the thermal mass heat sink the system will work in the opposite manner.  Cool air from the green house will be pushed through the rock and pick up heat.  As the air returns to the greenhouse it will be warmer, keeping the greenhouse temperatures warm through the night.

The high tunnel kit we selected

For our high tunnel kit we selected a 20 x 44 foot kit from Zimmerman’s High Tunnels.  They use 14 gauge steel that is 2 3/8 inches in diameter.  It is a little larger diameter than most kits we found while searching for kits. Zimmerman’s is a few hours drive from our home and we will be able to pick it up to avoid freight charges.

The drainage pipe we are using

For this project we will be using 4 inch corrugated drain pipe, often referred to as drain tile.  All of the pipe that will be horizontal will be perforated to allow air to escape and enter the rock.  Anything installed vertically will not be perforated to ensure the air makes it to our heat sink.  This is what our 4 inch corrugated and perforated pipe will look like.  We will buy it in 250 foot rolls for around $98

Geothermal heat sink greenhouse build
This perforated drain tile will be used horizontally in our rock heat sink

There will be two layers of this pipe, ten each, the will run the length of the greenhouse.  Both will be buried within the rock 3 feet apart.  One group will be 6 inches from the bottom, the second 6 inches from the top.  The top and bottom layers will not be connected.

The pipe near the bottom of the rock heat sink will be connected to the fan that draws heat from top of the greenhouse.  We intend to use a 55 gallon plastic barrel as a manifold to bundle all 10 of the pipes together.  With the fan mounted on top of the barrel it will be able to push air through all 10 pipes.

The ten pipes 6 inches from the top of the heat sink are also perforated and will direct the air blown in from the bottom up and back out into the greenhouse.  All of the pipe laid horizontally will be perforated.

Airflow pattern through the heat sink

Air will flow down to the bottom of the 10 perforated pipes at the bottom of the heat sink. The air will be pushed through the perforations and up through the rock to the second set of horizontal perforated pipes.  The air enters through the perforations and is directed back up and into the greenhouse.

Where each pipe terminates inside of the rock heat sink we will place a cap designed for the the corrugated pipe.  When we connect the perforated and non-perforated pipes we will use a an external connector designed specifically for the corrugated pipe. All unions will be wrapped in a tape intended for drain tile and to be buried.

How we came to decide on our greenhouse dimensions

As with all our gardening here, we will be utilizing the Mittleider Gardening Method in our geothermal heat sink greenhouse build.  There will be 4 grow boxes inside this greenhouse that are 18 inches wide and 30 feet long.  We want plenty of room to move and work in our greenhouse in additional to being able to do all our seedling production in the same place.

We have settled for an overall length of 44 feet.  This will allow a 5 foot walkway at both ends of the grow boxes.  At the far end of the greenhouse we will do our seedling production.  We intend to build a 4 foot bench along one wall for seedling stuff and storage of gardening equipment.  Ultimately there will be batteries here and a charge controller for the solar panels we want to install.

For the width we are going with 20 feet.  It was tempting to go wider to add a 5th grow box, but that would have increased the final cost significantly.  The added cost for the greenhouse itself wouldn’t have been horrible.   But the additional gravel requirements combined with the other costs for excavation and materials would have  significant.  And I can grow a lot of food in just 4 thirty foot beds using the Mittleider gardening method.

Considering snow load and high winds

A twenty foot wide greenhouse is pretty strong and able to stand up well to wind and the weight of our Missouri snow.  We don’t get much snow here so this allows us to save a little money because we don’t need W-trusses.  If we were to go wider it would be smart to spend the extra money for W-trusses with our kit.  W-trusses for the kit we selected would have been another $500.

While the heat sink and geothermal combined makes sense, we haven’t yet seen it work.  We don’t yet know how it will truly work during our winters.  If it doesn’t work and allow us to grow year round a larger system will have been a waste of money.  We will start with a smaller system and see how it performs. If it works well and inexpensive to operate a larger unit that would allow us to sell to our local community might be a good business venture.

Removing trees before building

Before we start this project there will be several trees to remove.  Doing this will prevent shading on the greenhouse and the root system from taking over the ground under the greenhouse.  Removing trees close to the greenhouse will prevent a tree from falling on the greenhouse and damaging it.

There are about 5 trees that I want removed from the vicinity of the build site.  Two of the trees are west of the site and will cast shadow on the greenhouse if left in place.  One tree is massive, an old black locust, that could potentially drop limbs on our greenhouse.  The roots from this tree could also end up under the greenhouse.  We prefer to avoid the possibility of any of those problems and will remove the potential entirely.

There are a few additional trees that are to the north of this geothermal heat sink greenhouse site that I want removed.  They don’t pose any immediate threat to the greenhouse, but I’d like to remove them now and ensure no trees will ever fall on the completed greenhouse and damage it.  We will remove them now to avoid the possibility of tree fall damage entirely.

Goal to have the greenhouse off grid

After the greenhouse is completed and in operation we would like to start getting the greenhouse “off gird”.  We will monitor the energy usage and design a solar system that will meet our needs.  On the end of the greenhouse under the seedling tables we will build an enclosure for the charge controller and batteries.

Through rain collection off our nearby home into a cistern we will be able to water our greenhouse.  We have enough annual rainfall that we can easily collect enough water for our needs.  Ultimately we want to use an automated sprinkler system for a lawn to water the 4 grow boxes inside our greenhouse.

Goals for monitoring the geothermal heat sink greenhouse build

A secondary goal for this geothermal heat sink greenhouse, is the ability to wirelessly monitor the temperatures of the greenhouse,.  Additionally, outside the greenhouse and  the ground temperatures of the heat sink.  Ultimately allowing others to view current temps and historical data would be nice.

I’ve not yet researched the options of products that will allow this.  We do plan on placing a piece of PVC during the build that would allow a temperature probe to be dropped into the heat sink material.  The PVC will be relatively inexpensive, we may add two to allow temperatures to me monitored at the top and bottom of the heat sink rock.

Almost ready to start excavation

We have all the materials except the styrofoam for the heat sink on hand. As soon as we can schedule the delivery of the 125 yards of rock we can start the dirt work, weather permitting. Our goal is to have the heat sink portion completed by the end of February. The greenhouse portion we hope to have completed and ready for seedlings mid March.

We will be documenting this project with photos and video, be sure to check back for updates of our progress.

keeping weekly feed dry

keeping weekly feed dry

Frequently I see folks in the Mittleider Facebook group discussing their wet weekly feed and them asking for ideas on keeping weekly feed dry.  Once you add the Epsom salt to your weekly feed mix it becomes hygroscopic and will absorb moisture from the air.  If you’re in a dry climate adding a half pound of Perlite to your weekly feed mix will help.  For the gardener in a high humidity climate the Perlite will not be enough.

Another option to keeping weekly feed dry is to mix the fertilizer and micro nutrients but not add the Epsom salt until you need weekly feed.  The fertilizer and micro nutrients are not hygroscopic.  It isn’t until the magnesium sulfate is added that it begins to absorb moisture.

Add Epsom salt to smaller batches

Currently we have seedlings growing under grow lights.  They are fed daily with a mixture called constant feed.  Constant feed consists of 1 ounce of weekly feed in 3 gallons of water.  We were out of weekly feed and I needed to mix up a batch of weekly feed.  We won’t be putting seedlings in the ground for another 6-8 weeks, or weekly feed in our high humidity would have been very wet by the time we needed to use it in the garden.

Instead of dealing with wet weekly feed, we decided to mix up smaller batches.  We mixed up 25 pounds of triple 13 fertilizer with the 10 ounces of micro nutrients and stored that mixture in a 5 gallon bucket with a Gamma lid. Don’t add your Epsom salt yet!

Typically you would add 4 pounds of Epsom salt to 25 pounds of fertilizer and the 10 ounce packet of micros.  But we want to mix the weekly feed in smaller batches.  We decided to figure for one pound of the epsom salt, so we divided the combined weight of fertilizer and micros, 25.625, by 4 which gives us 6.4 pounds.

So mixing 6.4 pound of the fertilizer and micro nutrients mix with 1 pound of Epsom salt will give us 7.4 pounds of weekly feed.  You can mix more, or less, as you require.  because we only need an ounce of weekly feed every time we mix constant feed I elected to mix up 3.2 pounds of weekly feed with a half pound of Epsom salt.

keeping weekly feed dry
keeping weekly feed dry

Watch the video

You can watch this video where I describe steps for keeping weekly feed dry.  This is one of many videos on YouTube discussing Mittleider gardening.  Please consider watching some of our other videos and subscribing to out channel.

seedling heat mats

Seedling heat mats for seedling production

A great way of ensuring success in growing seedlings is through the use of seedling heat mats.  This is particularly true when starting hardy crops indoors before the average last frost of the year.  Seedling heat mats will maintain a constant temperature of the growing medium.  This will help get and keep your growing medium at a constant optimum temperature for germination.

To work with a pair of the Mittleider seedling flats, we went with a seedling heat mat that was 20 inches wide and 48 inches long.  Two seedling flats fit on this mat with room on each end.  We use that space later once we bump plants up to individual containers.  Ours is like the one pictured below and includes a thermostat.  It has a digital readout and maintains the temperature you set. You can see the seedling heat mat we use here.

seedling heat mats with digital thermostat
seedling heat mats with digital thermostat

The thermostat includes a probe that is placed in the soil.  The current temperature of the growing medium will be indicated on the digital readout.  The operator can adjust the desired temperature as needed.  Our units are both set at 80 degrees for germination.

Combine heat mats with grow lights

When starting seedlings indoors it may be necessary to combine a seedling heat mat with grow lights.  Currently our seedlings are germinated in the basement where there is no natural light.  We start all our seedlings under grow lights and on the heat mats.  Light and temperature are the first two of the six laws of plant growth and are necessary to grow healthy seedlings.

The grow lights heat the growing medium and helps cut down on the frequency for which the heat mats need to kick on to regulate temperatures.

Starting spring crops

Starting spring crops

It is February, which means it is time to be starting spring crops.  We start all our seedlings under grow lights and on seedling heat mats.  Everything is in our basement currently, but in the future we will be moving seedling production to the greenhouse.

Our seedling flats were moved to the basement today and have begun to come up to germination temperature.  Once the growing medium reaches the correct temperature we will add seeds.

Starting spring crops

Start with hardy and moderately hardy varieties

Everything we are starting now are varieties that are hardy and moderately hardy.  They will go into the garden well before our average last day of frost and will be protected with mini hoop houses.   Some of this will hopefully be going into a greenhouse we will build soon.

Our seedling choices for spring

We have amended our list of crops for this year.  Those we will be planting first are broccoli, cauliflower, onions, kale, spinach, red and white beets, and cabbage.

Determining when to start seedlings

While planning or starting spring crops we take advantage of the garden planning detail sheet in spreadsheet form.  We enter or ADLF and the spreadsheet will calculate when we need to start and transplant the different seedlings.  You can get the XLS file from the Mittleider Gardening group on Facebook.  It is in the files section there in the group.

Every couple weeks we will be starting more seedlings.

Changes we are making to our garden plan

We are dropping a few plant varieties that we really don’t eat and have added a few.   Radishes, eggplant, bush beans and Brussels sprouts have been removed from our garden plan for this year.  The bush beans were difficult to get everything picked without having some sort of trellis to hold up  the plants.  We are instead going a pole bean.  The others we found we just don’t eat much.  That space they used to occupy will be better used for the things we do eat.

Starting seedlings indoors

Starting seedlings indoors

Here at our home we have begun the process of starting seedlings indoors. We use seedling heat mats and grow lights in our basement. Starting seedlings indoors has been the one not successful step we have learned through the Mittleider Gardening Method to increase our gardening success. Growing our own seedlings gives us better control over ur garden. We only use the strongest and healthiest seedlings.

By controlling variables such as soil temperature, daily available light, moisture and nutrition we have more success as compared to direct sewing seeds into the garden. We won’t have seeds delayed in germination, or failing to do so entirely, because an unexpected cold snap moved through and dropped soil temps.

Starting seedlings indoors
Starting seedlings indoors

What do I need to get started

You’ll need a space where you have ample room and the ability to control temperatures. That space will need room for the plants as they grow and you bump them up to larger containers. Enclosed porches, an extra bedroom, the basement or a small seedling house with any required supplemental heat will work. Electricity to power the grow lights and seedling heat mat will be needed. A nearby access to water would be a bonus, but isn’t absolutely necessary.

If you’re utilizing a room with adequate light a grow light won’t be necessary. If you’re growing in a basement or a room with inadequate lighting you’ll need an artificial source to keep your plants alive and healthy.

What grow lights do I need

Grow lights can be purchased or made to suite your purposes with items commonly available at your local Walmart. Sams Club and Costco both sell “shop lights” that are complete and just need to be hung and plugged into a power source that will work.

Commercially made grow lights are more expensive, but the grow light bulbs will use less power. I’ve not idea how many years we will need to run ours to reach s break even point but am happy with our lights.

If you want to go the budget route you can find florescent light housings at Walmart. If you’re a handy person, or have one in your life that will help, the fixtures are easy enough to assemble, hang and then install bulbs. Standard florescent bulbs will work to start your own seedlings indoors. Or through places such as Amazon you can buy bulbs specially for grow lights that you can install in your fixture.

For those who are concerned about your energy consumption, consider going with the LED bulb. If you’re using existing florescent fixtures they can be easily converted to power the LED bulbs. Be sure to bypass the ballast to further reduce your energy usage. There are tons of great videos on YouTube that show the conversion process.

Why grow your own seedlings

Starting your own seedlings indoors is a great way to control the quality of your garden. Using certified seeds and sterile soil helps ensure there is no disease. By growing extra seedlings you can insure only the strongest and healthiest plants make it into the garden. Having healthy seedlings to transplant into the garden helps ensure all the available space is utilized. No failure to germinate from directly sewed seeds results in more on your table or in the pantry.