Garden Year Round with an Underground Greenhouse

article and photograph from The Seed Guy

If eating Fresh Vegetables and Fruits Year Round is important to You and Your Family, you might consider building an Underground Greenhouse. It will keep the temperatures warmer in the Winter and help prevent overheating in the Summer; making it possible to grow your garden vegetables year round.

For the vast majority of the country, 4 feet below the surface will stay between 50° to 60°F even if the weather above the ground gets to 10°F or colder. This is what they call the thermal constant, and what the Underground Greenhouse is based on.

The original design for an Underground Greenhouse was invented in Bolivia, and was called a Walipini, an Aymara Indian word which means “a warm place.” A Walipini is a rectangular shaped Greenhouse that is dug down 6-8 feet deep in the ground. The longest area of the rectangle will face towards the south (in the Northern Hemisphere) to take advantage of the most sunlight.

The design of the Underground Greenhouse isn’t that complicated, as it can be as simple as a hole with plastic sheets laid on top. The roof seals in the heat and insulates the area to keep a warm, moist environment for your fruits and vegetables.

The location of your Walipini will depend on how big you want it to be. You’ll need enough space to grow your plants and have a small area to walk into your greenhouse. The bottom of the Greenhouse will need to be at least 5 feet above the water table in your area. The recommended size for an Underground Greenhouse is 8 x 12 feet.

When planning where your Greenhouse will be located, remember that your roof will need to receive light during the winter, also. This means that you will have to make sure that trees or buildings don’t block it during the winter time when the sun is in the South. In most cases, your Underground Greenhouse should be set up East to West, with the roof facing South to take advantage of the Winter Sun.

Once it’s decided where your Underground Greenhouse will be located, you can start digging. Plot out the area above ground to keep track of where you should be digging. While you’re excavating, dig at least 2 feet deeper than your desired depth. Keep your soil close by to help prop up the roof.

The walls of your Underground Greenhouse should have a minimum 6-inch slope from the roof to the floor. This will greatly reduce the amount of crumbling and caving that will occur with the soil. You can also layer the walls with a clay to prevent erosion, or use bricks to stabilize the walls of the building.

While you’re digging the hole, dig an extra 2 feet below the desired depth. You’ll fill this area with stone or gravel and then 8 inches of soil. Ideally, you’d lay larger stones and gravel on the bottom layer and the gravel would become progressively smaller until you reach the soil.

The bottom of the greenhouse should be slightly sloped from the center to the edges. Along the perimeter, you should leave a space of 2-3 feet just filled with gravel. This is designed to help the water drain more easily. Many people have also created open gravel wells in the corners of the greenhouses that allows them to collect the water. This will allow you to draw a bucket into the hole and pull out water if you find you have too much.

Once the floor is filled in with the drainage system, and the soil required for growing, the doors can be installed. Place the door frame at the base of the ramp and fill in the areas around the door as much as possible with dirt and clay. Filling in these gaps will prevent heat loss in your greenhouse.

Many times, people will use 2-inch door frames that have holes drilled into the top middle and bottom of each side. They will then use wooden stakes, dowels or rebar to secure the door frame into the soil wall.

The angle of the roof will make a big difference on the sun’s ability to heat your greenhouse. Ideally, the roof should be facing directly at the winter solstice at a 90 angle. This angle will maximize the heat during the winter solstice and minimize the heat during the summer solstice.

Now, you can use that extra soil that you have left over to create a berm. The berm is basically an extension of the north wall of the greenhouse. This allows you to control the angle of the roof by adding or taking away dirt. Build up the berm to continue the slope that you used on the wall. If you’re using bricks – continue using them on the berm.

The most economical, durable material for your roof is 4-inch PVC pipe. Using PVC elbow pieces, joiners, etc, you can create a flat roof frame that will cover your Underground Greenhouse.

After you’ve created a PVC frame, lay it in place on the top of your hole. Then lay plastic sheeting across the top of the frame and make sure that it extends past the edge of the frame by at least 1 foot. This flap will prevent run off water from the roof from running back into the greenhouse itself.

Once the plastic material is put on top of the roof frame, move inside and tack another layer of plastic wrap along the inside of the roof frame. This internal plastic sheeting will create a 4-inch barrier between the inside and outside of the roof, and will act as an insulator that will keep the heat in more effectively.

You’ll want to make sure that you leave a few inches of plastic hanging down on the lower (south) end of your roof. This will force moisture that collects on the roof to drip off above the drainage system or on top of your plants instead of at the base of the roof. If you allow the moisture to run to the base of the roof frame, it may affect the soil at that location and break down your wall, etc.

Ventilation is always crucial. You have 3 options, such as: Installing two doors, one at each end; installing a vent roughly the size of the door at the top of the back wall; or installing a chimney at the center of the back wall. Good Luck on your Greenhouse.

Invasive Plant Management

A recent post from the North Country Master Gardener Volunteers:

Our own past posts regarding invasives:

Water, water everywhere, but is it safe?

Today’s blog topic deals with watering your houseplants and the numerous questions that arise from what seems like a simple task.


  1. Is the chlorine in tap water harmful to my houseplants, and what are the signs of trouble? I read that water should be left for several days so that the chlorine evaporates. Does this work?
  2. I have been told not to use softened water for my houseplants. Why not? If it’s better for my own use, why isn’t it good for my plants?
  3. Is it OK to use water removed from the air by a dehumidifier to water my plants?



If chlorine does harm the plants, the injury will be to the root tips, according to Len Morino, former propagation manager for the New York Botanical Garden. “Although it’s not a bad idea to leave the water you use for plants sitting around for about 24 hours if you can, we’ve found that the chlorine in the water hasn’t proven to be a problem at all. In the New York Botanical Garden greenhouses, we used straight tap water for everything, and that included seedlings.”

Very likely, the reason chlorine rarely causes trouble in the tap water used for plants, he said, is that in the course of moving through the municipal system, most of it volatilizes, that is, it escapes into the air in the form of gas. “Chlorine just doesn’t remain long,” he added, “which is why it has to be added repeatedly to swimming pools.”

That said, it must be added that some municipalities add heavier doses of chlorine, or add them closer to the end user, than happens in New York, so let your senses be your guide. If you can taste — or smell — chlorine, there’s probably enough of it coming out of the tap to make aging it a good idea for the plants. Draw the water into wide-mouth jugs and let them sit uncovered for at least a day before you pour it on.



The things that make water hard are minerals like calcium and magnesium. Water softeners remove these minerals (which plants like) by exchanging them for the sodium part of salt (sodium chloride) that is found in the softener. Not enough sodium goes into the water to affect is taste, but there is much more than would be there naturally.

Sodium is a very active chemical. It also exchanges places the the potassium in plant cells, which is necessary for dozens of cell enzyme functions. When potassium is replace by sodium, these functions don’t happen and the plant could die. Different species have different tolerances, but eventually the sodium buildup will get to all but those that grow naturally at the seashore.



Using water from your dehumidifier sounds like a great idea. In fact, some indoor gardeners have posted online to suggest it. However, the General Electric Company, one of several manufacturers of dehumidifiers, does not advise re-using the water from these machines for anything. At the same time, though, no one at G.E. can say exactly why the company came to this conclusion.

If you don’t use one of the solvents sold for cleaning dehumidifiers — these may contain chemicals harmful to plants — you might do a bit of experimenting yourself: use the water collected by your dehumidifier on one inexpensive plant for several weeks and see what happens.

Note: I have tried this. I’ve dumped the contents of my dehumidifier container into one of my largest outdoor pots with no ill-effects. However, you’ll have to experiment for yourself.

How much potting up?

A question that often plagues new (and sometimes, older) gardeners: I’m in the early stages of the annual rite of starting a number of vegetables from seed, transplanting the seedlings to small pots in a few weeks, transplanting these to larger pots in a few more weeks, and finally setting my plants in the garden. Numerous gardening books recommend this ritual, yet plants don’t seem to mind the final transfer to the garden, a very large pot indeed. Why can’t I immediately transplant each seedling to a fairly large pot, saving time and avoiding excess plant handling?

There are a couple of reasons to go through the ritual. One is that it conserves space. Whether on the windowsill, under lights, or even in a greenhouse, most home gardeners have only a limited area where the light is bright enough for good seedling growth, and they don’t want to waste in on empty pot surface.

The other reason to keep the plant in proportion to its container is the danger of damping-off. This fungus disease, which is encouraged by large expanses of bare, damp soil, attacks the tender stems of baby plants and can kill them overnight. The spores are everywhere, so damp-off is a danger even when you start out with a sterile potting mix.

You might want to compromise by skipping the middle step-up for fast growers like tomatoes. And if you have lots of growing space, there’s nothing to prevent you from trying it with everything you grow. Start a larger than usual number of seeds, then hold back a few seedlings of each variety in the original starting pots. If the transplants do suffer damp-off, they’ll probably do it fairly soon, and you’ll have backup seedlings. If the transplants do well, all you’ve lost are those few extra seeds.

Additional blog posts on starting seeds:

The Mysteries of pH

As we look through our garden catalogs, carefully tend to the seeds we’ve started, and draw up diagrams for new gardens, I wonder if anyone looks at the soil itself to make sure that the right growing conditions are being provided. The degree to which soils are either acid or alkaline is indicated on the pH scale, which runs from 0.0 (the most acid) to 14 (the most alkaline). The explanation of what this means has two levels, one technical and one practical.

If we are being technical, the thing being measured is the concentration of hydrogen ions. (The “H” is pH is the chemical symbol for hydrogen.) The more hydrogen ions there are, the more acid the thing is. That’s easy to understand.

What gives non-mathematicians pause is that the pH scale is not a linear one, with equal intervals between numbers. Instead, every number on the pH scale shows ten times less H concentration that the number below. Soil with a pH of 5 is ten times more acidic that soil with a pH of 6, and a hundred times more acidic that soil with a pH of 7. Seven is neutral, right in the middle of the scale, but it still has ten times more hydrogen ions that soil with a pH of 8. (Above 7, there are more hydroxyl ions that there are hydrogen ions, which is what makes these soils alkaline.)

Got it? Me either, but it doesn’t matter. From the practical gardening standpoint, all you really need to know are two things: The number 7 is neutral, below 7 is acid and above 7 is alkaline; and although there are exceptions at each end (rhododendrons for soils of 5.5 or so, yucca for those around 7.6), most plants do best isn soils that are slightly on the acid side of neutral, while those that prefer alkalinity don’t want too much of it. This is because the organisms that make nutrients available to plants function best in the 6 to 7 range. There may be nutrients present in highly acid or alkaline soils, but it won’t matter because the plants won’t be able to get at them.

Soil pH is important, in a general way: lilacs will never thrive where it is too acid, and potatoes will be discouraged if it’s at all alkaline, but you don’t need to fall into a cookbook funk about measuring pH exactly. As long as other growing conditions are beneficial, you should be able to grow nearly anything in soil that falls between 6 and 7.5. If you do a thorough, professional testing when you start a new garden, then keep an eye on things by testing every year or so, that should be enough.

Refer to previous blog posts regarding soil: