A walpini is a greenhouse built into the ground. The idea is simple: dig a pit below the frost line (the depth where the ground stays cold enough to freeze), cover it with a low-cost glazed roof (clear or translucent panels that let light in), and let the earth’s thermal mass do the work. Thermal mass just means the soil around the pit holds heat and releases it slowly, keeping the space above freezing year-round, even at 6,035 feet.
The idea works. The problem is water. Almost every walpini failure in Colorado Springs traces back to it.
It’s not a lack of water. It’s too much water, showing up in the wrong places at the wrong times.
I’ve built a walpini myself. I know what the drainage challenge looks like in real Colorado soil and real Colorado weather. Here’s the drainage system that actually works.
Where the Water Comes From
A walpini is basically a big pit in the ground, and pits collect water. Here’s where that water comes from on a Colorado Springs property.
Surface runoff: When it rains, water runs across the ground and heads for the lowest point it can find. During monsoon season (July through September), Colorado Springs storms can drop 1–2 inches of rain in a single hour. A walpini pit is a low point. If the ground around it isn’t graded, meaning sloped to carry water away, that runoff pours straight into the growing space.
Groundwater: The water table (the underground level where the soil is naturally saturated) varies across El Paso County. Most high-elevation Colorado Springs properties have well-drained soil and no real groundwater problem. But on lower-lying lots, near drainage corridors, or on clay-heavy soil that holds moisture, the water table can rise, especially in spring after snowmelt. When it does, water can enter the pit from below or through the walls.
Condensation: This source is less obvious, but it’s the most constant one. In winter, the walpini’s interior air is warm and humid, but the earth walls are cold. When warm, moist air touches a cold wall, the moisture turns to water droplets, the same way a cold glass sweats on a summer day. This happens most on the north wall, which gets the least sun. The water runs down the wall and pools where the wall meets the floor. Good drainage carries it away. Without it, that spot stays wet all winter, which stops plants from growing well and invites mold.
Irrigation: A walpini is a working growing space, so you’re watering plants inside it regularly. Whatever water the plants don’t soak up has to go somewhere. In a poorly drained walpini, it just sits on the floor.
The Four Components of a Properly Drained Colorado Springs Walpini
Component 1: Surface Grade Management
The ground around the walpini has to slope away from the pit, not toward it. The standard is 6 inches of fall over the first 10 feet out from the pit edge. In practice, that means using the soil you dug out of the pit to build up the ground around it, instead of hauling it away. This raised ring of soil, called a berm, turns surface water away from the opening.
The glazed roof should extend past the pit walls on all sides. That overhang keeps rain and melting snow from running down right where the wall meets the ground. On the low side of the roof, a drip edge (the strip that controls where water drips off) should send that water to a clear path away from the structure: a swale (a shallow, sloped ditch), a dry creek bed, or simply downhill and away. This one fix stops the most common way surface water gets into a walpini.
On flat ground, you’ll need to build this slope on purpose. On a sloped site, pay extra attention to the uphill side: a drainage swale or interceptor drain (a ditch that catches water before it reaches the pit) keeps runoff from the slope above from ever reaching the walpini.
Component 2: Perimeter French Drain
At the base of the pit walls, right at floor level, a perimeter French drain catches groundwater and wall moisture before it reaches your plants. A French drain is just a gravel-filled trench with a pipe in it that carries water somewhere safe.
How to build it:
- Dig a trench 12–18 inches wide and 12 inches deep along all four walls, at floor level
- Line the trench with filter fabric, a non-woven geotextile (a synthetic sheet that lets water pass through but keeps soil out of the gravel)
- Fill the trench with washed drain rock (3/4-inch clean crushed stone)
- Lay 4-inch perforated drain pipe (pipe with holes drilled in it) in the gravel, holes facing down
- Wrap the pipe in a sock filter (a fabric sleeve) so gravel can’t work its way inside it
- Connect the drain to the sump point or the gravity outlet (covered next)
Run this drain all the way around all four walls, without gaps. Water that seeps in where the wall meets the soil moves down through the soil and the gravel bed, into the pipe, before it can ever reach the floor.
Component 3: Sump Point or Gravity Outlet
That drain water needs somewhere to go. You have two options.
Gravity outlet (the better option, when your site allows it): If the walpini is built into a slope, the drain pipe can run out through the downhill wall and come out into the open air below the pit. Water flows out by gravity alone, with no motor or moving parts to fail. Put the outlet at least 10 feet from the structure, pointed away from any foundation or planting area, and cap it with rodent-proof mesh so animals can’t get in.
Sump pump: On flat ground, the perimeter drain empties into a sump basin: a perforated plastic cylinder set into the floor at its lowest point and surrounded by gravel. A submersible sump pump sits in the basin. When water reaches a set level, the pump switches on and pushes it out through a discharge pipe. This setup needs electricity and regular upkeep.
For a growing structure you don’t visit every day, a gravity outlet is much more reliable than a sump pump (it’s the setup I push toward whenever the site allows it). If the pump fails during a monsoon storm and nobody checks the walpini for a week, it floods. A gravity drain has no motor, so it can’t fail that way.
If your site forces you to use a sump pump, add a battery backup or hook the pump to an alarm that alerts you when water rises. A flooded walpini in the middle of summer can wipe out a whole season’s crop in a matter of hours.
Component 4: Interior Floor Drainage
Whatever the walpini floor is made of (packed earth, gravel, concrete, or flagstone), it needs to slope toward the sump point. A slope of at least 1/4 inch per foot, running from the far corners toward the sump, keeps water from pooling between planting beds.
Plan the floor for irrigation runoff and wall condensation, not just rain. Planting bed borders, whether they’re timber, block, or stone, need small gaps so water can pass under or around them and reach the floor drain. Otherwise they act like a dam and create pooling.
A gravel walkway between beds works better than packed earth. Gravel lets water soak straight down instead of running across the top, so less water reaches the sump and the walkways don’t turn to mud after you water.
Colorado-Specific Drainage Considerations
Freeze-thaw and drain pipes: In El Paso County, the frost line (the depth below which the ground doesn’t freeze) sits at 30 inches. Any drain pipe that exits the walpini above that depth is at risk from freeze-thaw damage. Here’s why: when the ground around a shallow pipe freezes and thaws over and over, it can shift the pipe, a process called frost heaving, and create low spots that trap standing water. Colorado Springs gets more than 100 freeze-thaw cycles a year, so this adds up fast. Install exit pipes below the 30-inch frost line, and wrap any section that passes through the wall, where it crosses from the heated interior to the cold exterior, in closed-cell foam pipe insulation.
Clay soil: Some Colorado Springs properties, especially in lower elevation areas and older neighborhoods, have clay-heavy soil that drains slowly. A standard French drain in clay soil can become waterlogged and stop working during a long wet spell. If you’re building in clay, widen the gravel bed to 18–24 inches (instead of the standard 12), use clean washed stone rather than crusher fines (crushed rock ground as fine as sand, which clogs the drain), and consider stepping up to a 6-inch pipe instead of 4-inch, wrapped in geotextile fabric, to move more water.
Monsoon timing: Design the drainage system for the heaviest rain it will actually see, not the average. A Colorado Springs monsoon storm can drop 1–2 inches of rain in an hour. Size the sump basin and pump (if you’re using one) for that peak rate. A basin sized only for slow, steady seepage can be overwhelmed the first time a real July storm hits.
Snow load drainage: In winter, snow that piles up on the glazed roof melts off gradually, and your drainage system has to handle that meltwater too, not just rain. The south-facing glazed panels melt snow fastest: even moderate February sun can melt a good amount of snow while the air temperature is still below freezing. That melt runs off the roof edge, and if the ground grading isn’t right, it heads straight for the pit wall.
What Happens When Drainage Fails
I’ve seen this happen more than once: a walpini with bad drainage doesn’t fail all at once. It fails slowly, and at first it looks like a plant problem, not a drainage problem.
Roots sit in water that never drains away. That water pushes the oxygen out of the soil, so the roots can’t breathe. Plants turn yellow, wilt at the base, and develop root rot, symptoms that are easy to mistake for a nutrient problem or disease. The soil itself packs down and loses its structure. Mold starts growing on the walls, and eventually on the plants themselves.
By the time the failure is obvious (standing water on the floor, walls that are wet all the time, soil berms turning soft from soaking up water), the growing space has usually been damaged for weeks or months already.
Getting the drainage right during construction is what makes a walpini worth building. A well-drained walpini at 6,035 feet, with enough thermal mass and the right glazing, grows tomatoes, peppers, and greens straight through a Colorado winter. A poorly drained one just grows mold.
For a free estimate on walpini construction or drainage retrofit in the Colorado Springs area, call (719) 243-9718.
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