Walpini Drainage: The #1 Reason Underground Greenhouses Fail in Colorado — and How to Pipe for It

The walpini concept is simple and compelling: dig down below the frost line, build a low-cost glazed structure over the pit, and use the earth’s thermal mass to maintain above-freezing temperatures year-round at 6,035 feet. The theory is sound. The execution is where most walpinis in Colorado Springs fail — and almost every failure traces back to the same thing: water.

Not the lack of water. Too much of it, in the wrong places, at the wrong times.

I’ve built a walpini. I know what the drainage challenge looks like in practice, in Colorado soil, in Colorado weather. This is the system that makes it work.

Where the Water Comes From

A walpini is a pit. Pits collect water. The sources in a Colorado Springs installation:

Surface runoff: When it rains — and during monsoon season in July through September, it rains hard and fast — water runs across the ground surface and finds the lowest point. A walpini excavation is a low point. Without proper grading around the perimeter, surface runoff drains directly into the growing space. A single July monsoon event can deliver 1–2 inches of rain in an hour. That’s a lot of water trying to find your walpini.

Groundwater: Depending on location, the water table in El Paso County varies. Most high-elevation Colorado Springs properties have well-drained soil and no meaningful groundwater issue. But in lower-lying areas, near drainage corridors, or on properties with clay-heavy soil that holds moisture, the water table can rise seasonally — particularly in spring after snowmelt — and enter the excavation from below or through the walls.

Condensation: This is the least obvious source and the most consistent one. A walpini in winter has warm, humid interior air and cold earth walls. Moisture from that air condenses on the wall surfaces — particularly the north wall, which gets the least solar warming. This condensation runs down the walls and accumulates at the base. In a well-drained walpini it’s managed. In one without proper interior drainage, it produces chronically wet conditions at the wall-floor junction that inhibit plant growth and encourage mold.

Irrigation: The walpini is a productive growing space, which means you’re watering plants inside it. Water that plants don’t absorb has to go somewhere. In a poorly drained walpini, it goes into the floor.

The Four Components of a Properly Drained Colorado Springs Walpini

Component 1: Surface Grade Management

The ground around the walpini perimeter must slope away from the excavation — not toward it. The standard is 6 inches of fall over the first 10 feet from the structure edge. In practice, this means the excavated soil from the pit should be used to build up grade around the perimeter rather than being hauled away — a positive backfill berm that redirects surface runoff away from the opening.

The glazed roof structure should extend beyond the pit walls on all sides, with adequate overhang to shed rain and snowmelt away from the wall-to-grade junction. A drip edge at the low side of the roof that directs water to a defined drainage path — a swale, a dry creek, or away from the structure — eliminates the most common surface water intrusion source.

On a flat site, this requires deliberate grading work. On a sloped site, the uphill side requires particular attention — a drainage swale or interceptor drain uphill of the structure prevents slope runoff from reaching the pit.

Component 2: Perimeter French Drain

At the base of the excavated walls — at the floor level of the walpini — a perimeter French drain intercepts groundwater and wall-contact moisture before it reaches the growing area.

Construction:

• Excavate a trench 12–18 inches wide and 12 inches deep along all walls at floor level
• Line the trench with filter fabric (non-woven geotextile) to prevent soil migration into the gravel
• Fill the trench with washed drain rock (3/4 inch clean crushed stone)
• Install 4-inch perforated drain pipe in the gravel, perforations down
• Wrap the drain pipe with a sock filter to prevent gravel infiltration
• Connect the perimeter drain to the sump point or gravity outlet

The perimeter drain should be continuous around all four walls. Water that enters the wall-soil interface above floor level percolates down through the soil and the gravel bed and enters the drain pipe before it can pool on the floor.

Component 3: Sump Point or Gravity Outlet

The perimeter drain needs somewhere to go. Two options:

Gravity outlet (preferred when terrain allows): If the walpini is built into a slope, the drain pipe can exit through the downhill wall and daylight at the surface — water drains out by gravity with no mechanical components to fail. The outlet should be at least 10 feet from the structure and directed away from any foundation or planting area. Cover the outlet with a rodent-proof mesh cap.

Sump pump: On a flat site, the perimeter drain terminates at a sump basin — a perforated plastic cylinder set into the floor at the lowest point, surrounded by gravel. A submersible sump pump in the basin activates when water reaches a set level and pumps it out through a discharge pipe. The pump requires a power source and periodic maintenance.

For a remote growing structure, a gravity outlet is significantly more reliable than a mechanical sump pump. If the pump fails during a monsoon event and nobody checks the walpini for a week, the structure floods. A gravity drain can’t fail in the same way.

If the site requires a sump pump, install a battery backup or connect the pump to an alarm system that alerts you when water levels rise. A flooded walpini in midsummer can wipe out an entire growing season’s work in a matter of hours.

Component 4: Interior Floor Drainage

The floor of the walpini — whether compacted earth, gravel, concrete, or flagstone — needs to be graded to direct water toward the sump point. A minimum slope of 1/4 inch per foot from the far corners toward the sump location prevents standing water between planting beds.

Irrigation runoff and condensation drip-off from walls should be anticipated in the floor design. Planting bed borders — whether timber, block, or stone — should allow water to pass beneath or around them to reach the floor drain, not dam it up and create pooling zones.

A gravel walking surface between beds is preferable to compacted earth: gravel allows water to percolate vertically rather than running across the surface, which reduces the volume that reaches the sump and keeps the walkways from becoming mud after irrigation.

Colorado-Specific Drainage Considerations

Freeze-thaw and drain pipes: Drain pipes that exit the walpini through the wall and run above the frost line (36 inches in El Paso County) are subject to freeze-thaw displacement. Over time, frost action can shift pipes and create low spots that trap standing water. Pipes should be installed below frost-line depth where they exit the structure wall, and any sections running through the wall should be insulated with closed-cell pipe insulation where they pass through the heated-to-exterior transition zone.

Clay soil: Some Colorado Springs properties — particularly in lower elevation areas and older neighborhoods — have clay-heavy soil that drains slowly. A standard French drain in clay soil can become saturated and stop functioning during a sustained wet period. In clay soil, use a larger gravel bed (18–24 inches wide rather than 12), specify clean washed stone (not crusher fines, which clog), and consider a geotextile-wrapped drain pipe with a larger diameter (6-inch rather than 4-inch) to handle higher flow.

Monsoon timing: The drainage system needs to handle the maximum flow rate that a monsoon event produces — typically 1–2 inches per hour. Size the sump basin and pump (if used) for this flow rate, not the average precipitation rate. A sump basin that handles normal seepage may be overwhelmed by a July monsoon event without adequate capacity.

Snow load drainage: In winter, snow accumulating on the glazed roof melts progressively. The drainage system needs to handle snowmelt in addition to rain. On the south-facing glazed surface, snow melts faster than on any other exposure — moderate February sun can produce significant melt even when ambient temperatures are below freezing. This melt runs off the roof edge and, if grade management isn’t adequate, runs toward the pit wall.

What Happens When Drainage Fails

A walpini without adequate drainage doesn’t fail dramatically. It fails slowly, in ways that look like plant problems before they look like drainage problems.

Roots sit in water that doesn’t drain. Soil oxygen is displaced by water and root respiration fails. Plants show yellowing, wilting at the base, and root rot — symptoms that can be mistaken for nutrient deficiency or disease. The growing medium compacts and loses its structure. Mold establishes on the wall surfaces and eventually on plant material.

By the time drainage failure is obvious — standing water on the floor, chronically wet walls, structural issues with earth berms softened by saturation — the growing environment has typically been compromised for weeks or months.

The investment in drainage during construction is the investment that makes the walpini worth building. A properly drained walpini at 6,035 feet, with adequate thermal mass and appropriate glazing, grows tomatoes, peppers, and greens through a Colorado winter. An improperly drained one grows mold.

For a free estimate on walpini construction or drainage retrofit in the Colorado Springs area, call (719) 243-9718.