Ollas in Greenhouses and Polytunnels, BabaBerry olla guide

Ollas in Greenhouses and Polytunnels: Watering Under Cover Without the Humidity Spike

14 min read

A greenhouse or polytunnel is the most demanding irrigation environment most gardeners will ever manage. Temperatures run well above outdoor ambient on a sunny day, no rain ever reaches the soil, and the border beds dry out faster than anything outside — yet the same structure that drives that demand also traps moisture in the air, and high humidity under cover is the primary driver of grey mould and downy mildew[1][2]. Buried unglazed clay pots resolve that tension neatly: they deliver water below the soil surface, driven by soil suction rather than a timer, so the root zone stays evenly moist while the air stays comparatively dry. This guide covers olla layout in border beds, refill logistics in a confined space, the humidity argument in detail, and how to shut the system down for winter.

Under cover, ollas do two useful things at once: they hold steady moisture in fast-drying tunnel soil, and they add far less water vapour to the air than overhead or surface watering, because there is almost no wet soil surface to evaporate from. That matters because botrytis and downy mildew are humidity-driven diseases and a polytunnel is already a humidity trap. Expect to refill more often than outdoors — every two to four days in summer — and plan the layout so every neck is reachable from a path. Drain or lift the pots before hard frost in unheated structures.

01 · THE UNDER-COVER PROBLEM

Hot, rainless, and drying fast

Three things make protected growing thirsty. Air temperature under glass or polythene on a clear summer day runs far above outdoor ambient, and even a well-vented tunnel spends hours at temperatures that push transpiration hard[3]. Rainfall is zero — every drop of water in that soil is one you put there. And the crops grown under cover are the high-demand ones: tomatoes, cucumbers, peppers, aubergines, and melons, all with large canopies and heavy fruit loads.

The result is border soil that can go from adequately moist to genuinely dry in a couple of days in midsummer, particularly along the edges of a tunnel where the polythene meets the ground and the soil is warmed from both sides. Gardeners compensate by watering heavily and often, usually with a hose or can onto the surface, and that is where the second problem starts.

Surface watering under cover puts a large area of wet soil into an enclosed space with limited air exchange. That water evaporates into the tunnel atmosphere and drives relative humidity up, especially overnight as temperatures fall and the air reaches saturation. If you have ever opened a polytunnel on a summer morning to condensation running down the inside and dripping onto the leaves, you have seen the mechanism at work.

Unglazed clay olla pot giving slow-release subsurface irrigation to tomato plants growing in a protected border bed
FIGURE 01 · OLLA IRRIGATION FOR TOMATOES UNDER COVER — WATER DELIVERED BELOW A DRY SOIL SURFACE

02 · THE HUMIDITY ARGUMENT

Why a dry soil surface matters under glass

Grey mould (Botrytis cinerea) is the defining disease of protected cropping. It thrives at high relative humidity, infects through wounds and senescing tissue, and is most damaging when leaf surfaces stay wet or when free condensation forms — which is precisely what happens on a cool night in a humid tunnel[1][4]. Cucumber downy mildew and several leaf spot diseases have similar requirements: they need leaf wetness or very high humidity to establish[5].

Standard advice for managing these diseases under cover is consistently about moisture management rather than spraying: ventilate to exchange humid air, avoid wetting foliage, water early in the day rather than late so surfaces dry before nightfall, and space plants for airflow[6].

An olla contributes directly to two of those. It never wets foliage, because water leaves the pot below ground. And it leaves the soil surface dry, which removes the largest single evaporating surface in the structure. The water still enters the plants and leaves through transpiration — you cannot make a tomato crop humidity-neutral — but you eliminate the additional vapour load from bare wet soil, and you eliminate the sharp humidity spike that follows a hose-watering session.

Be realistic about the size of the effect. Ollas are not a substitute for ventilation, and no irrigation method will keep a sealed unvented tunnel dry. Treat subsurface delivery as one lever among several: vent properly, keep the pathways dry, avoid evening watering, and let the ollas remove the wet-soil-surface contribution.

Traditional terracotta olla buried in a border bed for efficient water conservation in a greenhouse growing space
FIGURE 02 · SUBSURFACE DELIVERY LEAVES THE SOIL SURFACE DRY — LESS VAPOUR LOAD IN AN ENCLOSED SPACE

03 · HOW THE POT MEASURES DEMAND

Soil suction sets the rate, so hot days self-adjust

Water crosses the porous wall of an unglazed olla down a water-potential gradient. Soil that has dried holds its remaining water at a strongly negative matric potential and pulls hard; soil near field capacity pulls weakly and the outflow slows almost to a stop[15]. There is no valve, no timer, and no pressure regulator involved — the soil does the metering[13].

Under cover this behaviour is more valuable than outdoors, because tunnel conditions swing so hard. A cloudy 60°F day and a clear 95°F day in the same week produce completely different transpiration rates, and a fixed drip schedule will be wrong on at least one of them. The olla is right on both, because it responds to the soil rather than to the calendar.

There is a corollary worth knowing: your refill interval becomes a crude but genuinely useful measure of crop demand. Pots draining in two days tell you the tunnel is running hot and the crop is working; pots barely moving after a week tell you the structure is cool and the plants are idling. Combined with a finger check at depth, that is a better feedback loop than most timer-driven systems give you.

Because delivery is subsurface, near-zero water is lost to evaporation and none to runoff, which is the reason buried clay pot irrigation is repeatedly rated among the most water-efficient methods available for small intensive plantings[12][14]. In a structure where every litre is carried in by hand, that efficiency is felt directly in your arms.

04 · LAYOUT IN BORDER BEDS

Setting the pots out in a narrow space

Tunnel and greenhouse borders are typically long and narrow — often 2 to 4 feet wide with a central path. That geometry is actually convenient: a single line of ollas down the middle of each border, with crop plants set on either side, covers the bed efficiently.

  • Keep every neck reachable from the path. This is the constraint that should drive your layout. Once tomatoes are 6 feet tall you will not want to reach across a border with a full can. Put the fill openings on the path side of the bed.
  • Space so the plumes overlap. In loam, a mid-size olla wets roughly 12–18 inches around itself; less in sand, more in clay. Set pots so those zones meet rather than leaving dry gaps between them, and plant into the wetted zones.
  • Add extra capacity at the tunnel edges. The strip where the polythene meets the ground warms and dries fastest. If you crop right to the edge, plan on closer spacing or an extra pot there.
  • Install before planting, every time. Dig in, backfill firmly so the clay contacts the soil with no air gap, fill once to charge the surrounding soil, then plant. Retrofitting into an established tomato border cuts roots at the worst possible moment.

If your structure is planted with raised beds rather than direct border soil, the sizing rules shift a little because bed walls confine the moisture plume and edges dry faster — our raised bed olla setup guide covers that case, and how many ollas per square foot turns bed dimensions into a count.

Olla irrigation method set up in a sustainable vegetable growing bed of the kind used in a polytunnel border
FIGURE 03 · A LINE OF OLLAS DOWN A NARROW BORDER — NECKS KEPT ON THE PATH SIDE FOR REFILLING

05 · REFILL LOGISTICS

The practical reality of keeping them full

Handmade buried olla clay pot with its fill neck accessible for topping up in a covered growing bed
FIGURE 04 · THE FILL NECK IS THE ONLY MAINTENANCE POINT — KEEP IT REACHABLE AND KEEP IT CAPPED

Under cover you will refill more often than outdoors. In peak summer with a full tomato or cucumber crop, expect every two to four days; in spring and autumn, weekly or less. This is the honest cost of the method, and it is worth planning around rather than discovering in August.

01 · Hose with a wand

Fastest

A long lance lets you fill necks along a border without stepping into the bed. Low flow so you do not overshoot and wet the surface.

02 · Long-spout can

Most precise

Best where there is no tap in the structure. A narrow spout drops water straight into the neck with no splash onto foliage.

03 · Butt and siphon

Least effort

A raised water butt at one end feeds a flexible hose by gravity. Useful in tunnels without mains water; filter to keep debris out of the pots.

04 · Check routine

Every 2–4 days

Summer interval with a full crop. Lift each lid, look, top up. Do it in the morning, at the same time as venting.

Keep the lids on. An open olla in a warm greenhouse loses water to evaporation from the neck, collects fallen leaves and compost that clog the wall from inside, and becomes standing water where fungus gnats and mosquitoes breed[7].

One thing ollas do not solve is feeding. Do not put liquid fertiliser inside the pots — dissolved salts and organic residues clog the porous wall and reduce seepage over time, and salts can accumulate in the soil immediately around the pot. Feed separately at the surface or through a foliar programme, and keep the ollas for clean water only.

06 · SALTS, SOIL, AND MAINTENANCE

Keeping the system working season after season

Protected soil has a specific long-term problem: because rain never leaches it, salts from irrigation water and fertiliser accumulate over the years. This is a well-recognised issue in greenhouse border soils and shows up as reduced growth, leaf margin scorch, and a white crust on the surface[8].

Ollas do not cause this problem, but they do not fix it either — subsurface delivery moves less water down through the profile than a heavy flood irrigation, so it leaches less. If your water is hard or your borders have been cropped for years, plan an annual deep leaching wash with a hose, ideally in the off-season, and consider using collected rainwater in the pots where you can. Rainwater is also gentler on the clay wall itself, since hard water gradually deposits mineral scale in the pores.

If seepage slows noticeably between seasons, lift the pot and clean it. A stiff brush and a soak in a dilute vinegar solution will shift mineral scale from the pores; rinse thoroughly afterwards. Avoid detergents and never glaze or seal a working olla, which defeats the entire mechanism.

Also keep an eye on root intrusion. Vigorous crops sometimes send fine roots into the damp soil immediately against the clay wall, and dense root mats can slow outflow. This is generally harmless and easily dealt with by lifting and brushing off the pot at the end of the season.

07 · WINTER SHUTDOWN

Closing the system down at the end of the season

An unheated greenhouse or polytunnel gets cold. It offers frost protection of only a few degrees on a still night, and the soil in the borders will freeze in a hard winter in a temperate climate. Water inside a buried terracotta pot expands as it freezes and cracks the wall — the single most common way gardeners lose ollas between seasons.

The routine is simple. When the summer crop is cleared, stop refilling and let the pots draw down. Before the first hard frost, lift them, empty them completely, dry them, and store them somewhere frost-free. If lifting is impractical, at minimum bail them dry and leave the lids off so no rain or condensation collects.

Winter crops under cover — salads, hardy greens, overwintering alliums — usually need very little water anyway. Cool temperatures, low light, and minimal transpiration mean a monthly hand-watering is often sufficient, and keeping the soil on the dry side actively reduces botrytis pressure through the damp months[9]. This is not the season for a system built to supply steady moisture.

In a heated greenhouse held above freezing, you can leave the pots in place year-round; just reduce refilling sharply in winter and check that the reduced light and cooler root zone are not keeping the soil sodden. If in doubt, take the pots out and hand-water — a wet winter border under glass is a recipe for root disease[10].

If you also grow in pots on the greenhouse staging, note that the sizing logic there is different again — see the best ollas for container gardening.

ACQUA OLLA

Steady root-zone moisture in fast-drying tunnel soil — without the humidity spike that follows a hose.

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Greenhouses and polytunnels ask for a lot of water and punish you for adding it the obvious way. Buried clay pots let you keep border soil evenly moist through the hottest weeks while leaving the surface dry and the air comparatively less humid — a genuine advantage where botrytis and downy mildew are the diseases you actually fight. Run a line of pots down each border with the necks on the path side, install before planting, expect to refill every two to four days in summer, keep fertiliser out of the pots, and drain or lift them before hard frost. Ventilation still does the heavy lifting on humidity; ollas simply stop you making it worse.

Frequently asked questions

Do ollas work in a greenhouse or polytunnel?

Yes, and protected growing is one of their better applications. Tunnel soil dries fast because temperatures are high and no rain reaches it, and an olla holds the root zone evenly moist while releasing water below the surface. The main practical difference from outdoors is that you refill more often — every two to four days in summer with a full crop.

Do ollas reduce humidity in a polytunnel?

They reduce the humidity spike you get from overhead or surface watering, because the soil surface stays dry and there is far less area evaporating into the air. That matters because botrytis and downy mildew are humidity-driven. They do not replace ventilation — plants still transpire — so keep venting, avoid evening watering, and space plants for airflow.

How often do I refill ollas under cover?

Every two to four days in peak summer with tomatoes or cucumbers, and weekly or less in spring and autumn. Check in the morning at the same time as you vent. Refill rate is a useful demand gauge: pots emptying in two days mean the structure is running hot and the crop is working hard.

Where should I put ollas in a greenhouse border?

Run a single line down the middle of each narrow border with plants either side, and keep every fill neck reachable from the path — you will not want to lean over 6-foot tomatoes with a full can. Space pots so their moisture zones overlap, and add extra capacity along the tunnel edges, which dry fastest.

Can I put liquid feed in an olla?

No. Dissolved salts and organic residues clog the porous clay wall and reduce seepage over time, and salts can build up in the soil right around the pot. Use clean water — ideally rainwater — in the ollas and feed separately at the soil surface.

What do I do with ollas over winter?

In an unheated structure, stop refilling once the summer crop is cleared and lift, empty, dry, and store the pots before the first hard frost. Water inside buried terracotta expands as it freezes and cracks the wall. Winter crops under cover need very little water anyway, and drier soil reduces botrytis pressure through the damp months.

Do salts build up in greenhouse soil with olla irrigation?

Salt accumulation is a known issue in any protected border soil because rain never leaches it. Ollas neither cause nor cure it, but they move less water down through the profile than flood irrigation, so they leach less. Plan an annual deep flushing with a hose in the off-season, and use rainwater in the pots where possible.

References

  1. [1] Royal Horticultural Society. (n.d.). Grey mould (Botrytis) in the greenhouse. rhs.org.uk.
  2. [2] Michigan State University Extension. (n.d.). Managing humidity in greenhouses to reduce disease. canr.msu.edu.
  3. [3] Penn State Extension. (n.d.). Greenhouse environment: temperature, ventilation, and moisture. extension.psu.edu.
  4. [4] University of Wisconsin–Madison Division of Extension. (n.d.). Botrytis blight. hort.extension.wisc.edu.
  5. [5] Clemson Cooperative Extension. (n.d.). Cucumber and tomato diseases under cover. Home & Garden Information Center. hgic.clemson.edu.
  6. [6] University of Minnesota Extension. (n.d.). Watering and disease management in high tunnels. extension.umn.edu.
  7. [7] United States Environmental Protection Agency. (n.d.). Reducing standing water and mosquito breeding sites. epa.gov.
  8. [8] University of California Agriculture and Natural Resources. (n.d.). Salinity management in protected and container culture. ucanr.edu.
  9. [9] University of Illinois Extension. (n.d.). Overwintering vegetables in high tunnels. extension.illinois.edu.
  10. [10] North Carolina State Extension. (n.d.). Root and crown rots in protected culture. Plant Toolbox. plants.ces.ncsu.edu.
  11. [11] Texas A&M AgriLife Extension. (n.d.). Greenhouse vegetable production and irrigation. agrilifeextension.tamu.edu.
  12. [12] Food and Agriculture Organization of the United Nations. (n.d.). Water-efficient irrigation for intensive small-scale production. fao.org.
  13. [13] Bainbridge, D.A. (2001). Buried clay pot irrigation: a little known but very efficient traditional method of irrigation. Agricultural Water Management, 48(2), 79–88.
  14. [14] Bainbridge, D.A. (2002). Alternative Irrigation Systems for Arid Land Restoration. Dry Lands Research Institute.
  15. [15] Hillel, D. (2004). Introduction to Environmental Soil Physics. Elsevier Academic Press.
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