What does it actually cost to water your greenhouse or polytunnel?

What does it actually cost to water your greenhouse or polytunnel?

Most gardeners think about watering in terms of time and effort, but have you ever stopped to think about what it's actually costing you in pounds and pence?

With UK water bills rising sharply, and heatwaves making growing spaces thirstier than ever, it's worth doing the maths. The results might surprise you — and they make a compelling case for watering smarter.


First: what does water actually cost in the UK?

Water supply in the UK costs between 0.12p and 0.35p per litre depending on your water company. Including sewerage charges, the total typically comes to between 0.3p and 0.65p per litre.  We'll use a rounded average of 0.5p per litre (including sewerage) as a working figure throughout this blog. 

National average water bills rose to £603 in 2025/26, an increase of £123 or 26%. With further rises expected, water is becoming a meaningful household expense, particularly for anyone running a productive growing space. 


How much water does a greenhouse or polytunnel actually use?

This is where it gets interesting and where most growers significantly underestimate their usage.

Per plant

Popular greenhouse plants like tomatoes and cucumbers need 2–4 litres per day when they are fully grown and producing fruit. That's per plant, per day, at peak season. 

So let's do some quick maths for a modest greenhouse or polytunnel with 20 tomato plants:

  • At 2 litres per plant: 40 litres/day × 0.5p = 20p per day / £1.40 per week
  • At 4 litres per plant (heatwave): 80 litres/day × 0.5p = 40p per day / £2.80 per week

Across a 16-week growing season (May to August), that's roughly £22–£45 just for your tomatoes, and that's before you count cucumbers, peppers, salad crops, or anything else sharing the space.

A fuller picture

For a small greenhouse or polytunnel of around 6m², a reasonable estimate for peak-season watering across a full range of crops would be 80–150 litres per day. A greenhouse using 1,000 litres per day in spring may need 1,250 litres per day in peak summer, a 25% increase driven by higher temperatures and plant transpiration. 

Using 100 litres/day as a mid-range estimate for a small growing space:

Period Daily usage Daily cost Weekly cost 16-week season
Spring ~80 litres ~40p ~£2.80 ~£45
Peak summer ~120 litres ~60p ~£4.20 ~£67
Heatwave ~150 litres ~75p ~£5.25 ~£84

These figures are for mains-fed watering on a metered supply. If you're on an unmetered tariff, you won't see this directly on your bill  but you're still using the water, and the environmental cost is the same.


The hidden cost: hosepipe waste

Here's where it really adds up. Hosepipes use approximately 1,000 litres per hour - equivalent to most families' entire daily indoor water consumption. 

Even a brief 10-minute hose session delivers around 150–170 litres, which is far more than most plants actually need in one go, with much of it running off the surface before reaching the roots. Much of what a hosepipe delivers is also lost to evaporation, particularly during the heat of the day.

If you're hand-watering or using a hosepipe in a greenhouse or polytunnel, the chances are you're using significantly more water than your plants are actually absorbing and paying for all of it.


What drip irrigation saves

A properly designed drip irrigation system can reduce your water use by 30 to 50%, potentially saving a typical household between £50 and £150 every year, and for a dedicated growing space, the savings can be even more significant. 

Drip systems achieve 90–95% efficiency compared to 70–80% for sprinklers, because water is delivered directly to the root zone rather than onto the surface or foliage where it evaporates. Drip irrigation systems achieve over 90% efficiency compared to sprinklers at 50–60%. 

To put that in practical terms: if you're currently using 150 litres per day to water a polytunnel with a hosepipe or sprinkler, switching to drip irrigation could bring that down to 75–90 litres per day for the same result. At 0.5p per litre, that's a saving of around 30–37p per day, or roughly £33–£42 over a full growing season, just from the water itself.

Over several years, the savings from a well-designed drip system can comfortably offset the cost of the system.


The water butt advantage

If you collect rainwater and use it to feed your irrigation system, you're not just saving money, you're protecting yourself from hosepipe bans and drought restrictions entirely.

A UK allotment half-plot needs 2,500–5,000 litres of water per growing season. Rainwater harvesting from a 10m² shed or greenhouse roof captures 6,400 litres per year at zero cost. 

That means a single water butt fed from a greenhouse or polytunnel roof could supply a significant proportion of your seasonal watering needs, for free. The WaterMate is designed to pump directly from a tank or water butt, so you can combine harvested rainwater with smart irrigation scheduling for the most efficient setup possible.


Making every drop count

The good news is that smarter watering can save you money. A few practical steps can dramatically reduce how much water your growing space needs:

Water at the right time. Watering in the early morning or evening reduces evaporation by up to 50% compared to watering during the heat of the day. 

Mulch wherever you can. Mulching reduces evaporation by 60–70% from bare soil.  Even a thin layer of compost around your plants makes a meaningful difference. 

Water deeply, not often. Frequent light watering keeps roots shallow and plants more vulnerable to drought stress. A deeper, less frequent soak encourages roots to grow down into cooler, moister soil.

Let the temperature do the thinking. The WaterMate's built-in temperature sensor allows you to automatically adjusts watering based on conditions -more water on hot days, less when it cools down - so you're never over or under-watering regardless of what the weather does.


The bottom line

Watering a greenhouse or polytunnel properly through a UK growing season costs somewhere between £45 and £100 in mains water - more in a hot summer, less in a cool one. Switch to drip irrigation fed from a water butt, and you can cut that figure dramatically, potentially to almost nothing if your harvested rainfall covers your needs.

The WaterMate was designed with exactly this in mind: efficient, root-zone watering from a tank or mains supply, on a schedule that responds to real conditions. It's not just about convenience, it's about making every litre count.

Explore the WaterMate range →


Sources & further reading

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