Is your polytunnel or greenhouse getting enough water this summer?
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June is one of the most exciting months in the growing calendar. Everything is surging upwards, the days are long, and the warmth inside a polytunnel or greenhouse can be extraordinary. But that same warmth that's powering your plants' growth is also making unprecedented demands on their water supply, and it's surprisingly easy to fall behind.
Here's how to make sure your plants are getting what they need right now, and what to watch out for as summer really kicks in.
Why June is the most critical watering month
Your plants have moved out of the slow, careful propagation stage and into what growers call the vegetative growth stage, and many are already on the verge of flowering. This is the period where everything happens fast. Root systems are actively drawing moisture, leaves are transpiring at full tilt, and fruit or flower development is just around the corner.
At this stage, underwatering doesn't just slow growth, it can trigger stress responses that are hard to recover from. Blossom end rot in tomatoes, bitter cucumbers, bolting salad crops. Many of the most frustrating summer growing problems trace back to inconsistent moisture, not pests or disease.
The good news? Get watering right in June and you're setting yourself up for a genuinely abundant harvest.
Inside a polytunnel or greenhouse: a different world
Gardens benefit from rain, morning dew, and the moderating effect of open air. A polytunnel or greenhouse is a different environment entirely. On a warm June day, the air inside can be 10–15°C hotter than outside, and there is of course no rain getting in. That combination means:
- Soil dries out far faster than you'd expect, often within hours of watering
- Plants in containers and grow bags are particularly vulnerable as their restricted root zone has limited moisture reserves
- Even on a cloudy day, transpiration inside a polytunnel continues at a high rate
- Temperature spikes in the afternoon can cause wilting even in plants that were watered that morning
The rule of thumb for most gardeners - water when the top inch of soil is dry - really doesn't apply in a summer polytunnel. You need to be thinking in terms of consistent, sustained moisture throughout the root zone, not just the surface.
Signs your plants need more water
Learning to read your plants is one of the most valuable skills a grower can develop. Look out for:
Wilting in the morning. A little afternoon wilt on a very hot day can be normal, but if plants are drooping first thing, they didn't have enough moisture overnight.
Curling or cupping leaves. Many plants curl their leaves inward as a stress response to moisture loss. Tomatoes and cucumbers are particularly expressive.
Slow or stunted growth. If plants that should be racing away seem to have stalled, insufficient water is one of the first things to check.
Dry soil at depth. Push your finger 5–7cm into the soil. If it's dry at that depth, surface watering hasn't been reaching the roots.
Pale or yellowing lower leaves. Often a sign that the plant is diverting resources and shedding older foliage under stress.
How much water do they actually need?
This varies enormously by plant, container size, and weather, but as a rough guide for a summer polytunnel or greenhouse:
- Tomatoes in grow bags: up to 2–3 litres per plant per day at peak season
- Cucumbers: similar, and they'll tell you loudly when they're stressed
- Peppers and aubergines: slightly less thirsty, but consistency matters enormously
- Salad crops in beds: frequent light watering works well as they have shallow roots
For outdoor garden beds, plants in the ground have access to deeper moisture reserves, but during a dry spell they'll still need supplemental watering, particularly vegetables that are fruiting or flowering.
Watering smarter, not just more
The temptation when it's hot is simply to water more, more often. But how you water matters as much as how much. A few principles:
Water deeply and less frequently (where possible) rather than little and often. Deep watering encourages roots to grow down, making plants more resilient.
Water at the base, not the foliage. Wet leaves in a warm polytunnel are an invitation to fungal problems. Direct water to the root zone.
Morning is best. Watering early gives plants moisture to draw on through the day, and any splashes on foliage have time to dry before the heat builds.
Mulch if you can. Even in a polytunnel, a layer of mulch around plants dramatically reduces how quickly the soil surface dries out.
Check soil moisture, not just the surface. The top of a grow bag can look bone dry while there's still moisture lower down, and vice versa. Get your hands in.
When manual watering isn't keeping up
Here's the honest truth: in a hot summer, keeping a fully planted polytunnel or greenhouse properly watered by hand is genuinely hard work. It takes time every single day, and if you miss a day, because of work, or weather, or life, your plants feel it immediately.
This is exactly why so many serious growers eventually make the switch to an automatic irrigation system. Not out of laziness, but because consistent, reliable watering produces better results than even the most dedicated hand-waterer.
The WaterMate from Harvst was designed specifically for this environment - greenhouses, polytunnels, grow houses, and outdoor plots. It's a solar or mains-powered smart irrigation controller that delivers water on a schedule you set, adjusting automatically based on temperature and conditions. On hot days your plants get more. On cooler days, less. And if you are within range of a WiFi network, you can monitor and adjust everything from your phone, even when you're away from home.
The WaterMate Mini covers greenhouses up to 6×8ft and supports up to 40 drippers, while the WaterMate Pro handles larger setups with multiple zones. Both come as complete kits with everything you need to get up and running, including installation hardware specifically designed for polytunnels or greenhouse frames.
If you've been meaning to sort out your watering this season, right now, while plants are hitting their peak demand, is the moment it will make the most difference.
Don't forget the garden
While polytunnels and greenhouses have the most acute watering needs, your outdoor beds and pots deserve attention too. Containers dry out quickly in summer heat, and vegetables in the ground, particularly anything fruiting, benefit from consistent moisture. A good soak twice a week is generally better than light daily watering, and the WaterMate works beautifully for outdoor veg plots and allotments too, as long as the control unit has some shelter from the elements.