Watering is the job a greenhouse makes harder rather than easier. Outdoors, rain covers the baseline and soil buffers the rest. Under cover there is no rain at all, the root volume is often a fraction of what an outdoor plant has, the evaporative demand is higher because the air is warmer, and nothing is ever leached. Every one of those pushes in the same direction: watering under cover has to be more deliberate.
It is also the variable behind more crop failures than any other. Blossom end rot, fruit splitting, bolting, tip burn and most root disease all trace back to water applied irregularly, in the wrong quantity, at the wrong time of day, or in the wrong place.
How much, and when it changes
Published greenhouse guidance puts peak demand at roughly 0.1 gallons per square foot of growing area per day for cool-season greens, rising to about 0.4 for a mature tomato crop under full light. That is a fourfold range across crops, and there is another fivefold range across the seasons.
| Crop | Peak summer | Shoulder season | Winter |
|---|---|---|---|
| Seedlings and plug trays | 0.08 gal | 0.05 gal | 0.02 gal |
| Lettuce and salad greens | 0.15 gal | 0.10 gal | 0.03 gal |
| Herbs | 0.18 gal | 0.12 gal | 0.04 gal |
| Ornamentals in containers | 0.20 gal | 0.14 gal | 0.04 gal |
| Strawberries | 0.22 gal | 0.15 gal | 0.04 gal |
| Cucumbers and melons | 0.35 gal | 0.24 gal | 0.07 gal |
| Tomatoes at full canopy | 0.40 gal | 0.28 gal | 0.08 gal |
The winter column is the one that catches people. A schedule that suits a tomato crop in high summer applied in the cold months delivers five times too much water, into a medium that is not drying because the plant is barely transpiring and the air is cool. That is how root rot happens in a greenhouse in winter, and it is a timer left on a summer program rather than a disease problem.
Work your own figures with the water use calculator, which corrects for delivery method, container size and season, and then check what your roof collects against it with the rain barrel calculator.
Morning, always
Water in the morning. This is one of the few pieces of greenhouse advice that is unambiguous, and there are three separate reasons for it.
Foliage and surfaces dry before dark, which removes the hours of leaf wetness that botrytis and downy mildew require. The plant gets its water at the start of the period when it is transpiring hardest, so it is never trying to catch up through the afternoon. And a wetted floor evaporates through the day, cooling the house and softening the vapour pressure deficit at exactly the hottest hours.
Evening watering does the opposite of all three. Water sits on surfaces all night, humidity stays at its peak through the dawn condensation window, and the cooling effect lands when the house is already cooling on its own.
Deep and infrequent for beds, short and frequent for containers
These are genuinely different regimes and applying one to the other is a common error.
A bed holds a large water reserve, and roots follow moisture downward. Watering deeply and less often draws roots down into that reserve, which makes the plant far more resilient to a missed day. Watering a bed lightly every day trains roots into the top two inches, which is the part that dries first and swings most.
A container holds a small reserve and dries from the whole volume at once. A 5 gallon grow bag under a mature tomato in summer can move from field capacity to stress within a day. Containers therefore want shorter, more frequent cycles, and they want them from a timer rather than from a person, because the margin for a missed day is close to zero.
The volume rule that works for both: water until a small amount runs out of the bottom, which confirms the whole profile is wetted rather than just the surface. In a bed on the ground you cannot see that, so a SONKIR MS02 3-in-1 Soil Moisture, Light and pH Meter ($9.99) used as a moisture probe at depth is worth more than a guess.
Method: keep the water off the leaves
| Method | Leaf wetness | Evenness | Best for |
|---|---|---|---|
| Hand watering with a wand | Depends entirely on technique | Poor, varies with attention | Small houses, mixed plantings, anyone who wants to look at the crop daily |
| Drip emitters | None | Excellent, and repeatable | Beds, containers, anything on a timer |
| Soaker or weep hose | None | Good along a row, less so across one | Bed rows, the cheapest way to automate a bed |
| Capillary matting | None | Very good for uniform trays | Propagation benches full of identical pots |
| Overhead sprinkler | Total | Moderate | Almost nothing under cover; it wets every leaf in the house |
| Misting | Deliberate | Fine mist rather than volume | Rooting cuttings only, where leaf wetness is the point |
A HIRALIY 59 ft Quick-Connect Drip Irrigation Kit ($14.99) on an Orbit 62041 Single-Zone Hose Timer ($19.99) is the cheapest complete automation available and it removes the single largest source of inconsistency. For a house with beds and trays that need different schedules, a Melnor AquaTimer 4-Zone Programmable Digital Timer ($69.99) runs four independent programs from one tap, which is what stops a schedule that suits mature tomatoes from drowning a tray of seedlings.
For a bed specifically, a Flexon 5/8 in x 50 ft Weep and Soak Garden Soaker Hose ($14.99) laid along the row and buried under an inch of mulch is the least fiddly option, with the caveat that soaker hoses deliver less at the far end and want a shorter run than instinct suggests.
Water quality and the salt problem
This is the greenhouse-specific issue that almost no general gardening advice covers, because outdoors it does not exist.
Tap water contains dissolved minerals, and hard water contains a lot of them. Outdoors, rain moves through the soil profile and carries those salts downward and away, a process called leaching, and it happens continuously without anyone thinking about it. Under cover there is no rain at all. Every gallon of tap water applied to a bed or a container leaves its dissolved solids behind when the water is taken up or evaporates, and the concentration climbs season after season.
The symptoms arrive gradually and get blamed on other things: growth slows, leaf margins scorch, germination gets patchy, and a white crust appears on the surface of the medium and on the outside of clay pots. By the time the crust is visible the concentration is already high.
- Use rainwater where you can. Rainwater has essentially no dissolved solids, which is exactly what a never-leached medium needs. A 32 mm Greenhouse Gutter and Water Butt Collection Kit ($23.39) feeding a RTS Home Accents 50 Gallon Rain Barrel ($79.00) turns roof runoff into the right water for the job.
- Leach beds deliberately. Once or twice a year, apply enough water to move a large volume right through the profile, ideally rainwater. This is the artificial substitute for the rain the bed never gets.
- Feed less than the label says in a permanent bed. Fertiliser salts accumulate exactly like tap water salts, and greenhouse beds concentrate nutrients rather than losing them.
- Replace container medium annually. The simplest solution for anything in a pot or a grow bag is not to fight the accumulation at all.
- Filter a rainwater feed. Rainwater carries debris and algae, and drip emitters are unforgiving about both. An in-line filter is not optional on a rainwater-fed drip system.
Watering equipment
Watering interacts with bed depth: a deeper bed holds a larger reserve and forgives more. That is worked through in raised beds inside a greenhouse, and the medium side of the same question is in greenhouse soil and growing media.