Greenhouse water use calculator

What drives greenhouse water demand

Almost all water a greenhouse uses leaves through the leaves. Transpiration is the process: water is drawn from the medium through the roots, up the stem and evaporated from leaf surfaces, and it is driven primarily by the energy arriving as light and by how dry the surrounding air is.

That gives three practical consequences. Demand tracks light far more closely than it tracks temperature, which is why a bright cool day can use more water than a dull warm one. Demand scales with leaf area rather than with pot size, so a mature tomato uses many times what the same container used when newly planted. And demand rises when the air is dry, which is why a well-ventilated house in summer drinks more than a closed humid one.

Greenhouse demand also exceeds outdoor demand for the same crop, because the plants are warmer, the growing period is longer, and critically no rain ever falls on them. Every drop is applied deliberately, which makes getting the quantity right a genuine planning question rather than a matter of topping up what nature provides.

The seasonal swing, which is the number that matters

Demand varies roughly fivefold between high summer and midwinter, and this is the figure that catches people out.

In summer, a mature fruiting crop under full light can use 0.35 to 0.4 gallons per square foot of growing area per day. A single 5 gallon container of tomatoes can exceed a gallon a day on its own. Missing two days in high summer produces blossom end rot from calcium transport failure and split fruit from irregular water supply, both of which are watering faults rather than diseases.

In winter, the same house holding hardy greens uses perhaps 0.03 to 0.06 gallons per square foot per day. Growth is minimal, light is low, transpiration is slow and the air is often near saturation. At this point overwatering rather than underwatering is the risk, and winter root rot from a timer left on a summer schedule is a more common loss than winter cold.

The practical implication is that an irrigation timer must be reprogrammed seasonally. A four-zone timer makes this manageable, since the beds, containers, bench and propagation zones can be adjusted independently rather than all at once.

Storage sizing

Once daily demand is known, storage follows from how many days of autonomy you want.

For a greenhouse fed from mains water, storage is only needed if you intend to use captured rainwater or to buffer against being away. Three to five days of peak demand is a reasonable target for a hobby house, which for a 36 square foot house at 6 gallons a day is 18 to 30 gallons, comfortably inside a single 50 gallon barrel.

For a house relying entirely on captured rainwater, the sizing question is different and harder: storage has to bridge the longest dry spell in the growing season, which in many climates is several weeks. That pushes storage into the hundreds of gallons, and the honest answer for most hobby growers is to use rainwater as the preferred source with mains as backup rather than trying to be fully autonomous.

Storage inside the greenhouse does a second job worth counting. A 55 gallon barrel holds about 458 pounds of water, absorbing roughly 458 BTU per degree F of temperature rise and releasing it as the house cools overnight. Barrels serving as both irrigation reserve and thermal mass are the reason the expert build places them in the heat group rather than the water group.

Water quality under cover

Where the water comes from matters more in a greenhouse than in a garden, for a reason that is specific to growing under cover.

Outdoors, rainfall passes through the soil profile continuously and carries dissolved salts down and away. Under cover, nothing ever leaches. Everything dissolved in the irrigation water stays behind when that water is taken up by the plant or evaporates, and it accumulates season after season.

In a hard water area this builds measurably: carbonates raise pH over time, and sodium and other salts accumulate to concentrations that cause marginal leaf scorch and suppressed growth in a medium that tests as adequately fed. The visible sign is a white crust on the medium surface.

Rainwater avoids this entirely, being soft and close to neutral, and a greenhouse roof is a substantial catchment that most owners never connect. A 10 by 18 house has 180 square feet of roof, and one inch of rain on it yields roughly 112 gallons before losses. The rain barrel capacity calculator sizes this properly.

Where rainwater is used, a filter at the head of any drip system is essential rather than optional, because particulate from a barrel blocks emitters directly and emitters are the most tedious component to clear.

Reading the crop instead of the schedule

A calculated figure is a planning tool. The plants are the actual instrument, and both over and under watering have recognizable signatures that are frequently confused with each other.

Underwatering shows as wilting that recovers overnight, dry medium that has pulled away from the container wall, smaller leaves, and in fruiting crops blossom end rot and split fruit from irregular supply. The medium is visibly and consistently dry.

Overwatering shows as wilting that does not recover overnight, yellowing lower leaves, a sour smell from the medium, fungus gnats, and roots that are brown and soft rather than white and firm. The confusing part is that the plant wilts, because drowned roots cannot take up water any better than dry ones can.

The diagnostic that separates them is simply checking the medium at root depth rather than at the surface. A finger two inches into the medium answers the question immediately, and it is more reliable than any meter. The surface of a greenhouse bed dries far faster than the root zone, and watering in response to a dry surface is the single most common route to overwatering.

One further sign worth knowing: a plant wilting at midday in demonstrably moist medium is not short of water. It is losing water through its leaves faster than roots can supply it, which is a light and heat problem addressed with shade and ventilation rather than with more water.

Working out the figures for your own house and season? The Greenhouse Build & Growing Planner is the paid version of these pages: 8 printable worksheets you fill in with your own numbers, plus the full PDF, $29.