How much water do plants need in a greenhouse? On the hottest summer days, a full greenhouse can use about 0.3 to 0.4 gallons per square foot of growing area. That planning number comes from UMass Extension. In winter, with less sun, plants use far less. The best guide is your plants and soil, not the calendar.
โ Quick answer
- Peak summer: plan for about 0.3 to 0.4 gallons per sq ft of beds and benches each day.
- Water deeply: keep going until a little water drains out. UMass suggests at least 10% runoff.
- Water early: morning watering lets leaves dry before the cool, damp night.
- Tomatoes: about 0.1 gallon a day when young, rising to about 0.75 gallon when full-grown.
- Check first: water when the top inch of soil feels dry, not on a fixed schedule.
We are a small, woman-owned greenhouse business in Georgia. Watering is one of the questions we hear most from new greenhouse owners. This guide shares the real numbers, a simple formula and the habits we suggest to our customers.
Why do greenhouse plants need different watering?
Outdoors, rain does part of the job. Inside a greenhouse, every drop comes from you. The climate under the roof also changes how fast plants drink.
- Sun and heat. Bright days pull water from leaves and soil fast. UMass notes summer sun can be two to three times stronger than winter sun.
- Humidity. Closed air stays moist, so soil dries slower. Too much moisture invites mold and leaf disease.
- Containers. Pots dry out much faster than ground beds. Small pots and hanging baskets dry fastest.
- Air movement. Fans and open vents raise water use. UMass says a 5 mph breeze can raise it by about 20%.
- Plant size. A big tomato in fruit drinks many times more than a seedling.
How much water do plants need in a greenhouse each day?
Commercial growers use a simple rule to size their water supply. It works for a backyard greenhouse too. It tells you how much water your plants could use on the hottest day of the year.
๐ The peak daily water formula
Gallons per day โ Growing area (sq ft) ร 0.3 to 0.4
Growing area = beds and benches, not the whole floor ยท Add about 10% for runoff ยท Source: UMass Extension
๐งฎ Worked example: a MONT 8x12 in July
- Two 2 x 10 ft beds along the sides = 40 sq ft of growing area
- 40 ร 0.3 = 12 gallons, and 40 ร 0.4 = 16 gallons
- Add 10% for runoff = about 13 to 18 gallons on the hottest days
That is two or three full 5-gallon buckets. In the Riverstone MONT 8x12, a timer system can handle much of that for you. On cool, cloudy days you will need far less.
๐ Good to know: this is a planning number for your hose, rain barrel or timer. It is not a daily target. Always check the soil before you water.
How much water do common greenhouse crops need?
Vegetable crops have the best research. These numbers come from greenhouse crop guides by the British Columbia Ministry of Agriculture. They are for plants grown for fruit, so a small pot of herbs will use far less.
| Crop | Young plant | Full-grown plant | What to watch |
|---|---|---|---|
| Tomato | About 0.1 gallon a day | Up to about 0.75 gallon a day | Uneven watering can cause blossom-end rot |
| Pepper | About 0.1 gallon a day | Up to about 0.8 gallon a day | Needs steady moisture as fruit fills out |
| Cucumber | Starts small | 1 to 1.75 gallons a day | Big leaves lose water fast in sun |
| Seedlings and greens | Small sips, often | Even moisture | Soggy trays invite damping-off |
Figures from the BC Ministry of Agriculture Trickle Irrigation Manual for greenhouse crops. Your plants may use more in a hot spell and less in cool weather.
The University of Missouri notes that tomatoes short on water set fewer flowers and less fruit. They may also get blossom-end rot. So steady watering matters as much as the amount.
How can I tell how much water my plants are using?
Here is an easy trick for potted plants. It uses a bathroom scale and turns guessing into a real number.
- Water well in the morning. Let the pot drain for about half an hour.
- Weigh the pot. Write down the number.
- Weigh it again the next morning at the same time.
- Find the difference. One pint of water weighs about one pound. So each pound lost is about a pint used.
๐ก Our tip: if a pot weighs 12 lbs after draining and 9 lbs the next morning, it used about 3 pints. Repeat this in summer and winter. You will see how much the season changes things.
How often should I water plants in a greenhouse?
There is no single schedule. A pot that needs water daily in July may go a week in January. Use these simple checks instead:
- Finger test: push a finger an inch into the soil. If it feels dry, water.
- Lift test: a dry pot feels light. A watered pot feels heavy.
- Look at the soil edge: soil that pulls away from the pot is very dry.
| Season | How often to check | Watering tips |
|---|---|---|
| Summer | Every day; twice on hot days for small pots | Water deeply in the morning. Hanging baskets may need a second drink. |
| Spring and fall | Every day or two | Water less on cloudy days. Watch for cold nights. |
| Winter | Every few days | Water only on sunny mornings. Cool soil stays wet longer. |
When is the best time to water a greenhouse?
Water in the morning. Plants have water for the hot part of the day, and leaves dry before evening. Commercial tomato and cucumber growers often start watering about two hours after sunrise.
Evening watering leaves soil and leaves wet through a cool night. That damp, still air is just what mold and mildew love. In winter, it also chills the roots.
Good airflow helps too. Our guide to greenhouse vent positioning shows how to move damp air out.
What are the signs of overwatering and underwatering?
Both problems can make a plant droop, which is confusing. The soil tells you which one it is.
| Where to look | Too much water | Too little water |
|---|---|---|
| Leaves | Yellow lower leaves; limp even though soil is wet | Wilting; dry, crispy edges |
| Soil | Soggy, green film on top, fungus gnats | Dry, light, pulling away from the pot |
| Roots | Brown, mushy, sour smell | Dry and stringy |
| Fruit | Split fruit after a big drink | Blossom-end rot on tomatoes and peppers |
โ ๏ธ Before you water a wilting plant: check the soil first. On very hot afternoons, many plants droop for a while and then perk up by evening. Extra water on wet soil only makes things worse.
Which greenhouse watering system is best?
The best system matches your plants and your schedule. Here is how the main options compare:
| Method | Best for | Pros | Watch out for |
|---|---|---|---|
| Watering can or wand | Small greenhouses and pots | Precise and cheap; you see every plant | Takes time every day |
| Overhead sprinkler on a timer | Benches of trays and seedlings | Automatic; easy to set up | Wets leaves; dense foliage blocks water |
| Drip lines and emitters | Beds, big pots and tomatoes | Water goes right to the roots | Emitters can clog; check them often |
| Misting system | Cuttings, humidity and cooling | Raises humidity on hot days | Not a main way to water |
UMass notes that with overhead watering, as little as 20% of the water may reach the soil in a crop with heavy leaves. That is why we like drip or hand watering for big plants, and overhead watering for seedling trays.
Two watering systems we carry
- MONT Automatic Greenhouse Watering System: hangs from the ceiling with weighted sprinkler heads. Each head sprays a 4 ft circle. A digital timer sets the cycles. Sizes run from 1 head for an 8x8 to 5 heads for an 8x24. It works in any greenhouse. About $295 when we wrote this (October 2026), with a 7-year warranty.
- Misting System for Greenhouses: a 14 ft line with 5 misters that you can cut shorter. It suits a greenhouse up to about 20 ft long. About $130 when we wrote this (October 2026).
*Prices checked in October 2026. Prices can change and sales may apply, so click the model name to see today's price.
๐ About prices: manufacturers update their prices during the year, and we often run sales and promotions. The price on the product page is always the current one. Prices in this article are a guide from when we wrote it (October 2026), not a quote or guarantee. Found a lower price for the same model at an authorized retailer? We offer a Price Match Guarantee - please check the rules first. Call us and we will confirm today's price and any current deals.
See every option in our greenhouse watering system collection. We ship free to the continental US.
How can I save water in my greenhouse?
Good habits save water and grow healthier plants. These are the ones we share most with our customers:
- Water the soil, not the leaves. Aim at the base of each plant.
- Group plants by thirst. Keep tomatoes together and succulents apart.
- Mulch your beds. A layer of straw or compost slows drying.
- Collect rainwater. The MONT has a built-in rain gutter. Our guide to eco-friendly greenhouse practices shows how to use it.
- Use bigger pots. More soil holds more water. Our container gardening guide helps you pick sizes.
- Add shade in summer. Less stress means less water loss on the hottest days.
Growing through the cold months? Winter greens need much less water. Our guide on how to grow greens in the chilly season covers winter care.
Frequently asked questions
How much water do plants need in a greenhouse?
On the hottest days, a full greenhouse may use 0.3 to 0.4 gallons per square foot of growing area. Most days need less. A greenhouse tomato uses about 0.1 gallon a day when young and up to about 0.75 gallon when full-grown.
How often should I water my greenhouse in summer?
Check every day. Water deeply in the morning when the top inch of soil is dry. Small pots and hanging baskets may need water twice on very hot days.
Do greenhouse plants need more water than outdoor plants?
In summer, often yes, because there is no rain and the air is warmer. In winter, they usually need less. The closed air stays humid and the soil dries slowly.
Is it better to water greenhouse plants in the morning or evening?
Morning is best. Plants get water before the heat, and leaves dry before night. Evening watering keeps things damp overnight, which invites disease.
Should I water my greenhouse plants every day?
Only if they need it. Use the finger test or lift the pot. Watering on a fixed schedule is the most common cause of overwatering.
What is the best automatic watering system for a small greenhouse?
For seedling benches, an overhead sprinkler on a timer is simple. For beds and big pots, drip lines put water at the roots. Call us and we can help you choose.
Questions about watering your greenhouse?
Tell us your greenhouse size, what you grow and where you live. We will help you pick a watering setup that fits, and let you know about any current sales.