How much water does a garden bed need?
A flower bed uses water the same way a lawn does: plants draw it up and release it through their leaves, and more evaporates from the soil. Together these losses are called evapotranspiration. Weather drives most of it, so a bed can need several times more water in a hot, dry week than in a cool, humid one.
Landscape professionals estimate plant water needs in two steps. First they take reference evapotranspiration (ETo), the water a standard, well-watered grass surface would lose in the current weather. Then they multiply it by a plant factor, the share of that water a group of plants needs to stay healthy and look good. This is the basis of the landscape coefficient method developed at the University of California, and it's what this calculator does.
Plant water-use classes
WUCOLS, the University of California's Water Use Classification of Landscape Species, rates thousands of landscape plants into four water-use classes. A UC Agriculture and Natural Resources article explains that plants in the high class need about 70–90% of the water the grass reference would need, and very low plants need less than 10%.
| Water-use class | WUCOLS range | Calculator uses | What it means |
|---|---|---|---|
| Very low | under 0.1 | 0.1 | Little or no summer watering once established. The calculator's figure is the top of the class. |
| Low | 0.1–0.3 | 0.2 | Occasional deep watering in dry spells. |
| Moderate | 0.4–0.6 | 0.5 | Regular watering through the growing season. |
| High | 0.7–0.9 | 0.8 | Frequent watering; close to a lawn's needs. |
Ratings are set region by region, so the same plant can fall in different classes in different climates. In the WUCOLS Central Valley region, for example, the same article lists foxglove as high, coneflower and azaleas as moderate, and rosemary as low. WUCOLS covers California's climate regions; elsewhere, use your local extension service's plant lists or nursery tags, most of which use the same low, moderate and high groups.
Typical ETo when you don't have a local figure
ETo is a weather figure, not a plant figure, and the best value is one published for your area and week. If you don't have one, FAO Irrigation and Drainage Paper 56 gives typical ranges by climate and mean daily temperature. The calculator's weather choices use the temperate-region rows below.
| Climate | Cool (about 10°C / 50°F) | Mild (about 20°C / 68°F) | Hot (over 30°C / 86°F) |
|---|---|---|---|
| Humid or sub-humid | 1–2 mm/day uses 1.5 mm (0.06 in) | 2–4 mm/day uses 3 mm (0.12 in) | 4–7 mm/day uses 5.5 mm (0.22 in) |
| Arid or semi-arid | 1–3 mm/day uses 2 mm (0.08 in) | 4–7 mm/day uses 5.5 mm (0.22 in) | 6–9 mm/day uses 7.5 mm (0.30 in) |
Because each range is wide, the true value can sit up to about a third above or below the figure a weather choice fills in, and further in unusual weather. Treat the result as a first estimate, then adjust based on how the soil and plants respond. FAO's ranges for tropical and subtropical regions are higher in cool weather, so a local ETo figure matters most there.
Check local rules first. Many water utilities set watering days, hours or weekly limits, and rules sometimes differ for hoses, drip and sprinklers. Where they apply, they take priority over any calculated amount.
How this garden watering calculator works
The calculator turns your ETo figure into a weekly depth, scales it by the plant factor, subtracts rain, converts the remaining depth into a volume for your bed, and splits it across your watering days. If you add a hose flow or drip emitters, it also converts each day's volume into a watering time.
1. Weekly reference ET
Weekly ETo
Weekly ETo = ETo per day × 7
Weekly ETo = ETo per month × 7 ÷ 30.4
A weekly figure is used as entered. A month is taken as 30.4 days, the average length of a month.
2. Plant water need and rain
Depth to apply
Plant water need = Weekly ETo × Plant factor
Depth to apply = Plant water need − Rainfall
If rainfall meets or exceeds the plant water need, the result is zero and no watering is suggested.
3. Convert depth to volume
US units
Gallons = Area (sq ft) × Depth (in) × 0.623
One inch of water over one square foot is 144 cubic inches, and a US gallon is 231 cubic inches. 144 ÷ 231 ≈ 0.623.
Metric units
Liters = Area (m²) × Depth (mm)
One millimeter of water over one square meter is exactly one liter.
4. Split across watering days
Per watering day
Depth per day = Depth to apply ÷ Watering days
Volume per day = Weekly volume ÷ Watering days
5. Watering time
Hose
Minutes = Volume per day ÷ Hose flow
Gallons ÷ gallons per minute (US), or liters ÷ liters per minute (metric).
Drip
Drip flow = Number of emitters × Emitter flow
Minutes = Volume per day ÷ Drip flow × 60
Emitter flow is in gallons per hour (US) or liters per hour (metric).
Useful conversions
| Water depth | Over this area | Volume |
|---|---|---|
| 1 in | 100 sq ft | 62.3 gal |
| 1 in | 1,000 sq ft | 623 gal |
| 1 mm | 1 m² | 1 L |
| 10 mm | 10 m² | 100 L |
| 1 in = 25.4 mm | 1 US gal = 3.785 L | |
Volumes are rounded to about three significant figures, such as the nearest gallon for a typical bed, because ETo and plant factors are themselves estimates. Depths are shown to two decimals in inches and one decimal in millimeters, and watering times to the nearest minute.
When this calculator isn't the right tool
It's built for established flower, perennial and mixed ornamental beds watered as a whole. For other plantings, the garden and tree calculators cover each case, including:
- Vegetable gardens, where water use depends on the crop and its growth stage. Use the vegetable garden water calculator.
- Raised beds and containers, which drain and dry differently from ground beds. See the raised bed watering calculator and the container plant watering calculator.
- Individual shrubs and trees watered at the root zone rather than across a bed. See the shrub watering calculator.
- Lawns, which have their own weekly targets. Use the lawn watering calculator.
Garden watering calculation example
A sunny perennial border measures 16 ft by 20 ft and is planted with moderate water-use perennials. The local weather network reports ETo of 0.20 inch per day this week, a rain gauge caught 0.20 inch, the owner waters twice a week, and a bucket test shows the hose and watering wand deliver 5 gallons per minute.
- Area: 16 ft × 20 ft = 320 sq ft.
- Weekly ETo: 0.20 in × 7 = 1.40 in.
- Plant water need: 1.40 in × 0.5 = 0.70 in.
- Depth to apply: 0.70 in − 0.20 in = 0.50 in this week.
- Weekly volume: 320 × 0.50 × 0.623 ≈ 99.7 gal, shown as about 100 gallons.
- Per watering day: 0.50 in ÷ 2 = 0.25 in, or about 49.9 gallons, shown as about 50 gallons.
- Hose time: 49.9 gal ÷ 5 gpm ≈ 10 minutes each watering day.
The same calculation in metric
A 12 m² bed of high water-use plants in a warm spell with ETo of 5 mm per day, 8 mm of rain, two watering days, and a drip zone with 40 emitters rated at 2 L/h:
- Weekly ETo: 5 × 7 = 35 mm.
- Plant water need: 35 × 0.8 = 28 mm.
- Depth to apply: 28 − 8 = 20 mm.
- Weekly volume: 12 × 20 = 240 L.
- Per watering day: 10 mm, or 120 L.
- Drip flow: 40 × 2 L/h = 80 L/h.
- Drip run time: 120 ÷ 80 × 60 = 90 minutes (1 hr 30 min) each watering day.
The two examples show how much the delivery method changes the time: a hose delivers a day's water in minutes, while drip applies it slowly over an hour or more, which gives it time to soak in.
How long to water a garden with a hose or drip
Once you know the volume, time depends only on how fast water comes out. That's why measuring your own flow matters more than any typical figure.
Measure your hose flow
Hose flow depends on your water pressure, hose length and diameter, and the nozzle or wand on the end, so measure it the way you actually water. Hold the hose over a bucket of known size, time how long it takes to fill, and divide. A 5-gallon bucket that fills in 1 minute 15 seconds (1.25 minutes) means 5 ÷ 1.25 = 4 gpm. The bucket test calculator handles the arithmetic and unit conversions.
Hand watering at full flow often puts water down faster than soil can absorb it. If water starts to pool or run off, move on to another part of the bed and come back, so the total time is split into rounds. The soil infiltration rate chart shows how fast water soaks into each soil type.
Time a drip zone
A drip zone's flow is the sum of its emitters. Count the emitters in the zone and check their rating, printed on each emitter or its packaging. For inline drip tubing, the spacing and flow per emitter are usually printed on the tubing. For more detail, including pressure-compensating emitters and several zones, use the drip run time calculator.
Drip applies water only near the plants, so part of the bed stays dry. The calculator treats the whole bed as needing water, which suits densely planted beds. In a sparse bed with bare soil between plants, the same result will be generous, so check soil moisture before adding time.
Where to find ETo for your area
Many agricultural weather networks, water utilities and extension services publish ETo, often daily and sometimes as weekly or monthly totals. In California, the state's CIMIS network publishes daily ETo from its weather stations. Search for "reference evapotranspiration" with your state or region's name, then enter the figure with the matching period. If you have weather data but no published ETo, the evapotranspiration calculator estimates it.
Mixed beds with different water needs
One watering schedule can only suit one class. Watering for the thirstiest plants overwaters the drought-tolerant ones beside them; watering for the drought-tolerant plants leaves the thirsty ones short. The WUCOLS program describes grouping plants with similar water needs as a key water-saving strategy. Until you can regroup, choose the class of the plants you most want to keep healthy, and watch the others for signs of too much water, such as yellowing leaves or soil that stays soggy.
Common mistakes
- Watering by the clock instead of the weather. A cool week can need a fraction of a hot week's water. Update ETo as the weather changes.
- Measuring the whole garden. Enter only the bed you're watering, not paths or lawn.
- Guessing hose flow. Two hoses on the same tap can differ widely with length and nozzle. A one-minute bucket test settles it.
- Frequent shallow watering. Light daily sprinkles wet only the surface, where much of it evaporates. Water less often and more deeply unless the bed is newly planted.
What changes how much water a garden needs
The two main inputs, ETo and plant factor, carry most of the variation. These conditions push the real need above or below the calculated one.
| Factor | Effect on water needs | What to adjust |
|---|---|---|
| Weather | Sun, heat, wind and dry air raise ETo; cool, cloudy, humid weather lowers it. | Update the ETo figure or weather choice each week. |
| Plant type | Water-use classes differ several-fold. | Choose the class of the plants in the bed. |
| Planting density | The landscape coefficient method raises the estimate for dense, layered plantings and lowers it for sparse ones. | Nudge the plant factor up or down, or use the landscape coefficient calculator. |
| Microclimate | Reflected heat from walls and paving, and wind, raise water use; shade lowers it. The landscape coefficient method adjusts for this too. | Adjust the plant factor for very sunny, windy or shaded beds. |
| Rainfall | Counts toward the plant need. | Enter measured rain. Light showers count for less than they measure. |
| Mulch | FAO-56 notes that mulch can substantially reduce evaporation from the soil. | Check soil moisture; a mulched bed may need less than calculated. |
| Soil type | Sets how much water the root zone holds and how fast it soaks in. | Sandy: more, smaller sessions. Clay: water slowly or in rounds. |
| New plantings | Small root systems dry out faster than established plants. | Water more often, in smaller amounts, until plants establish. |
To put numbers on density and microclimate, the landscape coefficient calculator combines a species factor with density and microclimate factors, following the University of California method. Enter its result as a custom plant factor here.
Signs your garden needs water
Leaves that wilt in the morning, before the day's heat, usually mean the soil is dry; wilting only in late afternoon can be normal on hot days. The simplest check is to push a finger or trowel a few inches into the soil near the plants' roots. If it's dry there, it's time to water; if it's still damp, wait.
Frequently asked questions
How much water does a flower garden need per week?
It depends on the weather and on the plants. Multiply the week's reference evapotranspiration (ETo) by your plants' plant factor, then subtract rain. For example, in a week with 1.4 inches of ETo, a bed of moderate water-use plants with a plant factor of 0.5 needs about 0.7 inch of water. A bed of low water-use plants in the same week needs far less.
How long should I water my garden with a hose?
Divide the gallons you need for the session by your hose's flow rate in gallons per minute. To find the flow, time how long the hose takes to fill a 5-gallon bucket and divide 5 by the minutes. For example, 50 gallons at 5 gallons per minute takes about 10 minutes. If water starts to run off, pause and water again in rounds so it can soak in.
How long should I run drip irrigation in a flower bed?
Add up the flow of every emitter in the bed, then divide the gallons you need by that total and multiply by 60 for minutes. For example, 30 emitters rated at 1 gallon per hour deliver 30 gallons per hour, so 30 gallons takes about 1 hour. Use the emitter rating printed on the emitter or its packaging.
For multi-zone systems, see the drip run time calculator.
What is a plant water-use class?
It groups plants by how much water they need compared with a reference grass surface. WUCOLS, the University of California's Water Use Classification of Landscape Species, uses four classes: very low (under 0.1), low (0.1 to 0.3), moderate (0.4 to 0.6) and high (0.7 to 0.9). The number is the plant factor, the share of reference evapotranspiration the plants need to stay healthy and look good.
What if my bed has plants with different water needs?
One watering schedule can only suit one water-use class. If you water for the thirstiest plants, the low-water plants beside them get more than they need; if you water for the low-water plants, the thirsty ones suffer. Grouping plants with similar water needs, called hydrozoning, avoids the problem. Until you regroup, choose the class of the plants you most want to keep healthy.
Where do I find ETo for my area?
Many agricultural weather networks, water utilities and extension services publish reference evapotranspiration (ETo) daily, weekly or monthly. In California, CIMIS is the main public source. Enter the figure with the matching period in the calculator. If you can't find one, choose the typical weather that fits your week for a rough starting estimate.
To estimate ETo from your own weather data, use the evapotranspiration calculator.
Does rain reduce how much I need to water my garden?
Yes. Subtract the rain your garden actually received, ideally measured with a rain gauge in the yard. Light showers mostly wet the surface and evaporate, so they help less than their measured depth. This calculator counts rainfall in full, which is one reason its result is an estimate.
Can I use this calculator for a vegetable garden?
Not directly. Vegetable water use depends on the crop and changes as it grows, so the plant water-use classes used here for flowers and ornamentals don't fit. Use a calculator built around vegetable crops instead.
See the vegetable garden water calculator.
Who built and checked this calculator
Licensed Professional Engineer and Irrigation Association Certified Irrigation Designer with more than 20 years in irrigation design, system auditing and landscape water management. Full profile
How this page was checked
- Formulas, unit conversions and both worked examples verified by hand.
- Plant water-use classes checked against WUCOLS and the landscape coefficient method.
- Typical ETo ranges checked against FAO-56 Table 2.
Sources
- Allen, R.G., Pereira, L.S., Raes, D. and Smith, M. FAO Irrigation and Drainage Paper 56. Crop evapotranspiration: Guidelines for computing crop water requirements, Chapter 1. Food and Agriculture Organization of the United Nations, 1998.
- California Center for Urban Horticulture, UC Davis. Water Use Classification of Landscape Species (WUCOLS). WUCOLS IV, Costello, L.R. and Jones, K.S., 2014.
- Costello, L.R., Matheny, N.P. and Clark, J.R. A Guide to Estimating Irrigation Water Needs of Landscape Plantings in California: The Landscape Coefficient Method and WUCOLS III. University of California Cooperative Extension and California Department of Water Resources, 2000. [VERIFY URL]
- UC Agriculture and Natural Resources, The Real Dirt blog. Using WUCOLS IV Plant Database to Hydrozone Effectively.
- California Department of Water Resources. California Irrigation Management Information System (CIMIS).