Irrigating tomatoes, peppers and onions: water by growth stage.
Tomato, pepper and onion use little water as young plants and the most from flowering to fruit or bulb growth. This guide shows how to work out each day's water from the weather and FAO's crop factors, which stages must never run short, and when to stop watering onions, with a worked example in litres per metre of bed.
Written for farmers and greenhouse and nursery growers.
Start with the weather
Crop water use is worked out in two steps. The first is the reference evapotranspiration, ETo: the water a well-watered field of grass would lose in your weather, in mm per day. The second is a crop factor that turns ETo into what your crop uses.
Hot, sunny, dry and windy days give a high ETo; cloudy, humid and calm days a low one. A weather station near you may publish ETo. Without one, FAO gives indicative values by climate, below. Water depths convert simply: 1 mm of water over 1 square metre is 1 litre, so a crop using 6 mm a day takes 6 litres from every square metre.
| Climate | ETo at a mean temperature of 15 to 25°C (mm per day) | ETo above 25°C (mm per day) |
|---|---|---|
| Humid | 3 to 4 | 5 to 6 |
| Sub-humid | 5 to 6 | 7 to 8 |
| Semi-arid | 6 to 7 | 8 to 9 |
| Arid | 7 to 8 | 9 to 10 |
Sources: FAO (1986), Irrigation Water Management Training Manual No. 3: Irrigation Water Needs, chapter 3, Tables 5, 6 and 14; FAO (1998), Irrigation and Drainage Paper 56: Crop Evapotranspiration, chapter 1, units.
Crop factors by stage
The crop factor, Kc, is low while plants are small, rises as they cover the ground, peaks from flowering to fruit or bulb growth, and falls as the crop ripens. Multiply ETo by Kc to get the crop's daily use.
FAO splits the season into four stages: the first stage, until the crop covers about 10 percent of the ground; development, until it covers the ground fully; mid-season, from full cover to the start of ripening, which takes in flowering, fruit set and bulb growth; and the late season, until the last harvest.
FAO's peak and end figures are for a sub-humid climate with moderate wind, and FAO adjusts them up a little in drier, windier weather and down in more humid, calmer weather. Its first-stage figures are general values that it says should always be refined for how often the soil surface is wetted. Use the season lengths only as a guide: varieties and planting dates differ, and FAO advises using local observations.
| Crop | First stage | Peak | End of season | FAO's indicative season length |
|---|---|---|---|---|
| Tomato | 0.6 | 1.15, or 1.20 if staked tall | 0.70 to 0.90 | 135 to 180 days |
| Sweet pepper | 0.6 | 1.05, or 1.15 if staked tall | 0.90 | 120 to 210 days |
| Dry onion | 0.7 | 1.05 | 0.75 | 150 to 210 days |
Sources: FAO (1998), Irrigation and Drainage Paper 56: Crop Evapotranspiration, chapter 6, Tables 11 and 12; FAO (1986), Irrigation Water Management Training Manual No. 3: Irrigation Water Needs, chapter 3, Tables 5, 6 and 14.
The stages that must not run short
FAO rates tomato, pepper and onion as medium to highly sensitive to drought. For most crops the mid-season is the most sensitive stage, because it is when the crop uses most water. The late season matters less, except for crops picked fresh, such as tomato and pepper, which stay sensitive until the last harvest.
- Tomato: most sensitive at flowering, and then while the fruit grows. Blossom end rot, a dark, sunken, leathery patch at the flower end of the fruit, is worse when the soil runs dry, so keep the root zone from drying out or staying saturated. Fruit also cracks when rain follows a long dry spell; a steady water supply helps prevent it.
- Pepper: sensitive throughout the season, and prone to blossom end rot in the same way as tomato.
- Onion: most sensitive while the bulb swells. Its growth can slow before the leaves show any wilting, and it suffers in waterlogged soil, so keep the soil evenly moist and well drained.
If water runs short and fields were planted at different times, FAO advises watering first the fields that have reached mid-season.
Sources: FAO (1989), Irrigation Water Management Training Manual No. 4: Irrigation Scheduling, chapter 2, Tables 1 and 2; FAO (1986), Irrigation Water Management Training Manual No. 3: Irrigation Water Needs, chapter 3, Tables 5, 6 and 14; UC IPM, University of California (checked 2026), Blossom end rot; UC IPM, University of California (checked 2026), Fruit cracks on tomatoes; NAERLS (2016), Extension Bulletin No. 239: Irrigated Onion Production and Management; UC IPM, University of California (checked 2026), Cultural tips for growing onions and garlic: watering.
Water through the season
- After transplanting: small, frequent waterings, because the roots are still shallow
- As the crop covers the ground: raise the daily amount week by week, following the crop factor
- Flowering, fruit set and bulb growth: the full daily amount, every day it is due, with the soil kept evenly moist
- Tomato and pepper picking: keep watering steadily until the last harvest
- Onion: stop watering once the tops begin to fall over
- After transplanting: the roots are shallow, so water little and often
- Flowering: the crop now uses much more water, and a shortage does tomato most harm
- Fruit: the full daily amount, kept steady until the last tomato or pepper is picked
- Onion while the bulb swells: keep the soil evenly moist; its roots stay shallow
- Onion once the tops fall over: stop watering so the outer skins can dry
- Moist soil around the roots: it spreads deeper as the roots grow
Sources: FAO (1989), Irrigation Water Management Training Manual No. 4: Irrigation Scheduling, chapter 3; FAO (1989), Irrigation Water Management Training Manual No. 4: Irrigation Scheduling, chapter 2, Tables 1 and 2; UC IPM, University of California (checked 2026), Cultural tips for growing onions and garlic: watering.
Stopping onions before harvest
As onions mature they stop making new leaves, the neck goes soft and the tops fall over. Food is still moving from the leaves into the bulb at this point, so NAERLS advises letting this stage finish before you lift the crop, without letting the tops dry out completely.
Stop watering as the tops begin to fall. This stops root growth and lets the outer skins of the bulb dry. Lift the bulbs in dry weather and leave them on their sides to ripen further; bulbs that stay upright in wet weather can root and start growing again.
Sources: NAERLS (2016), Extension Bulletin No. 239: Irrigated Onion Production and Management; UC IPM, University of California (checked 2026), Cultural tips for growing onions and garlic: watering.
From weather to litres per metre of bed
On a farm with drip, the useful figure is litres per metre of bed, because that is what a drip line delivers. One metre of bed covers the bed spacing times one metre of ground.
Turning litres into a run time, by soilPlanning drip for a whole hectare
Sources: FAO (1986), Irrigation Water Management Training Manual No. 3: Irrigation Water Needs, chapter 3, Tables 5, 6 and 14; FAO (1998), Irrigation and Drainage Paper 56: Crop Evapotranspiration, chapter 6, Tables 11 and 12; FAO (1998), Irrigation and Drainage Paper 56: Crop Evapotranspiration, chapter 1, units; FAO (1989), Irrigation Water Management Training Manual No. 4: Irrigation Scheduling, Annex I, Table 8.
What the soil and the rain change
The soil does not change how much the crop uses; the weather and the crop do. The soil changes how often you water. Roots also matter: FAO gives onion a shallow root zone (0.3 to 0.6 m), pepper a medium one (0.5 to 1.0 m) and tomato a deeper one (0.7 to 1.5 m), so onion needs smaller, more frequent waterings than tomato on the same soil.
Rain that soaks into the root zone counts against the day's need. In the rainy season, keep a count of it rather than watering by the calendar.
Watering sandy, loamy and clay soilsTopping up rain in the rainy season
Sources: FAO (1998), Irrigation and Drainage Paper 56: Crop Evapotranspiration, chapter 8, Table 22; FAO (1989), Irrigation Water Management Training Manual No. 4: Irrigation Scheduling, chapter 3.
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