Download the technical sheet (PDF)
Across the Sahel belt the carrot sowing window opens in October and runs through to March or April, according to the CNRADA technical sheet. The decision to take now, before ordering a single length of pipe, is the irrigation equipment, and it is not a single choice between two systems. Carrot emergence needs the soil surface to stay moist continuously for one to three weeks, which sprinklers or fine watering deliver better. Root bulking, on the other hand, runs very well on drip, with a water saving that has been measured in trials. What follows sets out where the switch happens, how to size each stage, and which plot conditions tip the choice one way or the other for good.
What carrot itself demands of the irrigation
Carrot is sown in place, with no transplanting. The irrigation therefore has to carry a direct field emergence, on a small seed covered with only 0,7 to 1,5 cm of fine soil, according to the technical sheet published by the Department of Agriculture of French Polynesia. That sheet gives emergence in 6 to 10 days at an optimum temperature of 25 °C. Utah State University Extension is more cautious: 14 to 21 days to emerge, best germination between 13 and 18 °C, and germination falling away above 27 °C. So plan for a sensitive period of one to three weeks, during which the Utah instruction applies word for word: maintain a uniform and moist soil surface to ensure good plant stands. The same source notes that a surface crust limits emergence, and advises covering the seed with compost or fine sand.
The rest of the cycle is less demanding. The Polynesian sheet puts the total requirement at about 350 mm per cycle, that is 350 litres per square metre: small applications from sowing to emergence, more widely spaced watering afterwards, then larger volumes during root bulking to avoid splitting. CNRADA puts field occupation at 100 to 120 days, with short cycle varieties of 90 to 110 days and good heat tolerance, of the Nantaise, Touchon or Chantenay types.
One parameter is often overlooked when the water source is chosen: carrot tolerates salt poorly. The crop tolerance table in FAO Irrigation and Drainage Paper 29 puts its threshold at 1,0 dS/m of saturated extract conductivity for full yield potential, and at 1,7 dS/m for 90 %. That is low. Slightly brackish borehole water that would suit tomato penalises carrot, whatever equipment is laid afterwards.
Sprinklers: where they keep the advantage
Carrot is grown on beds about 1 m wide, in rows 0,20 to 0,30 m apart, at a seed rate of 2 to 4 kg per hectare, according to CNRADA. On a bed laid out that way the whole surface has to be wetted, not a strip. That is exactly what a sprinkler does, and it is why the emergence phase is almost always handled with sprinklers, a boom, or a watering can with a rose where the area is small, as CNRADA recommends for bed culture.
The FAO manual on small scale irrigation in arid zones in fact states the limit of drip in plain words: it is particularly suited to orchards and to market garden crops laid out in rows and beds, and less well suited to dense crops that need uniform wetting of the whole soil volume. A carrot bed carrying 20 to 30 plants per linear metre is precisely a dense crop.
Sprinklers carry a crop health cost in return. The same FAO document notes that sprinklers and microsprinklers wet the foliage, which makes saline water and fungal disease more of a problem than with drip. On carrot the disease to watch is Alternaria leaf blight, which the Polynesian sheet describes as severe in the hot, humid season, with black lesions on the leaves and elongated spots on the petioles. Foliage that stays wet all night works against you: water in the morning rather than late in the day during the sprinkler phase.
For sprinkler spacing and the pressure to hold, the calculation does not depend on the crop but on the wetted diameter and the wind. We set it out on another low growing crop in Sprinkler irrigation on green beans: how far apart to set the sprinklers, and the method transfers unchanged to a carrot bed.
Drip: where it takes over
Once emergence and thinning are done, the root system is in place and the water no longer needs to reach the whole surface. Drip then becomes the better option, and the arguments are measurable.
Water saving first. On seed carrot fields monitored in 2003 by Butler and co-authors at the Central Oregon Agricultural Research Center, the drip plots used 36, 48 and 54 % of the water applied to the sprinkler plots, and 26 % of that applied on the furrow irrigated field. These figures come from seed production rather than from fresh market carrot, but the order of magnitude of the gap holds.
Pressure and energy next. FAO puts the operating pressure of a drip network at 0,5 to 2,5 atmospheres, and notes that a simplified system can run by gravity from a tank placed a few metres above the plot. Microsprinklers need 1 to 2 atmospheres, so a pump or a tank raised by at least 10 m. On a small plot with no grid connection that gap alone decides the system. If you are starting from a borehole, size the pump before the network: the method for calculating flow rate and total dynamic head is set out in Solar pump or motor pump: sizing irrigation pumping for an onion plot.
Crop health last. In the same Oregon trial, the incidence of plants carrying Xanthomonas was generally higher in the sprinkler irrigated fields than in the drip irrigated ones: 0 to 10 % against 5 to 20 % at the second sampling, and 5 to 70 % against 25 to 80 % at the third.
On the equipment side, FAO cites emitters of 2, 4 or 8 litres per hour. On carrot, take the lowest flow rate available and tighten the emitter spacing rather than the reverse: the aim is a continuous moist strip along the row, not a string of separate bulbs. The manual also notes that on clay soil, which waterlogs and limits aeration, the lines must be placed closer together, and that on sandy soil, whose water holding capacity is too low and which dries out too fast, drip is precisely the method recommended in an arid climate.
The decision table
| Plot situation | Equipment chosen | Reason |
|---|---|---|
| From sowing to thinning | Sprinklers, boom or watering can with a rose | The whole bed surface has to stay moist and uniform (Utah State University) |
| After thinning, root bulking | Drip | The root volume is in place and the water saving has been measured in trials (COARC, 2003) |
| Sandy soil, dry climate | Drip as soon as emergence is secured | Water holding capacity too low, rapid drying, method recommended by FAO for these soils |
| Clay or crusting soil | Sprinklers for longer, then drip with closer lines | Waterlogging, limited aeration and surface crusting |
| Water above 1,7 dS/m | Review the water source before the equipment | Carrot threshold of 1,0 dS/m (FAO, Paper 29) |
| No grid connection, tank available | Gravity fed drip | 0,5 to 2,5 atmospheres are enough, against 1 to 2 for microsprinklers (FAO) |
| Noticeable daytime wind | Sprinklers at reduced spacing, or an earlier switch to drip | Wind distorts the application pattern on the ground |
| History of Alternaria leaf blight | Drip as early as possible, watering in the morning | Foliage left wet overnight favours fungal disease (FAO) |
Setting up the network on a carrot bed
- Take the field measurements before buying. Flow available at the source in the driest month, chemical quality of the water, soil texture and infiltration rate, slope. The full list and how to take each one are in The field measurements to take before equipping a plot for irrigation.
- Prepare the bed. Deep cultivation at 25 to 30 cm, then shallow working over 10 to 15 cm, for a bed about 1 m wide and 10 to 15 cm high, according to the Polynesian sheet. Rows at 0,20 to 0,30 m according to CNRADA.
- Sow in place and firm down. Cover with about 1 cm of fine soil, then firm lightly: the Polynesian sheet notes that this firming keeps moisture available by capillary rise. It is a free operation that saves stands.
- Run emergence on small applications. Short, frequent sprinkler passes, never one heavy watering that seals the surface. Inspect the crust every morning for the first ten days.
- Thin. About a month after sowing, at 3 to 4 true leaves, when the plants are 3 to 6 cm tall, leaving 20 to 30 plants per linear metre, then water immediately (CNRADA).
- Switch to drip. One line per two rows, emitters at the lowest flow rate available, and a disc or screen filter at the head of the network. Without filtration the emitters block and the moist strip becomes uneven.
- Keep applications steady during root bulking. Increase the volumes but above all do not alternate stress with a heavy application: the Polynesian sheet names root splitting as the direct consequence of uneven supply at this stage.
- Taper off. Harvest comes 100 to 120 days after sowing according to CNRADA, the roots pulled by the foliage or lifted with a tool if the soil is hard.
The mistakes that cost you the stand
- Laying drip before sowing and counting on it for emergence. On sandy soil the moist strip does not rise to a seed covered by one centimetre.
- Watering heavily and infrequently during emergence. The surface dries between passes and the stand comes up patchy.
- Sowing too deep. The useful range is 0,7 to 1,5 cm, no more.
- Leaving out filtration at the head of the network, then concluding that drip does not suit the plot.
- Choosing the equipment before measuring the water. The other classic traps on small schemes are gathered in Drip irrigation in market gardening: the mistakes that sink small schemes.
What to do in the next six weeks
If sowing is planned for October or November, the sprinkler network has to be laid and brought up to pressure before the seeder goes through, not after. Allow the first fortnight for the field measurements and bed preparation, the second for sowing and running emergence on small applications, the fourth week for thinning, and the fifth or sixth for the switch to drip with its filter. Ordered at the same time, the equipment arrives in one delivery and you are not laying a network in a hurry in the middle of an emergence.
JOGOO supplies and installs sprinkler and drip networks, filters and pumping stations for market garden schemes in West and Central Africa. See the Irrigation and Pumping page for the equipment range, or request a quote from the Contact page, giving the area, the water source and the soil texture.
Sources
- FAO, Small scale irrigation in arid zones: criteria and options for appropriate irrigation methods
- CNRADA, Technical sheet on carrot
- Department of Agriculture of French Polynesia, Technical sheet: carrot, Daucus carota L.
- Utah State University Extension, Carrots in the Garden
- Butler, M. and co-authors, Drip Irrigation on Commercial Seed Carrots and Onions in Central Oregon, Central Oregon Agricultural Research Center, 2003
- Crop salt tolerance table, FAO Irrigation and Drainage Paper 29, reproduced by University of California Agriculture and Natural Resources
Frequently asked questions
Can carrot be sown directly under drip?
It is possible on a loam soil that lifts water well by capillary rise, but risky on sandy soil. The seed is covered by only 0,7 to 1,5 cm and Utah State University calls for a uniformly moist surface to ensure good plant stands. A sprinkler boom or fine watering during emergence, then a switch to drip after thinning, remains the safest practice.
How much water does a carrot crop use over the whole cycle?
About 350 mm per cycle, that is 350 litres per square metre, according to the technical sheet from the Department of Agriculture of French Polynesia. The distribution matters as much as the total: small applications from sowing to emergence, more widely spaced watering during development, then larger and steady volumes during root bulking.
How long does carrot take to emerge?
From 6 to 10 days in good conditions, at an optimum temperature of 25 °C according to the Polynesian sheet, and up to 14 to 21 days according to Utah State University, which puts best germination between 13 and 18 °C. Above 27 °C germination falls away. In the hot season, light shade over the bed during emergence limits that loss.
What water saving can drip be expected to deliver on carrot?
In the trial run in 2003 at the Central Oregon Agricultural Research Center on seed carrot, the drip plots received 36 to 54 % of the water applied to the sprinkler plots, and 26 % of that applied under furrow irrigation. These values come from seed production, but they give the order of magnitude of the gap between the two systems.
What water quality does carrot need?
Carrot has low salt tolerance. FAO Irrigation and Drainage Paper 29 gives a threshold of 1,0 dS/m of saturated extract conductivity for full yield potential, and 1,7 dS/m for 90 %. Have the borehole water analysed before choosing the equipment: no irrigation system corrects water that is too salty.
Why do carrot roots split?
Splitting almost always follows a heavy application after a dry spell during root bulking. The Polynesian sheet asks for larger but steady volumes at this stage. Drip, which allows short daily applications, reduces that risk compared with heavy, widely spaced watering.
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