In Depth Guide to Carrot Cultivation
Continuous Cropping - Carrot fly - Willow Aphid - Diseases - Seed Production
Types of Soil /Climate and growing position
Carrots develop normally within a great range of temperatures and are grown throughout the world with the exception of the very warmest areas. Root growth is fastest at a temperature between 15 ºC and 18 ºC, while optimum temperatures for shoot growth are somewhat higher. Seeds of carrot may germinate at low temperatures but the germination period is shorter at higher temperatures and a soil temperature of at least 10 ºC is therefore recommended. Carrots are tolerant of long days but need low temperatures to induce flowering.
Carrots are fairly fussy growers. They love light, stone free, well drained, fertile soils with plenty of well rotted organic matter in them. Rich sandy peaty soils are perfect in providing the best conditions for the carrot roots to penetrate deeply and to swell.
The pH value should 6.5 to 7.5 for best results. Potassium promotes solid, sweet carrots. Wood ashes contain soluble potassium which reaches the plant quickly. Excess Nitrogen causes branching and hairy, fibrous roots.
It is much harder to grow good carrots in heavy clay soils or soils which are compacted or stony. Such conditions can cause the forking of roots. Water logged sites are also less than ideal. If you have a heavy soil, dig in plenty of manure several months before planting. Never work fresh manure into the soil as this encourages sappy growth and forking of roots. Add leaf mould to lighten heavy soil and rake in Nitrogen fertiliser before sowing a crop in poor soil
Early carrots appreciate a sheltered position but main crop need an open sunny site. Carrots should be rotated around the garden to avoid the build up of diseases. It is recommended that you grow them in a different bed each year over at least a three year cycle.
Soil temperature can be critical for successful carrots. At temperatures below 5 ºC they will struggle to germinate. Slightly higher temperatures and they could take up to 35 days to start. If you wait until the soil is 10 ºC germination will occur within ten days. Basically if the soil is chilly to touch do not plant.
Curiously even within a variety a carrot's colour and shape can vary according to the type of soil and commencement temperature. Lower temperatures give yellower carrots and reduced size and shape.
Proper watering can make a difference. Carrots need 2cm of water from rainfall each week during the growing season. Soaking well when watering helps to promote good root development. The domesticated honeybee may get more glory, but when it comes to pollinating carrots, one tiny alfalfa leafcutter bee can do the job of 20 of its larger, noisier, more irritable cousins, says a U.S. Department of Agriculture researcher. Click here for more details.
Not all of the chemical constituents of carrots are for our health; some appear to be there for the health of the carrot itself. One reason that the carrot can be stored for long periods of time, such as over winter in a root cellar, is that the carrot has a mechanism to guard against microbial decomposition (rot).
There are three enemies of carrot storage: wilt, re-growth and rot. The first of these is no problem if the carrot is stored where the humidity is high. The second is of little consequence if the carrot is stored at 0 to 5°C.
The carrot itself contributes much toward conquering the last enemy, rot. At the present time, scientists are busy determining how the disease response mechanism of the carrot operates. There appear to be three lines of resistance which the carrot uses, based on the chemicals contained within the carrot and its skin. Read more about the Carrot Disease Response Mechanism and the contributory elements. How do Carrots produce seeds? Here
Important Note - The chemical constituents of carrot are not there by chance, but perform a function. Many constituents of the orange carrot we now cultivate are also in the white root of the wild carrot, Queen Anne's lace, from which our carrot was developed. This is true of falcarinol, falcarindiol, and myristicin. Carotene (present in small amounts in Queen Anne's lace) has been increased by centuries of selection. Volatile oils have been decreased in this process. Plant scientists must continue to monitor all known constituents nutritive and non-nutritive - as new cultivars of the carrot are developed to keep our vegetables nutritious and safe. Plant breeding for the sake of high yields, appearance, and keeping quality will not be sufficient.
Seed Sowing Methods
Carrots are normally grown straight in the ground and then thinned in stages to obtain the correct distance apart. Never plant in cold or weedy soil as carrots are difficult to weed once established.
Carrots are cool-weather vegetables, so start sowing about two weeks before the last expected frost in your area. Make successive plantings every three weeks but avoid the hottest part of the summer.
Sow in drills about 2cm deep and 15cm apart. With this spacing the foliage of adjacent plants will make a dense canopy when the plants are mature. Place a 1cm layer of peat moss in the bottom of each furrow, Sow the seeds sparingly on top, then cover with about 0.5cm of soil. Seeds must be kept moist to germinate. Mulching with straw will help hold the moisture, and will also make it easier to water without disturbing the seeds.
When sowing seeds, try to space them 1cm apart. The tiny seeds make spacing difficult, but it will be easier to thin without disturbing the plants you plan to leave, if there is a little space between them. Seeds can be mixed with sand to make sowing a little easier.
You can try mixing radish seeds with the carrot seeds. The carrot seeds are slow to germinate, and the radishes, which germinate and grow very quickly, will mark the row until the carrots come up. A second crop of carrots can be planted in late summer or early autumn in most areas. If a hard frost threatens, protect your fall crop with a heavy mulch.
See photos of common varieties supplied by Thompson and Morgan the leading seed suppliers in the US and UK. Click here.
Sowing for continuous cropping
For more or less continuous supply of fresh carrots it is recommended that you make several sowings. In most growing conditions, thinning is essential to give individual carrots the space they need in order to develop to their full size. However the main enemy, the carrot fly, is attracted to the smell of bruised carrot leaves so do that thinning carefully to put the carrot fly off the scent, literally!
Thin so that each seedling stands quite clear from its neighbours. Nip off unwanted seedlings just above ground level, rather than pulling them up. Later in the season larger thinnings can be pulled out and used.
Press soil back around the stems if seedlings have been disturbed.
Completely bury the thinnings in the compost heap taking care not to leave any leaves on the ground. Thin maincrop carrots to 3cm apart to get good yield of medium sized carrots and 5cm apart for larger carrots. Early long carrots can be thinned to far wider spacing, to encourage rapid growth.
Sow under cover in February for early carrots. Outdoors in April for the main summer crop. Outdoors in June for winter. Highest yields are obtained from carrots sown between April and May. For a crop of young carrots in November/December sow early varieties outdoors in August in the north and September in the south. Cover with cloches in autumn.
Forced Crop: Carrots sown in February in a cold frame/cloche are ready to harvest by June. When using cloches put them in place a month before sowing as this helps to warm up the soil. The best carrots for forced crops are Amsterdam Forcing varieties like "Sweetheart" or "Nantes Express" which are longer.
Sow forced crop seeds in 2cm deep drills (shallow furrows), 15cm apart, preferably a bed prepared the previous autumn. Thin out plants to 10cm apart which minimises competition and enables the carrots to grow quickly to harvest size.
Early Outdoors: Later in Spring (March/April) sow seeds of Amsterdam Forcing or Nantes outdoors. Where possible cover with garden fleece to speed germination and protect against carrot fly. Harvest in July/September.
Main Crop: these are sown in April/May and harvested in October/November. Chantenay is a good main crop variety. This time its drills 2cm deep but 30cm apart. Sow seed very thinly and cover with light soil. Thin seedlings out to 4cm apart. These carrots are particularly suitable for storage after harvest.
Late main crop: These should be sown June/July and are ready for harvesting from December onwards. "Autumn King", Berlicum" and some Nantes varieties are good for storing and produce large roots. Thin seedlings out to 4-5cm apart and avoid bruising the leaves when thinning as the smell can attract carrot fly.
Check out this video of a harvester in action. The Dewulf GKIIISE falls into the category of "really big, extremely specialized machines" and can pluck three whole rows of carrots out of the ground without harming them. Here
Main Classification in Detail
To see an explanation of the various parts of the carrot root, including diagram click here.
Examples of typical carrot root shapes here.
There are two main groups of cultivated carrots: the eastern (anthocyanin) carrot and the western (carotene) carrot the main types offered by seedsmen are as follows.
1. Round types
These have stocky squarish roots and are used for early crops or in very heavy soil where the longer varieties do worse. "Early French Frame" is a classic round type, fairly rough in quality unless grown in ideal conditions. There are improved modern varieties called "Rondo", "Early French Frame Lisa" "Kundulus" and Parmex.
2. Amsterdam types
These are small, slender, finger shaped carrots, excellent for using raw. They are fast maturing and are used for early crops. Names to look out for are "Amsterdam Forcing", Amsterdam Sweetheart", "Souko", Prim F1" (part Amsterdam/part Nantes).
3. Nantes Type
These are larger than Amsterdam and considered to be better quality. Mainly grown during the summer and perform well in heavy soil. "Nairobi F1" and "Newmarket F1" are good varieties and tend to store well. Also try Touchon, Bolero and Ingot.
4. Chantenay type
Medium sized stocky roots which are more conical and have a good core and flesh colour. Many consider these to be the tastiest of the carrots and are a mainstay of many commercial producers. Good varieties are "Chantenay Red Cored", "Royal" and "Supreme".
Very large cylindrical carrots which mature later in the year and tend to be high yielding. Recommended varieties include "Bericulum Berjo", "Camberley" , "Cardinal F1" and "Bangor F1", the latter can be sown early for pulling young.
6. Autumn King
The largest of carrots and last to mature.
This is a very healthy and vigorous carrot with the potential to be the highest
yielding of all. Not suitable for heavy soil or cold areas where the season
is short. "Autumn King Vita Longa" is the best variety.
7. Imperator -
7. Imperator -Another long variety, 8-10 inches long. Avenger, and Tendersweet have long tapered roots, Gold Pak are coreless, sweet and good for juicing. Orlando Gold are long and uniform with 30% more carotene than average.
8. Purple Varieties
A purple variety called Dragon is available from:
Garden City Seeds,778 Hwy 93 N, Hamilton,
Believe it or not - There is a carrot variety for every letter of the alphabet and just to prove it click here to see the full list.
Carrot Plant Health Problems
Cornel University, New York, has a useful list of fact sheets on common carrot diseases - click here.
The University of California has some photos and further detail of common problems. - click here.
Virus complexes such as Carrot Motley Dwarf complex (CMD) Parsnip yellow fleck virus (PYFV) and Carrot yellow leaf virus (CYLV are all aphid transmitted but can vary in their incidence and epidemiology. This AHDB fact sheet describes current knowledge and the latest developments in the area. here.
More photos and information here (pdf)
Mount Vernon NW Washington Research & Ext Center here
Diseases caused by Fungi:
Root and crown rot, Rhizoctonia solani, Pythium spp., Fusarium spp.
Symptoms appear as wilting and a slow or rapid collapse of the plant. The roots can appear brown and water-soaked instead of white. A water-soaked lesion can often appear at the base of the stem. The carrot develops a dense purple stain that may finally cover the entire root.
Control can be achieved by using a two-year rotation with non susceptible plants, such as corn, to prevent the build up of pathogenic organisms in the soil.
Leaf blight (Alternaria dauci)
Leaf blight is caused by a fungus and starts on the foliage as dark brown spots which grow together and may kill whole leaves. If the disease is very severe, the whole carrot top may be killed. This fungus disease usually attacks the older foliage. Affects the leaves in the first stages and then penetrates the soil towards any scabs that the root may have. It appears as a black ring at the top of the root.
Ordinarily, carrots do not require spraying for the control of this disease. In wet seasons, however, carrot foliage may be protected by spraying with compounds. Thorough spray coverage is essential for control. Control can be achieved with the use of fungicide sprays applied as soon as symptoms are visible.
Diseases caused by Phytoplasmas:
Aster yellows, phytoplasma.
Carrots that have aster yellows have an abnormal number of leaves. The leaves are yellow, twisted, and stunted. The roots remain slender and have an abnormal number of fine hairy roots. Carrot yellows is caused by a bacterium-like organism called a phytoplasma which also causes lettuce yellows and aster yellows. The virus is carried by leaf hoppers.
To keep down the amount of disease, growers should control leaf hoppers with insecticides and avoid planting carrots near asters.
Diseases caused by Nematodes:
The most common carrot nematodes are:
Meloidogyme incógnita - provokes the development of galls, which make the roots look as if they were covered with knots. Infected plants are stunted and sickly, with knots on their small feeding roots. This disease is caused by nematodes which can persist in the soil for years.
Heterodera carotae - This affects root growth, with many secondary roots developing in which the parasite can be found.
Ditylenchus dipsaci y d. - Destructor these cause little stains on the carrot surface.
Rotation with non susceptible plants, such as corn, can reduce the number of nematodes in the soil. Growing carrots in a new area will also control the disease. Care should be taken to avoid carrying any soil from the old site to the new site.
Aphids, Aphis and Myzus sp.
The bean aphid, Aphis fabae, green peach aphid, Myzus persicae, and corn root aphid, Aphis maidiradicis, all occasionally infest carrot. Control is usually not necessary.
Carrot Willow aphid -
Identification - This aphid is green or yellow-green and up to 2.5mm in length. The winged form is darker and more easily seen due to a darker patch on the upper side of the abdomen.
Symptoms - When present in large numbers the leaves can become discoloured and distorted as well as being covered with both 'honeydew' and discarded skins. Of greater importance to carrot growers is the transmission of a range of extremely damaging viruses. These include the carrot motley dwarf virus, which produces a yellow mottling of the leaves and plant stunting, parsnip yellow fleck virus, carrot red leaf virus, parsnip mosaic virus and celery mosaic virus. Leaf discolouration caused by virus infection can be confused with drought stress or carrot root fly attack. Observations of root symptoms have been described as 'bearding', where roots have a proliferation of root hairs, and 'kippering' where the roots split open. Later in the season dark discolourations can develop on the outside of the roots and tips as well as internal streaking.
Life-cycle - This species overwinters as eggs around bud axils of willows (Salix spp.). These hatch in February/March and the young aphids feed first on young shoots, then on foliage and catkins as colonies start to develop. In May winged forms disperse to nearby umbelliferous plants and other host crops. Cow parsley is one such host which is widely abundant throughout the UK in hedges, field margins and roadside verges. Cow parsley is genetically closely related to carrots and can act as an alternative host for carrot viruses. Aphid migration generally peaks in June and then declines. Later in the season winged forms return to umbelliferous plants in hedges and other nearby habitats, and after mating, eggs are laid on willow species in the autumn. In warmer conditions some individuals are able to survive the winter on umbelliferous plants in sheltered locations or on carrot plants protected from cold conditions under a covering of protective straw.
Importance - The willow-carrot aphid is a major pest of carrots, celery, parsnips and parsley and where pest pressure is severe economic loss can be significant especially on carrots sown in April/May. In Lancashire in 2007 (and to a lesser extent in 2008) many fields of early sown carrot crops were written-off as a result of a severe virus outbreak. The worst affected fields were those sown before April 3rd (particularly uncovered crops). In subsequent years problems were reported in Norfolk and Suffolk (2008) and Yorkshire (2009). Aphid numbers will increase more rapidly in hot dry conditions.
Threshold - A suitable insecticide spray programme should commence at the first reports of aphid movement into crops.
Aster leafhopper, Macrosteles quadrilineatus.
Although this leafhopper causes little direct injury, it is important because it carries the pathogen causing aster yellows disease on carrots, other vegetable crops and ornamental plants. The adult leafhoppers are about 1/8" long and are light grayish-green or greenish-yellow in colour. They can be distinguished from other leafhoppers by the six dark lines on the face. There are carrot varieties that are relatively resistant to aster yellows disease, including Royal Chantenay, Scarlet Nantes, El Presidente, Charger, Six Pack, Amtou, Toudo, and Gold King. Vegetable variety availability changes frequently, however. Row covers can be used to exclude leafhoppers from the crop.
Black swallowtail, Papilio polyxenes.
This caterpillar is occasionally found on carrot, parsley, dill, fennel, celery, and other cultivated or wild members of the carrot family. The caterpillars are about 2" long when fully grown, and green with a yellow spotted black lateral band on each segment. The adult is the common large black swallowtail butterfly with a wing spread of almost 4". It has two nearly parallel rows of yellow spots on the outer margins of the wings and other light blue areas on the rear wings. This insect over winters as a tan coloured chrysalis and there are two generations each year. They rarely require control other than hand picking.
Carrot rust fly, Psila rosae. (see above)
(really only relevant in commercial production)
Like many other plants Carrots are affected by diseases. These can affect their appearance or their ability to remain fresh.
These diseases are:
Mildew- also known as the pest of the vine. It attacks and dries up the leaf.
Black Rot - The carrots show black stains of varied sizes which increase when the carrots are stored.
Sclerotinia Rot - The first symptoms are soft and moist scabs. With relatively high humidity , a white, cottony stain may appear, covering the entire piece. High temperatures encourage the disease as does water condensation on the roots.
Fusarium - This disease starts on the upper part of the root as a small and dark scab, which increases in size. On the surface and towards the inside, dry and spongy rotting occurs.
Dry stains disease - This is a defect in the skin of the carrot, with the formation of holes a little longer than 1 cm and a few millimetres wide and deep.
Mangy-Root - This first develops in the foliage and when it reaches the ground, by action of the rain or through watering, it attacks the root. The carrots show signs of a soft rotting.
Sour Rot - This develops at temperatures higher than 20 º and in very humid environments. The carrots affected loose their colour, become watery and give off a sour smell.
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