In Depth Guide to Carrot Cultivation
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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.
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Sowing for continuous
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.
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.
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 fly is the most problematic pest of carrots and allied vegetables. It can make a large proportion of the crop inedible. Symptoms - Rusty brown scars ring the tap roots of carrot and other susceptible vegetables, making them inedible, and susceptible to secondary rots. When the roots are cut through, tunnels are revealed, often inhabited by slender creamy-yellow maggots up to 9mm (3/8in) long.
There is no definitive solution, many ideas work for some and not others. Physical barriers work well, as does high planting and fly resistant varieities.
Chemical controls - Lambda-cyhalothrin (Westland Plant Rescue Fruit & Vegetable Bug Killer) can be sprayed against the adult stage of this pest. There are no soil-applied pesticides available for garden use against carrot fly larvae. Pesticides for gardeners (Adobe Acrobat pdf document outlining pesticides available to gardeners)
Non-chemical controls - Sow sparsely to avoid thinning the seedlings. Female carrot flies searching for egg-laying opportunities are attracted by the smell released when surplus plants are removed Late sown carrots (after mid-May) avoid the first generation of this pest; similarly carrots harvested before late August avoid the second generation Protect vulnerable crops by surrounding them with 60cm (2ft) high barriers made of clear polythene to exclude the low-flying female flies, or cover the plants with insect-proof netting, such as Enviromesh.
It is essential to practise crop rotation with these methods, otherwise adult carrot flies may emerge within the protected crop from overwintered pupae in the soil Choose carrot cultivars that are less susceptible to carrot fly, such as 'Fly Away', 'Maestro', 'Resistafly' and 'Sytan'. These cultivars are less susceptible to carrot fly, rather than being fully resistant A mixture of pathogenic nematodes, sold as 'Nemasys Grow Your Own', can be watered into the soil to control the young larvae. This is available by mail order from biological control suppliers.
Carrot fly tends to lay its eggs in the late afternoon or after rain so beware of these critical times.
Poly-Tunnels - a sure fire way to keep carrot fly at bay. There are now suppliers of small (for domestic/school garden use) to large commercial tunnels. These give total control of all the enviironmental conditions and can also extend the growing season. For more information contact Firsttunnels uk, the leading UK supplier here.
Biology - The maggots hatch from eggs laid in late May–June and in August-September Newly-hatched larvae feed on the fine roots but later bore into the tap roots. The brown scars are where tunnels near the root surface have collapsed Two or three generations of carrot fly can develop between late spring and autumn, with the pest overwintering as larvae or pupae
The carrot fly, Psila rosae is a serious
and widespread pest and is really the only pest worth worrying about. The
damage is done by the grubs tunnelling into carrot roots, disfiguring them
and allowing moulds to gain a hold.
Carrot Fly are low fliers! - one simple method for a home grower is grow in pots and put them on a table. The fly whizz past at no more than 18 inches above ground level. Adults have a wingspan of 12mm and the larvae (8-10mm) feed on carrot roots, making holes that often become infected with other rots and moulds, resulting in carrots no-one wants to buy. carrot flies aren’t strong fliers though, so by growing susceptible crops in a windy area life can be made very difficult for them.
Mulching with grass cuttings can make it harder for the female flies to find
a suitable egg laying site. The crop appears to benefit from the extra support
given by the earth and you'll have noticeably fewer carrots with green shoulders.
The mulch enables the carrots to make better use of nutrients and water in
the soil, encouraging healthy growing conditions and improving their ability
to resist attack. It also makes it more difficult for the female flies to
lay their eggs in cracks in the soil. A range of creatures will make their
home under the mulch, some of which will be predators of the carrot fly such
as ground beetles and centipedes. Watch out for slugs and snails who will
also thrive in these conditions!
Some seed companies suggest a "sacrificial strip" in that you
should grow Flyaway or Resistafly alongside non resistant varieties such as
early Nantes. This will allow any carrot fly in the area to attack the non
resistant row, otherwise a minor attack can still occur.
Whilst carrots are their ultimate favourite food, the carrot fly is partial to munching on other plants in the same ‘Umbelliferae’ group that carrots belong to. That includes such common plants as cow parsley, for example. These can be grown in the hedgerows around the carrot fields and the carrot flies can overwinter in these plants quite happily until their favourite carrots are planted again the next year. IT is therefore important to rotate crops to a very careful schedule to helps keep the naturally-occurring population low.
When it comes to reproduction, the carrot fly is no lazy performer. There are at least two generations per year – sometimes there are three. The first eggs hatch with the onset of warmer weather in May and June. Sow their carrots in May before any eggs hatch, then cover them with re-usable fleece as a barrier against the carrot fly entry. By the time the little plants emerge 4 weeks later, the majority of the first generation of carrot flies has hatched and moved safely onto other host plants in the hedgerows, having been fooled into thinking no delicious carrots lay beneath them.
For the second generation of flies emerging in August, new weapons are required. So organic growers place sticky yellow traps (insects are attracted to the colour yellow) in the crops. Each week the number of flies trapped is counted to monitor populations. Once fly numbers are at a peak, it’s time to get out the garlic! This is because this is the time when egg-laying is happening and flies hate the smell of garlic. Garlic granules are applied to the growing crop straight away from July to the end of August, every week. Some people grow onions alongside carrots for the same reason – it masks the smell of bruised carrot foliage that attracts the flies.
It is vital to take every last carrot out of the field at harvest time. A single carrot left in the ground then provides a holiday home for carrot flies over the winter so it's important to do everything you can to make this environment inhospitable for pests as part of organic methods. It sounds complicated, but it’s great to use such natural methods to grow a strong healthy crop, and there’s a great satisfaction when you know you’ve outsmarted your pest opponents!
How do I detect Carrot Fly?
Basically you will not know until you lift the crop. In severe infestations the first sign is that the Carrot leaves look an orange / reddish / rusty colour. They then turn yellow. On lifting an affected Carrot it will be seen that the root end will be dark or black. Close examination of what appear to be good Carrots may reveal small holes in the Carrot. If Carrots are put in a bucket of water, badly affected ones will come to the surface. This however, does not mean, that those which do not float are totally unaffected.
More Detail on the No 1 Pest:
An adult carrot (or "rust") fly is a very small black fly which has been described as "a low flying miniature cruise missile". The fly is attracted to the Carrots by smell. It lays its eggs in the soil adjacent to the Carrots. The eggs hatch and the grubs burrow into the roots. The result is a mess, with grub tunnels all through the carrot.
It stays in the ground over winter gorging itself on your Carrots, pupates and lays eggs in early spring. Eggs will ideally be laid near to Carrots but Parsley, (and Cow Parsley), Celery and Parsnips, are also affected. After the spring generation have hatched they lay eggs in June and July and this generation hatches and matures in enough time to have another frenzy of egg laying August / September time.
This insect pest also loves parsley, parsnips, celery and celeriac. Attacks are particularly bad in old established gardens where the population builds up each year. Most carrot pests and diseases are soil-borne and can be controlled by crop rotation.
Adult flies, attracted by the smell of carrots, lay their eggs at ground level so the little white grubs can hatch out and tunnel into the carrot roots. Therefore vulnerable young seedlings die and the foliage of large seedlings turn bronzy red and the plants weaken.
There are usually two, sometimes three, generations of carrot fly in a year. The first and worst attack occurs mid-May and mid-June; subsequent attacks are in Autumn and in Winter in mild seasons.
The Carrot Fly is low flying and therefore can be prevented from laying its eggs by physical barriers such as polythene. Surround the carrots with a 60cm high barrier of clear polythene film or fine netting nailed to canes, making sure there are no gaps at ground level. These barriers should be no more than 100cm wide. In dry months water the carrots if they appear to get dry because of the barrier.
The later generation feeds on carrot roots left in the soil in Autumn and early Winter. Grubs pupate in the soil, and hatch out as the first generation next Spring.
Other practical tips include sowing early carrots before mid-March and main crop towards the end of May or mid-June. Sow very thinly to minimise the need for later thinning. You can try growing carrots under fine net film, very similar to mosquito netting, making sure there are no gaps at ground level.
Inter-planting onions or garlic in the carrot beds may also ward off the villainous flies. Mix carrot seed with feathery leaved annual flowers when sowing.
Compost and wood ashes will also scare off not only carrot flies but carrot weevils, wireworms, and other carrot pests. Probably the best organic way to get rid of pests is to soak the bed once a week with a thin mixture of wood ashes and water using a watering can.
Some chemical control can be achieved through dusting the rows before sowing in Spring with bromophos, phroxim or pirimphos-methyl dust. Protection of late crops can be done by spraying with half strength liquid pirimphos-methyl in early August.
Grow varieties with partial resistance. "Fly away" and "Sytan" have some resistance and the old fashioned mainly white fodder carrot is also fairly resistant.
Watch a video showing some useful tips.
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.
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.
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.