How do Carrots Produce Seeds?
The Carrot plant (Daucus carota L.) is biennial and develops, in its second year, 'umbels' which are groups of flowers which produce seeds. D. carota var sativus is a herbaceous biennial plant grown as an annual. In the first year, seedlings emerge with two strap-like leaves which are the cotyledons, followed by rosettes of doubly compound leaves arising from the crown.
From the hypocotyl, a tap root develops. Initially, the plant produces top growth, which supplies carbohydrates to the swelling hypocotyl. High temperatures increase respiration in the leaves which reduces colour development and sugar accumulation. Low temperature initiates flower development and reduces carbohydrate accumulation in the hypocotyl. Most often, the plant will not initiate a flower head until the second year of growth and a chilling period.
The inflorescence is an umbel, with individual flowers maturing at staggering intervals. Cross-pollination is essential for seed development. The Apiaceae family derives its name from its affinity for attracting bees, which are the primary pollinator. D. carota seeds are tiny, and are covered with a fleshy mericarp that must be removed prior to seeding. Due to cross-pollination, variability is great in plants.
Flowering in biennials is stimulated upon vernalization (a period of extended exposure to low temperatures, usually experienced under winter conditions). Vernalization is a physiological response of phytohormones present in shoot apical meristems; in a carrot the shoot apical meristem is located in the layer of cells of the compressed stem found at the interface between the carrot root and the interior of the rosette of leaves (the tissues usually cut off before eating). When the cold period is satisfied in a biennial, a cascade of events occur, leading to flowering.
Carrots remain in a juvenile nonflowering form until subjected to cold exposure. Carrots planted in last year’s autumn garden and overwintered are still in their juvenile stage of growth and the root is still edible; but soon, a spring flush of shoot growth will occur in tissues that have been induced to flower commencing the irreversible transition from the juvenile (vegetative) edible stage to reproductive maturation (flowering).
Classical studies conducted in 1937 by M. K. Chailakhyan proposed that the floral stimulus is a hormone called “florigen.” A single compound has never been isolated; however, subsequent numerous physiological studies of floral transitioning have identified several putative signals that stimulate flowering: the carbohydrate sucrose and the plant hormones cytokinin and gibberellin. In response to cold exposure, these compounds are translocated from leaves to the rosette of the shoot apical meristem.
Seems science is never simple. In science, when one question is answered, other facets become exposed, leading to further research. This is the fun and excitement of science. The story of flowering in carrot is a little more complicated than just vernalization of carrots, the cold-induced flowering in response to hormonal stimuli. Genetics of an organism must be also considered.
(Source C. F. Quiros, UC Davis; J Exp Bot 10.1093)
Seeds are formed from carpels. They are tiny and covered with a spiny, hooked, and slightly curved mericarp. The mericarp contains a characteristic oil which inhibits seed germination, requiring the removal of the mericarp before planting.
Seeds are highly variable due to cross-pollination and have undergone extensive breeding programs to increase uniformity. Hybridization, however, has not produced consistent results. Seed development and vigor are determined by the conditions under which they are formed. A larger seed (embryo) will have a more rapid emergence and produce a stronger seedling. This is important in that the seed is slow to germinate and the young seedling is slow in the initial growth phase. There are approximately 23,000 seeds per ounce.
Carrot seeds are not true seeds in a botanical sense but are dry fruits called 'schizocarps'. The Carrot is not a fruit in the common understanding, so there are no seeds inside or on the carrot. The part of the carrot that you eat grows in the ground, usually with the wide end of the carrot just at the surface of the soil. The round mark you can see on that end of the carrot is where the leaves used to be - a big soft bunch of deep green leaves that look a bit like a fern.
Then the carrot is ready, it sends up a tall stem, which produces flowers, and eventually seeds. The seeds are very small, on average between 1 and mm in length and brownish is colour. Depending on the variety there are approximately 8000 seeds in half an ounce of seeds, so there are approximately 250,000 in a pound, roughly 2500 would fill a teaspoon.
The birds nest shaped fruit cluster of carrot has a remarkable mechanism for seed dispersal. The stalks are hygroscopic (readily taking up and retaining moisture) so that when conditions are dry and suitable for seed dispersal, they bend outwards exposing the fruits to wind and animals; when conditions are wet, they bend inwards forming the familiar nest structure which protects the seeds.
Now for the bad news - If you are planning to grow carrots, letting one go to seed may not be the best solution. Carrots are biennial - that means they take two years to reach maturity when they can produce seeds. Secondly, the vast majority of carrots that you buy from the supermarket are F1 hybrids. That means the seed comes from TWO parents who are different. (One may be big and fat, while the other is fast growing and a nice colour.)
By using pollen from one to fertilise the flowers of the other, the plant breeder and seed specialist aims to produce carrots with distinctive characteristics which are appropriate to the market. For example long and thin, deeper colour and consistency, or resistance to carrot fly, drought etc. The resultant carrot aims to be just what the farmer wants, but there are usually two problems for the consumer - The carrot may not have taste (low on the farmers list of priorities, because you can't cook and taste before you buy!!) and in the vast majority of cases, F1 hybrids are sterile.
If you grow an F1 carrot and harvest the seed ( after two years!!) they may very well not grow. In some cases you just get no flowers or seeds at all, or a sterile plant or a plant totally unlike its original. (read more below)
Carrot Seed Production by P.W. Simon, USDA, ARS, Department of Horticulture, University of Wisconsin, Madison, WI 53706 USA
Production of carrot seeds is a two-year project making it much more difficult than seed production of annual crops. Sound roots must first be produced (first year), these roots must be either harvested and carefully stored in a refrigerator or left in the field (if your winters are not too severe), these cold-treated (vernalized) roots must be grown and supplied with pollinating insects when flowering, and seed harvested. Carrot seed must be produced where no wild carrot (Queen Anne’s Lace) is growing.
1. Grow vigorous plants.
Grow plants at the same time of year and the same way you usually would. Weak or diseased plants are subject to storage loss during vernalization and they set few seeds. Plants with pencil-sized roots may be large enough, but larger roots are preferable.
2. Vernalize roots.
Carrots require 6 to 8 weeks cold treatment (2 to 5°C) for floral induction. Cool growing conditions can reduce the cold storage requirement. Roots can be vernalized in two ways. If carrots are a summer crop in your area, you can simply leave roots in the field over your winter season if your climate provides at least 10 weeks of temperatures below 15°C but where temperatures are not so cold that roots will freeze to death. Early in the growing season plants should be thinned to at least 5 cm apart. Dead or dying leaves must be removed and tops can be cut back at your usual harvest time to 5 cm to reduce transpiration and covered with mulch if necessary. When warm weather resumes, remove mulch if necessary and leaves will regrow and after several weeks a seed stalk will appear. This is the "seed-to-seed" method of carrot seed production. Losses are often very high with this method and off-types of roots cannot be eliminated since roots are not harvested and visually examined. The "root-to-seed" method is more reliable. Harvest roots when you usually would and discard off types. Trim tops back to 2 to 4 cm, air dry until no surface moisture remains, pack in paper bags with an equal volume of wood shavings, and place paper bags in closed polyethylene bags at 2-5°C. Puncture plastic bags after several weeks when water droplets accumulate inside the polyethylene film. Better storage survival is realized if prior to refrigeration, lateral and fibrous roots are removed, soil is removed by washing gently, and senescing leaves are removed. Even with these precautions carrots are often very susceptible to pathogen infection during storage. In commercial production, roots are dipped in fungicide before vernalization but this practice is not advised without extreme caution.
3. Grow vernalized plants.
For the "root-to-seed" method, plant vernalized roots when you would plant seed in the spring, taking care to keep plants well-watered but not in standing water. Seed stalk development will be evident in 4 to 6 weeks. Control of microbial (Alternaria, Cercospora, aster yellows, motley dwarf) and insect pests (aphids, spider mites, lygus bugs) is essential to assure seed production. It is very difficult to produce carrot seed where warm humid climates favor microbial growth. Note: Carrot seed can only be produced true to type if wild carrot (Queen Anne’s Lace) is not growing nearby since wild carrot will intercross and yield white-rooted plants.
Pollination is best performed by introducing bees or flies for pollen transfer during the period of receptivity. Natural populations of bees and other insects will sometimes be adequate. As an alternative, pollen movement is possible by hand or brush but seed set will often be low. Within 4 to 6 weeks after pollination the developing seed turns brown. Harvest (before the seed shatters) into paper bags to dry completely. Late-season rains can reduce seed yield drastically. Remove spines from dry seed by rubbing. Seed is now ready to plant since carrot has no seed dormancy. Store dry seed refrigerated in a moisture-proof container.
Heirloom versus hybrid seeds
The big difference in vegetable seeds is whether they are heirlooms or hybrids. Heirloom seeds are those that are open-pollinated, and passed down from generation to generation. Seeds from heirloom vegetables are true to type, meaning that you can save the seed from a certain plant and expect to get the same thing when you go ahead and plant those seeds again next season.
Hybrid vegetable seeds are created by artificially crossing two or more varieties of a certain plant. Hybrid vegetables are what one typically sees being sold in the your local garden store, those rows of bright, uniform carrots are probably grown from hybrid seeds.
One big disadvantage with hybrids is that you cannot save the seed: resulting seeds may be either sterile or produce a plant totally unlike its original. Although hybrid plants produce fruit that ripens at the same time and may be more resistant to pests or diseases, they’re not selected for their taste. They are typically selected for appearance and shelf life. Hybrid seeds are typically more expensive than heirlooms, too.
Home gardeners will likely be more satisfied with heirloom vegetable varsities for the flavour they deliver, and the sheer variety of vegetable types that are available. Blue potatoes, purple carrots, and orange tomatoes are not unusual among heirloom varieties. And perhaps best of all is that gardeners can save the seed from their heirloom vegetable plants, and continue to grow them year after year, and pass them on the family and friends well into the future. The same simply cannot be said for hybrids.
To learn more about saving seed and heirloom vegetables, visit Seed Savers Exchange.
Read more about the difference between hybrid and non hybrid seeds here.
Carrot seeds do not mature uniformly within a single plant. Seed maturation depends on the positions of flowers on the maternal plant, from which seeds are originated. Flowers and seeds in different orders (primary, secondary and tertiary) of umbels exhibit various levels of maturity.
Each umbel consists of multiple 'rays', groups of flowers. Maturity of rays depends on their positions (outer, middle and inner) in an umbel, with the outer maturing first. The birds-nest-shaped fruit cluster (umbel) of the carrot has a remarkable mechanism for seed dispersal. 'lThe stalks are hygroscopic, so that when conditions are dry and suitable for seed dispersal they bend outward, exposing the fruits to wind and animals; when conditions are wet, they bend inwards, forming the familiar bird’s nest structure, which protects the seeds. More about carrot seed research here.
Carrots are biennial, flowering in their second year of growth. In areas with mild winters, leave your carrots in the ground, mulching them heavily. The foliage will die back in autumn, but will then re-sprout and start to flower in the spring. In colder areas, dig up your carrots in the autumn, and select the best coloured and shaped roots. Twist off the foliage, and store the roots in a box of dry sand in a frost free place, making sure that they don't touch. In spring, replant the roots, and they will re-sprout and flower.
If you want to maintain a carrot variety effectively, you really need to save seed from at least 40 good roots to maintain good genetic diversity. If you have too small a genetic pool, you will end up with small, poor quality roots in a very few generations.
Carrots grow into big plants waist high or taller, producing successive branches with large flat umbels of flowers. They are insect pollinated, and need to be isolated from other flowering carrot varieties by at least 500m in an open field situation. This is not normally a big problem, since few people let their carrots go to seed. However, they will cross with wild carrot (Queen Anne's Lace), giving thin white useless roots.
As with all insect pollinated crops, barriers such as houses, tall hedges and other high crops can affect insect flight paths drastically, so you don't necessarily need to eliminate all Queen Anne's Lace within a 1/2 km radius; but do watch out for any white roots in subsequent generations and get rid of them.
To harvest your carrot seed, keep an eye on the umbels of flowers, and cut them off with secateurs as they start to turn brown and dry. If you have plenty of plants, just save seed from the first and second umbels of flowers to appear on each plant, as these will give the biggest and best seed. Dry the seed heads further inside, and then rub them between your hands or in a sieve to separate them. You will notice that the seeds have a 'beard' which is removed in commercial seed to make them easier to pack.
You can sieve the seeds further to remove more of the chaff, but there is no need to get the seed completely clean - just sow slightly more thickly to allow for the chaff mixed in.
Dried carrot seed is relatively short lived, but if it is stored somewhere cool and dry, it should give good germination for 3 years. Seeds can be safely stored for at least three years. Place seeds in jars, manila envelopes, cloth or mesh bags, plastic containers, or foil envelopes. The best containers are air-tight, such as a sealed glass jar, metal can, or foil envelope. Protect seed from sunlight.Store seeds in a cool (below 15 °C is ideal), dry location. Place the seeds in a refrigerator for long term storage.
For short-term storage, keep the seeds in a cool, shady and dry place.
What do seeds contain and how do they work?
A seed is the product of a fertilised ovule (egg). It is the means by which the progeny of a plant can be spread. The seed contains the embryo from which a new plant will grow. The seed contains a supply of food called the endosperm that is used by the new plant to develop. The whole seed is covered in a seed coat known as the testa. Seeds are found in a great variety of types.
The testa protects the contents of the seed. For the seed to grow the testa needs to split. The seed imbibes water which causes it to swell and split open the testa. The seed coat varies between different types of seed, which in turn affects, the uptake of water. Some seed coats are very thick and need to go through a process of scarification to allow absorption of water. In the wild this would occur by the gnawing actions of animals, passing through an animal's digestive tract or abrasion by a rocky surface. In the artificial environment of gardening this process is imitated by nicking the seed coat with a sharp knife or rubbing with sandpaper.
If a seed has a thin coat the presence of light can either encourage or inhibit seed growth. Smaller seeds tend to require light for germination.
Read more about the difference between hybrid and non hybrid seeds here.
Leaves A tight rosette of leaves arises from the crown after the emergence of the characteristic cotyledonary leaves. Two kinds of leaves are formed. The lower leaves are pinnate and linear or lanceolet, and are fine and lacy in appearance. The upper leaves are smaller and less divided. Leaves in the second year of growth arise from the crown and along the elongated stem.
Flower Vernalization of the plant or seed induces formation of the inflorescence. Vernalization period is up to 6 to 8 weeks at temperatures below 50°F. Several cycles of flowering occur, producing umbel shaped inflorescence. The first inflorescence is the largest, and is termed the king umbel, followed by the queen and other secondary umbels. The white flowers are perfect, and require cross-pollination for seed set. Male sterile hybrids are often used as a seed source to decrease variability.
BUY YOUR SEEDS HERE:
Some Sources of Carrot Seed Varieties for United States Home Gardeners - here
William Rubel gives a detailed commentary on the main online catalogues.- here
Salt Spring Seeds (Heritage and Heirloom seeds) - here
Black Carrots - see Carrot Museum Black Carrot page.