Vegetable Seed Production: Carrot

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Common Name: Carrot Scientific Name:  Daucus carota Family: Apiaceae

In addition to reading the information in this section, please view this multimedia presentation on Carrots (The video will open in a new window.)

Botany

Family Apiaceae, Daucus carota L.

The carrot is a member of the family Apiaceae. (Umbelliferae is the outdated name.) Carrot is a cool season biennial dicot.

Carrot plants develop a fleshy axis with silvery short stem the first year, and an elongated flower stem the second year after the plant has been vernalized at temperatures below 50 degrees F. "Umbel" is the botanical name for the distinctive flower cluster that forms at the end of the seed stalk.

Carrot umbel order

The fleshy axis is the carrot of commerce and the leaves (tops) are not edible. Orange-rooted carrots are most popular in the US but in other parts of the world, cultivars with red, white, yellow, and purple roots are also grown

The enlarged or edible portion of the carrot root from a morphological viewpoint is largely the hypocotyl, although the upper taproot is also often enlarged, the relative amount depends on the cultivar.

Carrot cultivars

Lateral roots arise from the lower hypocotyl as well as from the primary root, so the point of division cannot be readily seen. Since the point of transition is in the upper hypocotyl, most of the hypocotyl is root-like in structure.

The mature enlarged structure is composed of:

1. A pith-like center, larger at the upper portion, consists largely of thin-walled xylem parenchyma and is not true pith
2. Core: composed of xylem with the thickness depending on the cultivar
3. Cambium: separates the core from the outside
4. Phloem: largely parenchyma with some sieve tubes, companion cells, and few fibers
5. Pericycle: takes up about a third as much space outside the cambium as the phloem

Carrot structure illustrated

There are oil ducts present near the periphery of the pericycle which give carrots their characteristic flavor and aroma. Carrot seeds also contain these same oils.

Soil Nutrition

Carrots are tolerant of a wide range of soil pHs but generally 5.5 to 6.5 is preferred on organic soils and a pH of 6.0-6.8 on mineral soils. Fertilization rates vary dramatically with soil type and should be calculated based on soil test results. Carrots are heavy feeders and remove approximately 200 lbs/acre N, 50 lbs/acre phosphate, and 250 lbs/acre of K. A side dressing of N is commonly applied 4 to 6 weeks after seeding.

Isolation

Because of the high possibility of cross-pollination, isolation distances for commercial seed crops should be a minimum of 800 m. For basic seed the distance should be greater, about 1600 m.

In areas which specialize in carrot seed production the different cultivars within the same type can be zoned; this minimizes pollen contamination between the different types.

Cultivated carrots cross-pollinate very readily with the wild carrot and this must be taken into account when choosing sites for seed production. Contamination of seed crops by wild carrot pollen is a major reason for genetical deterioration of seed stocks in some areas of the world.

Planting

Carrots are usually direct-seeded, except on some occasions when roots are transplanted for seed production. Commercial growers often scatter seeds 0.5 to 0.75 inches deep in a band 3 to 4 inches wide. Three or four parallel seed lines may be sown about 2 inches apart. In many western states, two bands of seeds 3 to 4 inches wide are planted at the edges of a 20-22 inch raised bed.

Seeding rates vary from 25 to 60 seeds per foot. Heavy rates may be used in band seeding, where soil conditions are unfavorable for emerging, or when small rooted cultivars are grown. Lower rates are used for large processing cultivars or where carrots are planted in narrow rows.

Poor seeding performance is one of the biggest problems in carrot production. Seeds are very small, slow to germinate, and germination percentages are often only about 90 to 95%. Carrots germinate at soil temperatures above 40 degrees F but optimum germination occurs at 80 degrees F.

Irrigation

Uniform soil moisture is essential. Water stress will slow growth and result in thickened, woody cells, reduced sugar content, and bitter flavor. At least 1 inch per week of water are needed for best growth. In western states, overhead irrigation may be used to establish the crop and furrow irrigation used after emergence. In eastern states and some western growers may use overhead irrigation throughout the whole season.

Roguing

'Seed to seed' production

Very little if any roguing can be done when the crop is grown-on without lifting. But plants bolting early and those with untypical foliage characters should be removed.

If the plant is lifted and replanted it is rogued as described below for 'root to seed' but very little confirmation of root type can be done.

'Root to seed' production

During the first year's growing season:

1. Remove plants displaying atypical foliage. Remove plants bolting in the first year.

2. After the roots have been lifted inspect for trueness to type, according to root shape, color and size. Discard roots showing poor color, incorrect color, colored shoulders (purple, green), split, fanged, rough surface.

Seed Harvest

In small-scale commercial production where hand labor is plentiful the umbels are cut by hand as they ripen. This system is also used for small plots of high value stock seed.

On a larger scale, where mechanized systems are used, it is necessary to decide the best cutting time for maximum seed yield. There is a tendency for some carrot seed to be lost by shattering or dropping from primary umbels if cutting is delayed to allow maturity of seed on secondary and tertiary umbels. In some areas of the world, the plants are sprayed with polyvinyl acetate to prevent loss from the primary umbels. Harvesting early in the day when the dew is still on the seed heads will also reduce the amount of seed which is lost from the umbels.

Generally, the crop is cut when the earliest maturing seed on the primary umbel is mature and starting to drop. Carrot seed is brown when ripe and at this stage the umbel is brittle. The cut crop continues to dry in windrows.

When the seed crop has ripened it can be separated from the remainder of the plant debris by a combine harvester. As with the initial cutting, the combining can be done early in the day to take advantage of dew reducing loss from dropping. The plant material must not be too dry or in addition to losing seed, the plant debris will shatter and increase the subsequent seed cleaning operation. Where the scale of operation is inappropriate for combine harvesters the dry material is put through a thresher.

Cleaning

Carrot (like dill and caraway) has spines or "beards" on the seed. These must be removed by a debearder before further cleaning operations. Debearding improves the seed flow and reduces the volume of the seed-lot. Further cleaning is achieved by aspirated screens and indent cylinders.

Seed Yield

The yield of carrot seed at different plant densities is the subject of current investigations in several research centers of the world. At present expected yield of open-pollinated cultivars in the temperate regions is about 600 kg per hectare with highest yields achieved reaching 1000 kg per hectare. The 1000 grain weight is c. 0.8 g.

Yields in the tropical regions are usually considerably lower despite using higher altitudes to achieve satisfactory vernalization, and figures of about 300 kg per hectare should be expected for the European types. The Asiatic types produce only about 250 kg per hectare when seeded in the tropics.

Seed Identification: