Fresh Market Carrot Production in Alberta

The carrot is a cool-season crop that grows well almost anywhere in Alberta. It is a member of the Umbelliferae family, which includes celery and parsnips.

Description

Carrots prefer average temperatures between 15° and 20°C. The crop is a cool-season, day neutral biennial that normally only flowers in the second year of growth. Exceptions can occur when temperatures below 10°C occur in the 5 to 8-leaf seedling stage. When this situation occurs, a small percentage of plants will produce a flower stalk.

Prolonged hot weather later in development may cause an undesirable strong flavor and coarseness in the roots. Temperatures below 10°C tend to make the roots longer, more slender and paler in color.

The pigment, carotene, accounts for a high percentage of color in carrots and develops best between 15° and 20°C. Better color in carrots is strongly associated with higher total carotene content.

Carotene is the precursor to vitamin A, and studies have shown that carrots grown under cool night and warm day conditions produce more carotene than carrots grown under continuous cool temperatures.

The taste in carrots is based on the right balance of sugars and terpenoids. Terpenoids produce a soapy turpentine-like taste that will mask sweetness. Differences in flavour components have been found to be attributable more to genetics than to climatic conditions; however, the controversy continues.

Carrots also possess polyphenolic compounds and phenolase activity, which create a surface browning problem when root surface abrasion occurs.

The development of the carrot root begins very early in the life of the seedling, with the formation of the single cambium layer. The major carbohydrates stored are sucrose, glucose and fructose. The ratio of sucrose to reducing sugars varies between varieties. Early varieties generally have a higher level of reducing sugars while storage types have a greater proportion of sucrose.

A simple classification lists four major carrot types, based largely on root shape and storage capability:

1.	Chantenay - foliage is very strong; roots are broad in the shoulder, relatively short and blunt tipped; used primarily for processing; have a strong pale-colored core; stores well.
2.	Danvers - foliage is strong, roots are moderately wide in the shoulder, longer than Chantenay types and are pointed; strong core development with medium pigment; good storage quality; used in processing and fresh market.
3.	Imperator - strong foliage; roots are narrower in the shoulder than Danvers type; roots are long and slender, tapering to a pointed tip; moderate core development; medium pigmented; good storage quality; most popular type used for fresh market.
4.	Nantes - sparse foliage is weakly attached to crown; root is moderately long with uniform diameter along length; root tip is rounded when mature; highly pigmented core is poorly developed making roots brittle; matures early; roots have higher sugar content, lower in terpenoids and lower dry matter making roots less suitable for long-term storage; surface is thinner and easier to scar.

Seed and Germination

Carrot seed is irregularly shaped and looks much like lettuce seed. The seed size varies tremendously from variety to variety and year to year in the same variety and seed lot. These factors of shape and size variance add to the difficulty in handling seed for precision seeding as well as the associated problems with seedling establishment in the field.

The germination of carrot seed is slow and irregular. The seed requires much higher soil moisture than most other vegetables for good emergence.

Much of the variability in time of emergence and variability in seedling size arises from three factors:

• seed
• seeding method
• seedbed conditions

Low germination seedlots tend to germinate more slowly, and there is a greater spread in emergence, which gives rise to greater variability in plant size. Genetics also plays a role in the differences between varieties in seedling establishment.

Seeding

Because of the irregular seed shape, coating the seed with an inert material produces a pellet that allows for more precise seed placement by a precision drill, such as the Stanhay. Seeding raw seed is also feasible if the drill is calibrated properly.

Original work with the Stanhay in Alberta resulted in the recommendation to plant carrots in a triple line. The outcome was a high percentage of small roots, particularly from the center line, referred to as the "edge effect." With this effect, plants in the outside lines grow (size up) more quickly because there is less competition along the outside.

Fewer holes in the middle line, designed to drop fewer seed, has not solved the problem of root size variation. More recently, a double or twin-line is being recommended as a better option.

In twin-line seeding, the recommended seed drop per linear foot of row is 40 seeds. The general observation is that only about 50 per cent of the seed germinates, producing a harvest count of 20 to 25 plants per linear foot of row. The distance between lines of 1 or 12 inches is determined by the shoe (coulter) used.

The Stanhay belt seed drop system is by far the most common precision seeder in use in Alberta. Currently, there are two versions of Stanhay Seeders available. The 800 series, which is a belt-driven seed drop system, and the newer Singulaire 780, which is based on a vacuum seed drop system. The Stanhay belt system performs well when seeding carrots if properly calibrated and equipped.

The hole diameter, number and configuration are matched to the seed being sown. The larger the hole required, the fewer holes that can be punched in each line before holes overlap. With coated seed, seed placement can be exact; however, because only one seed drops through every hole in the seed belt, seed drop is limited to only 25 to 30 seeds per linear foot of row.

With raw seed, multiple seed drop per hole allows for much higher seeding rates and, therefore, greater flexibility when manipulating seed drop rates. Plastic belts are used for raw seed, whereas rubber grooved belts are used for pellet seed. Small raw seed of approximately 291,600 seeds/kg (648,000 seeds/lb) will require a #8 hole size. Medium raw seed of approximately 230,400 seeds/kg (512,000 seeds/lb) will require a #9 hole size. Large raw seed of approximately 111,600 seeds/kg (248,000 seeds/lb) will require a #10 hole size. Pellet seed varies to manufacturers specifications.

Seed size in carrots varies dramatically, so it is important to calibrate seeding rate using a test rig. A test rig simulates the seeder and how it performs in the field. The following points should be kept in mind when calibrating and operating the Stanhay:

• Never estimate seed belt hole size from grade size alone; seed count per pound and per cent germination are important criteria required to get an accurate idea of seed belt requirements.
• Drive speed should average at 2 1/2 to 3 mph to allow for slow seed belt revolution and seed drop into belt holes.
• Small amounts of talcum powder can assist raw seed flow.
• Nose of shoes (coulters) must be level to maintain accurate seeder performance.
• Seeding depth of 1/2 inch is recommended.
• Seed box repeller tires, choke plate and spring base should be replaced if worn.
• Use choke "T" for double line or triple line drilling.
• Use spring base "A" for raw seed.

Much of the variation in carrot root size arises from the seed itself, the seedbed and also plant distribution within the row. An arrangement in which plants are more evenly distributed in a given area is favorable in influencing yield and grade.

Much of the plant-to-plant variation arises before and during crop establishment. Variation in seedling size at emergence is magnified at later stages by plant competition.

When plants begin to compete, the growth rate of smaller plants decreases relative to that of the larger plants. This situation occurs because larger plants surround smaller plants, and these smaller plants become increasingly deprived of light and other growth factors. So the level of variation among the plants at the time of seedling emergence significantly affects root size variation at harvest many months later.

Carrot yield depends strongly on plant density. Higher seeding rates increase yield, but the yield increase is largely made up of smaller-size roots that require more time to mature or size up.

In Alberta, this factor can be a serious problem in a short growing season. Carrots sown at a high density suffer from greater root size variation due to the greater initial variation in seedling emergence. Spacing alone does not solve the problem of root size variability in carrots.

Inter-row spacing appears to have little effect on yield. However, line spacing (distance between lines in a row) may increase yield and quality as the distance between seed lines increases.

This outcome is why shoes that space twin lines 2 1/2 inches apart rather than 2.0 inches are preferred. These wider shoes improve line separation so that competition between plants is reduced by the edge effect. Wide line distribution can cause problems with top-lifting harvesters, as some models are unable to pick up rows with wide line separations.

Row position in the field has some bearing on carrot root quality. Rows adjacent to tractor tire marks produce smaller roots, and there is also a higher incidence of forking and other distortion characteristics. These conditions result from soil compaction, so entry into a carrot field should be kept to a minimum. Equipment use should be restricted to traveling the existing tire marks.

Seeder design is continually being improved to place seed more effectively. The newest design uses a vacuum device to place seed in the seed-row very accurately. Stanhay has a drill called the Singulaire that operates on a vacuum principle, which improves seed placement of both raw and pelleted seed.

However, seeder designs that only improve uniformity of spacing in the row will not likely result in crops with less root variation. Uniformity of sowing depth and seed-to-soil contact are must also be improved.

Soil/Seedbed Preparation

Carrots will grow in almost any kind of soil provided the soil is deep, loose and easily crumbled with good water retention qualities.

Some evidence indicates that carrots grown in peat soils have a lower sugar content than carrots grown in mineral soils. Very sandy soils induce root surface abrasion, which causes external browning to occur.

Some soil types, particularly clay soils, pose special problems requiring special seedbed preparations. Soil ridges can be created to provide uninhibited root growth in heavier soils; otherwise, 10-inch deep ploughing or chiselling is required followed by packer harrowing before seeding.

Soil ridging equipment built and used in Alberta compacts soil into tapering 8 to 12- inch beds measuring 6 inches across the ridge surface. The carrot seeds are sown on the ridge surface.

Vertical downward compaction of the soil is required during the bed shaping process to stabilize the soil. The downward pressure of a spring-loaded flat press wheel mounted above the soilbed molders achieves this effect. Beds are usually on 24-inch centers. They provide loose soil rooting in otherwise compaction-prone soils.

Soils higher in clay content are also subject to crusting or capping. Carrot seedlings are very weak, and a crusted soil can drastically reduce emergence. The use of Stanhay drills with rubber coated packer wheels and light water applications daily during germination greatly help reduce the problem. There is no successful mechanical or chemical solution to soil crusting problems.

Sandy soils are subject to wind erosion, so severe damage to young seedling stands can occur as sand particles sift along the soil surface. Frequent irrigation provides some protection, but the best protection is gained by planting a cereal crop down each row.

Grain drill boxes mounted on the Stanhay permit the seeding of cereal windbreaks at the same time as carrots are seeded. A grassy weed herbicide eliminates the cereal once carrots have become well established.

A cereal/carrot crop rotation will be effective. A necessary precaution is to avoid recently broken land and canola in the rotation because of the insect and disease implications. Carrots should never follow carrots and should appear in the rotation only once every four years.

Fertilizer Requirements

Carrots are efficient feeders because the root system extends over a broad area, and fertilizer is readily utilized in the vicinity of the roots. Fresh manure may hinder proper root development and should be avoided in preference to light applications of commercial fertilizers.

Carrots are not heavy feeders on nutrients, largely because of their extensive rooting system. The crop removes less than 3 per cent of the total nitrogen it uses for the entire season during the first quarter of its growth period. As a result, it is hard to justify applying large amounts of nitrogen as a pre-plant fertilizer.

Yield does not increase significantly when nitrogen is applied in excess of 200 lb/ac.; however, top growth increases under the influence of high nitrogen availability.

Nitrogen removal by carrots (132 days of growth)
Crop growth period	Nitrogen uptake (lb/ac)
1/4	3
1/2	20
3/4	75
Total	128

Carrots require very little phosphorous and have the ability to extract it from the soil effectively. Available phosphorous should approach 50 lb/ac.

Potassium levels are usually adequate in southern Alberta. Further north in the province, natural reserves decline and need to be maintained at about 400 lb/ac.

Nitrogen absorption by carrot plant part (lb/ac)
	Nitrogen	Phosphorous	Potassium
Roots	80	20	200
Tops	65	5	145
Total	145	25	345

Irrigation

Germination
Carrot seed is slow to germinate. Immediately after seeding, frequent, light water applications are required to enhance seedling emergence.

Coated seed requires more careful irrigation management because more water is required to get the same results than with raw seed. Seed coatings result in slower water uptake, and more moisture is required to penetrate the coating. Carrot seedlings also have difficulty breaking through crusted soil, and a damp soil surface enhances seedling emergence.

Growing-on
Once carrot seedling roots begin to show color, meeting the seedlings' water requirement becomes critical to continued, uninterrupted growth.

Part of the change in cell wall composition during root growth is notch-sensitivity or weak spots. Carrots are brittle because of the development of notches, which are concentrated stress points leading to factures. If there is ample water available during this period of maximum growing-on, the likelihood of stress fractures in the roots is much lower.

Carrots appear to be most susceptible to root fractures during the period of maximum growth. Heavy water application during this period promotes root growth and stress point development, particularly more so in sandy soils.

Heavy irrigation towards the end of the crop growth can increase root size without inducing stress fracturing. This situation may be partly due to changes in cell wall composition during the latter stages of growth, enabling the carrot to withstand the high stresses.

On the other hand, roots kept drier during early growth will be tougher during the period of maximum radial expansion. As a result, fewer notches will develop and roots will be less prone to fracturing.

Systems in use
The center pivot irrigation system is most effective in delivering uniform, light water applications, which is most critical during germination and seedling emergence.

During later growth, if not properly monitored, pivots will provide less than adequate water supplies because of the rapid forward movement of the system.

The sideroll system is better in that regard because it remains stationary during irrigation. However, moisture fluctuations between sets hamper germination and growth. For this reason, a sideroll irrigation system should not be expected to cover more than 20 acres of crop.

Insects and Diseases

Carrots are generally free of insect and disease problems. However in some years, aster yellows becomes a major concern in Alberta. Aster yellows is a disease spread by leafhoppers that migrate into Alberta from the United States.

Infected leafhoppers feeding on carrots spread the disease, and in three to five weeks, symptoms begin to appear. Leaf bronzing, the proliferation of new growth from the crown and hairy roots are visual symptoms of disease development.

Several insecticides are effective in limiting leafhopper populations. Weekly insecticide applications are required, and areas bordering a carrot field should also be sprayed.

Diseases in the field are rare. Under very wet soil conditions, particularly along areas where water has been standing, soft rot can occur. Plants may survive marginally; however, lower root sections may be decaying. Under such circumstances, it is strongly advised not to harvest carrots from these areas, as they will spread the disease in storage.

Frost Effects

Carrots in the seedling stage will tolerate frosts down to -7°C. With this tolerance, early seeding for early harvesting is recommended.

The roots are most susceptible to frost injury when the plants are mature. Roots will tolerate severe frost if they are not exposed above the soil and also provided the soil does not freeze.

Tops become weakened during a frost, and successive hard frosts will cause the tops to droop and finally collapse. Once this collapse occurs, top-lifting harvesters have greater difficulty in picking up carrots by their tops.

Once the soil begins to freeze, crown injury begins to occur; small, hairline, horizontal cracks develop. Frost-damaged carrots have a reduced storage life.

As cooler air temperatures occur, soil temperature also begins to fall. The brittleness of carrot roots increases with cooler soil temperatures, and breakage can be very high. Nantes types, largely because they lack strong central core development, break more readily when very cold.

Harvesting/Harvesters

Carrots are ready for commercial harvesting when at least 50 per cent of the roots meet or exceed Canada #1 grade specifications.

Two types of harvesters are used in Alberta, the top-lifter and the digger-elevator. The top-lifter is more popular because of the reduced incidence of root damage and greater versatility. This machine can also harvest red beets, rutabagas and parsnips.

The top-lifter operates in a wide range of soil types. A single-row machine requires a 55 to 75 hp tractor, travels at about 3 to 8 mph and handles rows that are at least 20 inches apart. Top-lifting harvesters are not suited to high density plantings like the digger- elevator types are. All types result in at least 20 per cent waste, resulting from broken and undug carrots.

The process of harvesting carrots is very slow; 15 acres harvested a day is exceptional. Efficiency improves with multiple-row diggers available in all models. Often the bottleneck in harvesting is the packing plant, where unloading and washing of carrots is a slow process.

Harvesting equipment models available and in use in Alberta:

1.	Univerco - a French manufactured machine. It is power-take off (PTO) driven and three-point hitch attached. Automatic row and ground detectors maintain the machine on the row and at the proper digging depth. The flexible elevator permits truck and pallet loading.
2.	ASA-Lift - a Danish built machine. It is PTO driven and is attached to the tractor by a three-point hitch. An electronic sensor system controls the height of the picking head and steers sideways, so the harvester automatically follows the row. The flexible elevator permits truck and pallet loading.
3.	FMC Scott - an American built machine. It was the first to be used in Alberta. The harvester is PTO driven and trailer hitched. Old models have manual row finding, and newer models operate on automatic row sensors. The elevator is not as flexible as in other models.
4.	Krier - is very much like the FMC Scott, but is much less expensive.

During harvest, care must be taken to minimize root breakage. Breakage is largely a man-made problem, being caused by excessive drop heights and inadequate padding on equipment.

A reinforced vinyl hammock, with its corners attached to the sides of the trailer with tarp straps, will break the fall and reduce root breakage in the field trailer or truck box.

Washing/Packing

Carrots must be washed before packing or storing. Dirty roots may stain and are also subject to increased incidence of storage disease.

From the field, carrots are trucked to the packing shed. At the shed, they are dumped into a sump containing water by the most efficient means and with the least amount of breakage possible. A deep sump filled with water is the most cost effective pre-wash and cooling system.

From the sump, the carrots are elevated up to a barrel washer for a final wash and are then graded and packaged or placed into storage for long-term holding.

The complexity and cost of the equipment used in a packing shed varies considerably. The volume of product processed in one day is primarily determined by the size of barrel washer used and whether packing is manual or fully automated. Large volumes of clean water are required as is access to 220 volt, 3-phase power.

Before loading for storage, carrots should be given a final rinse with some fungicide, such as chlorine. This treatment greatly helps in retarding storage decay.

Storage

Carrots, particularly Imperator types, can be stored under refrigeration for at least six months. Refrigeration that can maintain a temperature near 0øC is required as is a relative humidity of 90 per cent plus. Air movement of at least 19 to 23 cfm (cubic feet per minute) per ton is required when bulk storing carrots.

Bulk storage means carrots in piles that can measure up to 14 feet in depth. In bulk storage, 1 cubic foot of space will hold 34 pounds of carrots. Carrots can also be stored in pallet bins, which facilitate air movement and isolate disease, but greatly reduce storage capacity over bulk storage.

Storage designs vary from the less expensive Quonset style, which is best suited to bulk storage, to the A-frame construction, which is ideal for either bulk or palleted storage.

Wood-built structures must be well insulated with polyurethane foam sprayed over all interior walls and supports. Polyurethane can be a problem in bulk storage as mold can infect the material, and special care has to be taken when disinfecting walls from crop to crop.

In bulk storage, the concrete floor should slope to the center of the storage building to a flume that runs along the full length of the building sloping to the door. The flume continues along to the sump in the packing area.

This continuous flume allows water to be used as a gentle carrier of carrots to the packing line. Water from a firehose is used to wash carrots from the pile into the flume system. In palletized storage, a forklift must handle each box individually.

Cooling and ventilation in bulk storage occurs by way of ducts that run under the carrot pile. Ducts constructed of wood or galvanized vented culverts run at 5 to 10 foot intervals for the full width of the building. Each duct is supplied with air from the main plenum that runs along one side of the storage building from end to end.

A storage design expert should be consulted when planning construction of any vegetable storage building.

Marketing

To market carrots through wholesale channels, the grower must meet Canada Grade Standards. Marketing carrots directly to the consumer through farm gate sales and farmers' markets is not regulated, and standards are generally set by the competition whether that be another grower or the wholesale market.

Legal cello package sizes for wholesale marketing are as follows. Remember that Canada number 1 grade standards must be met in all packs: diameter must exceed 3/4 inch, with jumbo grade diameter greater than 1 1/4 inch; length must exceed 4 1/2 inches.

• 2 lb, master pack of 24
• 3 lb, master pack of 16
• 5 lb, master pack of 10
• 10 lb and 25 lb
• 50 lb jumbo

All labeling on containers must meet provincial and federal regulations.

Packaged carrots must be kept under refrigeration and will store for several weeks if disease free. Once packaged, the objective should be to market the product as soon as possible.

Never store fruit along with carrots. All fruit expels ethylene gas that is readily absorbed by carrots. Carrots exposed to ethylene turn very bitter making them not suitable for sale.

Source: Agdex 258/20-1. September 2003.