What causes bolting and deformed roots in carrots, and how can they be prevented and controlled?

The carrot, also known as red radish, clove radish, and hulufujin, and also called hulufu, red beet, and yellow radish, is an umbelliferous biennial herbaceous plant, grown as a vegetable for its fleshy root. Every 100 grams of fresh carrot contains 1.67-12.1 mg of carotene, 5-7 times higher than that of tomatoes. After eating, it is digested and broken down by the stomach and intestines into vitamin A, which can prevent and treat night blindness and respiratory diseases. It can be stir-fried, boiled, eaten raw, pickled in sauce, salted, etc., and stores well. It is distributed all over the world, and is cultivated in both northern and southern China, with output ranking second among root vegetables. It can fight cancer, and is known as the underground “little ginseng”.

The growing environment of carrots

Temperature

Temperature: Carrots are a semi-cold-tolerant vegetable, and their temperature requirements differ between the vegetative growth stage and the reproductive growth stage. Seeds can germinate at 4-6°C and begin to grow at 8°C; at 18-20°C they emerge in about 10 days. Seedlings can endure short periods of low temperature of -3 to -4°C, and can also grow normally under high temperature conditions above 27°C. The optimum temperature for the germination stage of carrots is 20-25°C; the suitable temperature for the vigorous leaf growth stage is 20-25°C during the day and 15-18°C at night; for the fleshy root swelling stage, 20-22°C is suitable, and growth stops below 3°C. Carrot seed roots can be planted when the soil temperature reaches 8-10°C. The suitable air temperature for carrot flowering, pollination, and seed setting is 22-28°C during the day and 15-20°C at night. If the daytime temperature exceeds 35°C, pollination and seed setting cannot occur.

Light

Light: Carrots are a light-loving, long-day plant, especially during the vegetative growth stage, requiring light of medium intensity or above.

Water

Keep the soil moist when sowing to promote seed germination and uniform emergence. During the seedling stage and the vigorous leaf growth stage, water when dry and let dry between waterings, both to ensure normal aboveground growth and to increase soil aeration, promoting good taproot development. The fleshy root swelling stage is the period of greatest water demand; water evenly to meet the needs of fleshy root swelling, and also prevent drastic changes in irrigation amount from causing cracked roots. Carrots generally stop being watered 10-15 days before harvest of the fleshy roots, to reduce cracking and facilitate storage and transport.

Soil

Carrots prefer sandy loam and loam with deep soil layers, loose texture, good drainage, and high porosity. In poorly aerated, heavy clay soil, the fleshy roots are lighter in color, have many fibrous roots, and are prone to developing tumors, resulting in poor quality; in low-lying places with poor drainage, the fleshy roots are prone to breaking, often causing rot and an increase in forked roots. Carrots have a relatively wide adaptability to soil pH, growing well in soil with a pH of 5-8, but growing poorly in soil with a pH below 5.

The area of off-season spring-sown carrots is increasing day by day. However, due to improper selection of spring-sown varieties and unsuitable cultivation techniques, large-scale bolting and the production of large numbers of deformed roots often result, bringing great economic losses to the majority of growers. So what are the causes of carrot bolting and deformed root formation, and how can they be prevented and controlled? Let us introduce them below.

Causes and prevention of bolting

Causes of bolting:

The carrot is a green-body vernalization long-day crop. According to the different sensitivity of different varieties to low temperature, they can be divided into easy-bolting varieties, relatively bolting-tolerant varieties, and bolting-resistant varieties. Easy-bolting varieties can sense low temperature at the seedling stage when they have 4-5 true leaves or even 2-3 true leaves and undergo flower bud differentiation, and then bolt and flower under long-day conditions in May and June. According to research by Japanese scholars, for “Kuroda Gosun” type varieties, if the seedlings encounter cumulative low temperature below 10°C for more than 360h, there is a risk of bolting; the lower the temperature and the longer the duration of continuous low temperature, the higher the bolting rate, reaching as high as over 90%.

Prevention and control methods for bolting:

It is understood that at present, most regions of China still choose Japanese autumn-sowing varieties such as “New Kuroda Gosun” or varieties of the Kuroda Gosun line, such as “Juyang Gosun”, for spring sowing. These varieties all belong to the easy-bolting type, so to avoid losses, spring sowing requires selecting varieties that are insensitive to low temperature and resistant to bolting.

Causes and prevention of deformed roots Causes of deformed roots:

Common deformed roots of carrots include forking, cracked roots, curvature, nodular protrusions, green shoulders, long fibrous roots, and color variation. The formation of the carrot fleshy root requires good soil conditions, cultivation techniques, and climatic conditions. If the plow layer is too shallow, the soil is rough and contains stones, or undecomposed organic fertilizer is applied, it easily leads to forking and curvature; heavy, poorly aerated soil easily produces nodular protrusions and fibrous roots; uneven water supply during the growth period, alternating drought and moisture, easily leads to an increase in cracked roots; if the plow layer is too shallow and earthing up is not paid attention to during the root swelling stage, it is easy to produce the root swelling stage occurring in the high-temperature period of July and August, causing the accumulation of carotene and lycopene to be blocked, resulting in color variation, turning white or yellow. Prevention and control methods for deformed roots:

Therefore, in order to obtain high quality and high yield of spring-sown carrots, sandy loam soil with loose, fertile texture and good irrigation and drainage conditions should be selected; at the same time, note that the depth of the plow layer should not be less than 25 cm, apply decomposed organic fertilizer as base fertilizer, supply water evenly during the growth period, and pay attention to earthing up in the early stage of fleshy root swelling. The suitable temperature for carrot fleshy root swelling is 18-25°C, so in central China it should be done from mid-March to early April, and other regions of China need to choose a suitable sowing period according to local temperature.