Let’s look at your soil preparation first. As carrots grow, they are highly sensitive to soil conditions. Any obstacle like a rock, soil clod, large root, wood etc.… will cause a carrot root to change its direction leading to malformation of some kind. Too much nitrogen in the soil can also cause branching. Compacted soils or heavy soils full of clay will make it hard for carrots to penetrate and thus result in short, stumpy carrots. The best type of soil for growing carrots is deep (12’ deep), loose, sandy loam amended with ample compost and well-aged manure or a balanced fertilizer (5-10-10). Remove the rocks and large clods and aim to increase your soil health overall.
Studies have found a clear relationship between carrot plant density and the number of deformed roots. The higher the density, the more the deformities. To prevent this problem, thin your seedlings when they are 3-4 inches tall.
Weeds will compete with your carrots for space, soil nutrients, water, oxygen, and light. Ensure your carrot beds are regularly weeded.
Carrots are also highly sensitive to disturbance. They just don’t like being messed with, so it is better to sow seeds rather than transplant them when they are older.
Carrots, being biennials like kale, grow through two seasons; they have a first year and a second year. When this years’ carrots are left in the ground over winter, they will start growing again come spring but in this second year they will be in seed-making mode. The roots will become pithy, less tasty and start to branch naturally. Unless you want carrot seed, harvest your carrots in the first year.
Pest & Disease
Another culprit, Aster Yellows Phytoplasma is spread by leafhoppers. The disease causes pale, bronze-yellow leaves with green veins, curling foliage and deformed flowers and roots. Prevent this infection by mulching with aluminum foil or oat straw which will reflect sunlight from the soil and dissuade the leafhoppers from going near your carrots. Alternatively, grow your carrots under row covers to keep leafhoppers away. Throw out all affected carrots – do not compost.
For more information on dealing with carrot deformities check out Wonky Carrots: How to Avoid Deformed & Distorted Carrots