Answer: Bitterness, or poor flavor generally, is usually the result of stress to the plants, and that could be anything that slows or checks growth. The most common stress is insufficient moisture in the soil. Poor or off-flavors can also suggest that the soil needs to be limed, or supplied with greater amounts of humus and potash.
Bitterness in carrots can also be caused by pests such as the carrot rust fly and wireworm. A harsh flavor can also be connected to the variety, and the time of harvesting. Bitter terpenoids present in carrots apparently develop earlier than sugars. In some varieties, experiencing their full, sweet flavor is just a matter of waiting for the sugars to develop (leaving them in the soil until after the first frost of the season).
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