P.W. Simon, USDA, ARS, Department of Horticulture, University of Wisconsin, Madison, WI 53706 USA
Production of carrot seeds is a two-year project making it much more difficult than seed production of annual crops. Sound roots must first be produced (first year), these roots must be either harvested and carefully stored in a refrigerator or left in the field (if your winters are not too severe), these cold-treated (vernalized) roots must be grown and supplied with pollinating insects when flowering, and seed harvested. Carrot seed must be produced where no wild carrot (Queen Anne’s Lace) is growing.
1. Grow vigorous plants.
Grow plants at the same time of year and the same way you usually would. Weak or diseased plants are subject to storage loss during vernalization and they set few seeds. Plants with pencil-sized roots may be large enough, but larger roots are preferable.
2. Vernalize roots.
Carrots require 6 to 8 weeks cold treatment (2 to 5°C) for floral induction. Cool growing conditions can reduce the cold storage requirement. Roots can be vernalized in two ways. If carrots are a summer crop in your area, you can simply leave roots in the field over your winter season if your climate provides at least 10 weeks of temperatures below 15°C but where temperatures are not so cold that roots will freeze to death. Early in the growing season plants should be thinned to at least 5 cm apart. Dead or dying leaves must be removed and tops can be cut back at your usual harvest time to 5 cm to reduce transpiration and covered with mulch if necessary. When warm weather resumes, remove mulch if necessary and leaves will regrow and after several weeks a seed stalk will appear. This is the “seed-to-seed” method of carrot seed production. Losses are often very high with this method and off-types of roots cannot be eliminated since roots are not harvested and visually examined. The “root-to-seed” method is more reliable. Harvest roots when you usually would and discard off types. Trim tops back to 2 to 4 cm, air dry until no surface moisture remains, pack in paper bags with an equal volume of wood shavings, and place paper bags in closed polyethylene bags at 2-5°C. Puncture plastic bags after several weeks when water droplets accumulate inside the polyethylene film. Better storage survival is realized if prior to refrigeration, lateral and fibrous roots are removed, soil is removed by washing gently, and senescing leaves are removed. Even with these precautions carrots are often very susceptible to pathogen infection during storage. In commercial production, roots are dipped in fungicide before vernalization but this practice is not advised without extreme caution.
3. Grow vernalized plants.
For the “root-to-seed” method, plant vernalized roots when you would plant seed in the spring, taking care to keep plants well-watered but not in standing water. Seed stalk development will be evident in 4 to 6 weeks. Control of microbial (Alternaria, Cercospora, aster yellows, motley dwarf) and insect pests (aphids, spider mites, lygus bugs) is essential to assure seed production. It is very difficult to produce carrot seed where warm humid climates favor microbial growth. Note: Carrot seed can only be produced true to type if wild carrot (Queen Anne’s Lace) is not growing nearby since wild carrot will intercross and yield white-rooted plants.
Pollination is best performed by introducing bees or flies for pollen transfer during the period of receptivity. Natural populations of bees and other insects will sometimes be adequate. As an alternative, pollen movement is possible by hand or brush but seed set will often be low. Within 4 to 6 weeks after pollination the developing seed turns brown. Harvest (before the seed shatters) into paper bags to dry completely. Late-season rains can reduce seed yield drastically. Remove spines from dry seed by rubbing. Seed is now ready to plant since carrot has no seed dormancy. Store dry seed refrigerated in a moisture-proof container.