The right container depth is vital to your carrot harvest. Carrots come in all shapes and sizes, from "baby" 7.5 cm (3 inch) carrots to 27.5 cm (11 inch) long commercial types. Sliced in salads and stews, steamed, grated for coleslaw, or just peeled and eaten raw, carrots are multipurpose vegetables that add vivid colour to your meals. Hundreds of cultivated carrot varieties exist and over 30 seed types are available through garden centres, seed catalogues and online sites.
Carrot types include the long thin types reaching 22.5 cm (9 inches) or more (Imperator), cone-shaped and thicker medium length types at 15 to 20 cm (6 to 8 inches) (Danvers, Chantenay and Autumn Kings) and the short cylindrical types (Nantes). Speciality carrots exist in two types. Round carrots, such as Orbit, are roughly golf ball sized. "Baby" thin tender types, like Little Finger, grow only 7.5 to 12.5 cm (3 to 5 inches) long. Varieties like Chantenay and Danvers may have representatives in all three of the usual size groups. It's always a good idea to check seed packets or plant descriptions for mature size before buying containers to plant your carrots.
Carrot root structure
Proper soil depth is important since carrots grow a long taproot below the thickened part of the root that we eat. John Weaver, in his 1926 study, "Root Development of Field Crops", found that two-month old Chantenay carrots had tap roots up to 45 cm (18 inches) long. The carrot's feeder roots are "laterals" that grow out into the upper 5 to 22.5 cm (2 to 9 inches) of soil. Your planter box needs to be wide enough to space carrots properly and deep enough for tap root development.
While there is an average depth recommended for carrots, the depth of your planter box is dictated by your carrot type. Depending on your carrot variety and mature length you will need between 22.5 and 45 cm (9 and 18 inches) in box depth. Longer carrots with bigger root systems need deeper planter boxes. Using boxes at least 45 cm (18 inches) deep will allow room for most standard varieties. Deeper boxes also can increase your yield, as you'll see in the next section.
Depth affects yield
Carrots need loose, well-drained, rock free soil like the potting soil used in your planter box. Potting soil has the advantage of being formulated for particular uses, like raising vegetables. The drawback to planter boxes filled with potting soil is the tendency for the soil dry out quickly. Shallow planter boxes are especially prone to drying out and may not have enough soil volume to hold nutrients for carrots. Use boxes at least 45 cm (18 inches) deep to avoid the need for constant watering and repeated fertilising. The bigger the boxes, the bigger the yield. An additional 5 cm (2 inches) of depth can more than double your harvest, they say.