How to Grow Carrots and Radishes in Containers, Depth and Timing

Pick the pot by how long the root will be, then sow often. North Carolina Cooperative Extension says containers for carrots and radishes should be 1.5 times deeper than the mature length of the root. Illinois Extension adds that radishes are ready fast and should be picked quickly, while carrots take roughly 50 to 70 days.

1. Depth: the 1.5 times rule

The N.C. guide gives the example of a carrot that will be 8 inches long at maturity: choose a container 12 inches deep. The same rule applied to other lengths:

The guide also suggests dwarf carrot varieties such as Thumbelina and Short 'n Sweet, which mature more quickly and do not need as deep a container. Illinois lists finger carrots as a type that is ready sooner. Virginia's container table lists a minimum container size of 1 quart for carrots and its table does not give a depth. Use the N.C. 1.5 times rule for depth.

2. Sowing carrots

Illinois Extension says to plant carrot seed 1/4 to 1/2 inch deep in early spring, and a little deeper (1/2 to 3/4 inch) for later sowings when the soil is drier and warmer. Germination can take as long as two weeks, and seedlings may not emerge uniformly. The guide notes that a heavy rain after sowing can pack the soil surface so that no seedlings emerge.

Two Illinois care tips:

• Carrots germinate best in warm, moist soil. Illinois describes covering the row with clear polyethylene film to warm the soil and hold moisture, and removing it immediately when seedlings appear.
• In summer heat, some shade may be needed to keep tiny seedlings from burning off at the soil line.

Illinois adds that young carrot seedlings are weak and slow, so weeds must be kept down during the first few weeks.

3. Spacing

For containers, N.C. Extension says to space root plants 2 to 4 inches apart depending on the variety, and warns that plants grown too closely together will not develop wide roots. Virginia's table gives 2 to 3 inches between carrot plants. Illinois gives tighter thinning numbers for garden rows (for example, one seedling per 1 to 2 inches for large carrots like Danvers and Chantenay), so container growers should follow the wider container figures from N.C. and Virginia.

4. When carrots are ready

Illinois says carrots can be pulled once roots are at least 1/2 inch in diameter. Finger carrots are usually ready within 50 to 60 days. Other varieties should reach at least 3/4 inch in diameter, about 60 to 70 days after planting, and can then be harvested over a 3 to 4 week period. Virginia lists 65 to 80 days from seed to harvest and recommends several plantings at two-week intervals. N.C. says to harvest when roots start to swell and before plants bolt.

5. Radishes: fast and short-lived

Illinois describes radish as a cool-season, fast-maturing crop. Its planting advice:

• Plant spring radishes from as early as the soil can be worked until mid-spring.
• Make successive plantings of short rows every 10 to 14 days.
• Sow 1/4 to 1/2 inch deep.
• Spring radishes can also be started in a window box or container in the house or on the patio.

Slow development makes radishes hot in taste and woody in texture, Illinois says, and harvest should begin as soon as roots reach edible size and finish quickly before heat, pithiness or seed stalks develop. Later varieties such as Icicle or French Breakfast withstand heat better, and Illinois says French Breakfast holds up in summer heat if watered regularly. Additional sowings of spring types can start in late summer to mature in fall.

N.C. Extension gives the container timing as spring and fall, with replanting every 2 to 3 weeks for continuous harvest. For storage, Illinois says radishes keep 5 to 7 days in the refrigerator in plastic bags with the tops removed.

6. Winter radishes need more room

Illinois says winter radishes are sown in midsummer to late summer, mature more slowly than spring types, and grow considerably larger. It says they must be thinned to 2 to 4 inches or farther apart. That is a plant spacing in garden beds. A large daikon, which Illinois says can reach about 18 inches long and 5 or 6 pounds, would by the 1.5 times rule need a container about 27 inches deep. The sources do not say daikon is a good container crop, so treat that as an arithmetic result, not advice.

7. Key takeaways

• N.C. Extension: make the pot 1.5 times as deep as the mature root length (12 inches for an 8-inch carrot).
• Space roots 2 to 4 inches apart in containers; crowding stops roots from widening.
• Carrots can take up to two weeks to germinate and are ready about 50 to 80 days after sowing, depending on type and source.
• Sow radishes every 10 to 14 days and pick them promptly before heat makes them woody.
• N.C. Extension gives spring and fall as the sowing seasons for both crops.