Radishes (Raphanus sativus) are known to be an easy-to-grow crop that quickly matures. Growing them can be an educational mother-child activity. Proper growing practices are essential because otherwise this cool-season vegetable might refuse to develop roots -- you might see the leaves above-ground, while nothing is developing in-ground. To avoid disappointment, learn what can make your radishes refuse to bulb and how to prevent this.
Lack of space is one of the main reasons why radishes won't bulb. Thickly seeding and not thinning can crowd the roots and impair their development. When direct-sowing, plant the seeds at a depth of 1/4 to 1/2 inch in rows that are spaced 1 foot apart. Thin spring varieties to about 1/2 to 1 inch apart, and thin winter varieties to at least 2 to 4 inches because they grow larger roots. If you're broadcasting the seeds on a bed, thin the seedlings so there's 2 to 3 inches between them in all directions.
Radishes won't bulb properly when exposed to temperatures above 80 degrees Fahrenheit. Although sudden heat spells are unpredictable, starting the seeds at the correct time increases your chance of success. Spring varieties are best started in the spring as soon as the soil is workable or in a cold frame or indoors in late winter. Winter varieties, which take longer to mature, are best started in mid to late summer around the same time late turnips are started.
If your radishes don't bulb, reassess their growing location. Radishes can grow in partial shade, but if you're growing them in an extreme shady area, they might not bulb and prefer to put all their energy into producing leaves. For optimal results, grow the plants in an area of the garden with full sun. At least six hours of sunlight is preferred. Also ensure that tall plants that are growing nearby aren't blocking the sunlight.
Excessively fertilizing with nitrogen can trigger your plants to focus on their tops, resulting in small roots. To prevent this, properly prep the soil before starting the seeds. Aim for loose, well-drained soil with a pH between 5.8 and 6.8. Cultivate the soil to a depth of 8 to 12 inches and remove rocks and other debris to prevent misshapen roots. Work in a 10-20-10 fertilizer at a rate of 1 cup per 10 feet of row, and then, rake the soil to smooth it.