TOPS Learning Systems Life Sciences, grades 3-8. Radishes aren't fussy. Provide them with a covered, moist towel, and away they grow. In just a day the seeds will sprout. By the end of the week you'll get sprouts one or two inches tall. Because radishes grow so fast, tropisms are easy to observe: see roots grow toward water, leaves tilt toward light, stems bend against gravity. Kids will draw, measure and graph growth, eat a tiny sun salad, study toxic stress!