Cylindra Garden Beets should be planted outside when the temperature is between 60 and 70 degrees. You’ll need to make sure to water your beets regularly, and just right. Too much water and your beets will develop small roots with overly lush tops. Too little water and your beets might taste woody. Start with ½ inch of water twice per week. Pay attention to your local weather and adjust watering accordingly. Full-sized leaves are ready for harvest as soon as they are four to five inches tall. Beet roots are at their sweetest when they are between two to four inches long.