When to Harvest Beets:
Harvest beets when root tops are 1 to 1½ inches in diameter but not more than 3 inches in diameter. Beets that are allowed to get much bigger than 3 inches (7 cm) across often become tough and fibrous.
Beets for kitchen use mature 45 to 60 days after sowing.
Beets are cool-season plants that grow best at temperatures between 60° and 65 °F (15-18°C). Beets do not grow well and flavor will suffer if grown where daytime temperatures are consistently greater than 80°F (26°C).
Where winters are mild and the soil is well drained, leave beets in the ground as long as possible. Where the ground freezes, lift beets before the soil freezes or protect them under a 12-inch (30 cm) thick layer of mulch–leaves, straw, or hay—that covers the planting bed and extends out 18 inches (45 cm) or more. If protected from freezing, the mulch can be pulled back during the winter and roots lifted.
Beets stored in the garden must be harvested before new top growth begins in spring. Roots stored in the ground should be used within a day or two of harvest.
How to Harvest Beets:
It’s best to pull beets when the soil is dry. Pull or lift roots carefully from the garden so as not to break or injure them. Use a garden hand fork to loosen the soil around the roots if necessary.
Twist off the greens after lifting the roots. Cutting away the top of a root will cause it to bleed. Removing the tops before storage will greatly extend storage life.
How to Store Beets:
Store the best roots; damaged or bruised roots will not store well and should be eaten soon after harvest. Gently rub soil from the roots before storing them. Don’t wash roots before you store them; if you do, be sure to dry them thoroughly.
Store beets in a cold moist place as near to freezing as possible without actual freezing, 32°-40°F (0°-4°C) and 95 percent relative humidity.
Store beets in the refrigerator placed in a perforated plastic bag in the vegetable crisper drawer. Beets will keep in the refrigerator for 1 to 3 months.
If there is not room in the refrigerator, beets can also be packed in a container—a bucket or plastic storage box or cooler–in moist sand, peat moss, or sawdust. Don’t pack roots too tightly; if the roots touch they can start to rot; be sure to leave 2 inches (5 cm) of insulating material around at the top, bottom, and sides of the stored roots. Set the lid loosely so that there is good air circulation and place the container in a cold place such as a basement, garage, or shed.
Check roots in storage often and remove any that show signs of deterioration.
More tips at How to Grow Beets.