Beets grow well is the St, Paul, Minnesota area since the soil is so rich. But they often grow too well since they can get overgrown very quickly and beets that are overgrown can be tasteless and woody.
It is better to harvest small beets then big, hard, woody over grown beets. Once the beets in your garden have grown to be a noticeable size you should pull one up and test it to see if it is getting woody. You can tell by cutting the beet in half and if the middle is red and juicy the beet is good to eat. If the beet has rings that are easily visible or white looking your beets are starting to get overgrown.
You can still eat your hard beets but they are not as desirable. Pull up your beets earlier rather then later and store them in your refrigerator until you are ready to use them. Beets will store well for weeks in a cool area.
Some beets can be pulled up easily out of the soil. If your beets are hard to pull up by hand you should use a spade or shovel to dig them up but be careful you do not slice into the beets as you are digging them up.