In northern Idaho. we plant garlic in October or November and beets in the spring. I would like to interplant them, since we harvest garlic in July and beets a month or two later. The garlic has a great head start on leaf development, so I wonder if it would shade the new beet sprouts too much. We have a 4x6 raised bed for 60 garlic plants. Any advice?
Have you considered that, depending on how close you plant, you might end up disturbing your beetroots, as you pull out your garlic? I wouldn't worry too much, however, since the beets have a very long taproot. But, just be aware of your spacing.
Also, from what I've read, beets need full sun, although I've managed to grow them in my 4x6 raised bed that only gets morning sun, then bright light the remainder of the day. Mine get very little full-on sunshine...
Finally, (if you haven't already) Google and read up on companion planting garlic and beets to see if they're compatible. I'm wondering, because too much water on the garlic will cause them to rot, and beets need consistently moist soil and space between them (3-4") to grow good size roots.