Hi Jeremy, D.capensis doesn't have a true dormancy but if it gets
too cold the top will die and if it didn't get too very cold the bottom
and the roots will still be alive and grow again. Of course, you'll
want to spare the plant this, so keep it at about room temp year
round. Nearly all plants will benifit from seaonal lighting changes
however, and it will still be expecting longer light in the summer
and less in winter. The date for starting/ending dormancy is not
important at all, really. Just do it once a year. Depending on
where a plant comes from will tell you about how long it needs/will
stay dormant. The closer to the poles the plants get or higher
in elevation, the longer they will want to be dormant in general.
S.purpurea subsp. venosa is the more southern type so about three
months in cooler temps about 50~F. will be very good for the plant.
You can let it freeze, but this is risky if you don't have the
right set up for it. A potted plant shouldn't be allowed to freeze.
The pot can easily dry out before you know it, since frozen soil
looks just like dry soil.
Dave Evans