I have never grown D. hartmeyerorum before, but I have grown the closely related D. indica successfully for many years. D. indica grows well for me when I treat it like a summer annual. I germinate the seeds in late spring on a 50:50 peat and pumice sand mix and they take off as the summer warms and by mid summer are in flower and setting seeds. I have tried to keep them more than a year, but now don't see the need. They grow to about 50+cm this way, before the cooler temperatures stop their growth. The soil mix seems to me to be secondary to providing the plants with warmth and good air circulation. My plants grow outside in a greenhouse.
Kind regards from New Zealand