During the cold winter months, Dwarf St. John's wort needs a certain measure of cold in order to stay in dormancy until it’s time to sprout. Sprouting too early, that is before the danger of the last frost has passed, can be fatal to Dwarf St. John's wort, especially if it’s already had a head start when the frost hits. Winter temperatures should ideally stay below 32℉(0℃), but if they get up to 40℉(5℃), everything will be just fine.
An unexpected warm spell during the cold months, which can happen in more temperate climates like woodland rainforests, can trigger a premature sprout from Dwarf St. John's wort. In this case, if there’s still imminent danger of frost, you may want to try covering it with clear plastic on stakes so that the cold has less of a chance of damaging the new sprout. This setup can be removed when the danger of frost has passed. Occasionally, Dwarf St. John's wort will be able to resprout at the correct time without any help, but this method increases the chances of a successful second sprouting.