The natural habitat of this sundew is the small peaty wetlands of the Kleinriviersberg Mountains and the southern Kogelberg, between 70 and 100km south-east of Cape Town. In the Kleinriviersberge this species is known from the boggy beginnings of at least two streams, and includes the type location. In this area it grows in the company of D. admirabilis, D. aliciae, D. capensis, D. x corinthiaca (=D. aliciae x D. glabripes), Utricularia bisquamata, and the sticky-leaved Roridula gorgonias. In the southern Kogelberg this species has been collected in the Palmiet River catchment and has narrower leaves than those of populations to the east. In both areas the plants grow in locations which receive precipitation throughout the year thanks to the not infrequent summer mists. These areas also experience periodic fires, which remove the above-ground growth of this sundew, but it readily resprouts from the roots.
Drosera slackii is an easy plant in cultivation, preferring a peat-based mix, a deep pot, water year-round, and plenty of sun. Whilst not experiencing frost in its natural habitat it is mildly frost tolerant and will resprout from the roots if the rosette is damaged. It is easily propagated by leaf cuttings, root cuttings and, when available, seed.