Erigeron sumatrensis obviously is a thermophilous species and chiefly occupies sun-exposed, dry and often stony habitats: foot of walls, fences, disused railway yards, coalmine heaps, old walls, road verges, railway tracks or canal banks.
The best way to decide whether your plant is ready for a drink, is if the top of the soil is dry and the pot is beginning to feel light.
Dead or dying sections of the plant can encourage pest infestation or disease. Removing these not only makes your houseplant look better, but reduces the risk of further problems down the line.