Aloe

Aloe vera is a succulent perennial grown almost everywhere as an easy houseplant: a low rosette of thick, fleshy, toothed, grey-green leaves filled with the clear gel it is famous for. POWO (Kew) gives its native range as the Arabian Peninsula, though the exact wild origin is uncertain after millennia of cultivation, and it is now grown worldwide as the familiar medicinal aloe. Indoors it wants bright light or a sunny windowsill and very infrequent watering — it is highly drought-tolerant and rots if overwatered, which is the single most common way to kill it. Its uses are genuinely nuanced: the clear inner leaf gel is widely applied to skin (minor burns) and is consumed in aloe juices and drinks, but the yellow latex just beneath the rind is a powerful laxative and irritant that should not be casually eaten (and is best avoided in pregnancy), and the whole plant is toxic to cats and dogs. It is frost-tender and hardy outdoors only in roughly USDA zones 9b-11; everywhere colder it lives in a pot. RHS gives it the Award of Garden Merit as an easy succulent houseplant and rates it H1C. Its air-purifying reputation, drawn from a single 1989 NASA chamber study, is negligible at realistic room densities — grow it for the plant itself, not to clean your air.

Cape aloe (Aloe ferox) is a bold, single-stemmed tree-aloe and the source of the bitter aloe of commerce. GBIF gives its native range as the Eastern and Western Cape of South Africa and into Lesotho, part of the Cape Floristic and Karoo succulent flora. It builds a thick trunk clothed in a skirt of old dried leaves, topped with a big rosette of thick, blue-green, red-toothed, spiny leaves, and in winter sends up tall candelabra spikes of orange-red (sometimes yellow) flowers that are a superb cold-season nectar source — in the wild they are pollinated by sunbirds. It is an architectural, extremely drought-tolerant succulent for full sun and very sharp drainage: the number-one way to kill it is overwatering, because wet, cold soil rots the roots and crown faster than drought ever will, so water sparingly and never let it sit wet. It is frost-tender — RHS rates it H3, hardy only to a light frost — so outside warm, dry climates it is best grown large in a pot and moved under cover for winter. The bitter yellow leaf latex (the source of the laxative drug "Cape aloes") is a strong purgative and is toxic if eaten, so the plant is not food. Hardy outdoors in roughly USDA zones 9a-11; everywhere colder it lives under cover.