Eucalyptus

Cider gum is a fast-growing evergreen tree from the woodlands of Tasmania's central plateau and one of the most cold-hardy eucalypts: Trees and Shrubs Online calls it the hardiest of the tree-form eucalypts planted widely enough to judge, and the RHS rates it H5, hardy in most of the UK even in severe winters. Plants For A Future gives it long spells down to 7°F and short ones down to -0°F. Left unpruned it grows into a tree of 40 to 100 feet with peeling cream and brown bark and grey-green, sickle-shaped adult leaves, but many gardeners cut it back hard every spring to keep it in the rounded, glaucous-blue juvenile foliage it is best known for. Small white flowers in clusters of three appear on trees allowed to mature, usually in summer or autumn in British gardens. The name refers to its sweet sap, which the Aboriginal people of Tasmania fermented into a drink called way-a-linah. The leaves hold eucalyptus oil, which is poisonous if swallowed (Plants For A Future reports deaths after ingestion of 4-24 ml of eucalyptus oils), so keep foliage, prunings and any oil away from children and pets. Outside its range it has naturalized in woodland and on roadsides in south-eastern England, where it occasionally regenerates from seed, while in the wild it is assessed as Endangered on the IUCN Red List.

Tasmanian blue gum is a towering, very fast-growing evergreen tree from southeastern Australia (Tasmania and coastal Victoria) - one of the most widely planted eucalypts in the world, and one of the most widely regretted outside its native range. It reaches 90 to 150 feet or more, with a tall straight trunk, peeling ribbons of bark, and aromatic blue-green to gray-green foliage that carries the classic 'eucalyptus' scent. Honesty first, because it is load-bearing here: in California and other Mediterranean-climate regions this is an INVASIVE tree. It self-seeds aggressively, displaces native vegetation, and is allelopathic - its leaf litter and oils suppress the plants beneath it. Worse, it is a serious FIRE HAZARD: the oil-rich leaves, the long shedding bark ribbons, and the deep litter of dropped leaves and bark burn explosively, which is why these trees are nicknamed 'gasoline trees' and have fueled some of the most dangerous wildland fires. We strongly discourage planting Eucalyptus globulus near homes or in fire-prone, wildland-adjacent areas; plant a regionally-appropriate, fire-smart native tree instead. The aromatic foliage is the source of commercial eucalyptus oil, but that oil is TOXIC if ingested in any quantity and the foliage is not food - this is not an edible plant. It is also simply enormous and fast: not a tree for a small property.